Target Delineation Research Articles

DART: diagnostic-CT-enabled planning: a randomized trial in palliative radiation therapy (study protocol)

Palliative radiotherapy (PRT) is an effective treatment for managing symptoms of advanced cancer. At least half of all radiation treatments are delivered with palliative intent, aimed at relieving symptoms, such as pain or shortness of breath. Symptomatic patients must receive PRT quickly, therefore expeditious treatment planning is essential. Standard radiation planning requires a dedicated CT scan acquired at the cancer centre, called a 'CT simulation', which facilitates treatment planning (i.e. tumor delineation, placement of radiation beams and dose calculation). However, the CT simulation process creates a bottleneck and often leads to delays in starting treatment. Other researchers have indicated that CT simulation can be replaced by the use of standard diagnostic CT scans for target delineation and planning, which are normally acquired through the radiology department as part of standard patient workup. The goals of this feasibility study are to assess the efficacy, acceptability and scalability of diagnostic-CT-enabled planning, compared to conventional CT simulation planning, for patients receiving PRT to bone, soft tissue and lung disease. This is a randomized, phase II study, with 33 PRT patients to be randomized in a 1:2 ratio between conventional CT simulation (Arm 1), and the diagnostic CT enabled planning workflow (Arm 2). Patients will be stratified by treatment target volume (bone and soft tissue metastasis vs. primary or metastatic intrathoracic disease targets). The primary endpoint is the amount of time the patient spends at the cancer centre. Secondary endpoints include efficacy (rate of plan deliverability and rate of plan acceptability on blinded dose distribution review), stakeholder acceptability (based on patient and clinician perception of acceptability questionnaires) and scalability. This study will investigate the efficacy, acceptability and scalability of a "sim-free" PRT pathway compared to conventional CT simulation. The workflow may provide opportunity for resource optimization by using pre-existing diagnostic imaging and requires minimal investment due to its similarity to current PRT models. It also offers potential benefit to patients by eliminating an imaging procedure, reducing the amount of time spent at the cancer centre, and expediting time to treatment. Clinicaltrials.gov identifier: NCT05233904. Date of registration: February 10, 2022; current version: 1.4 on April 29, 2022.

The importance of standardized treatment planning and decision-making in radiation oncology for non-small-cell lung cancer—are current guidelines sufficiently strict for uniform target delineation?—a narrative literature review

The importance of standardized treatment planning and decision-making in radiation oncology for non-small-cell lung cancer—are current guidelines sufficiently strict for uniform target delineation?—a narrative literature review

Current Radiotherapy Considerations for Nasopharyngeal Carcinoma.

Radiotherapy is the primary treatment modality for nasopharyngeal carcinoma (NPC). Successful curative treatment requires optimal radiotherapy planning and precise beam delivery that maximizes locoregional control while minimizing treatment-related side effects. In this article, we highlight considerations in target delineation, radiation dose, and the adoption of technological advances with the aim of optimizing the benefits of radiotherapy in NPC patients.

MRI-only radiotherapy from an economic perspective: Can new techniques in prostate cancer treatment be cost saving?

Background and PurposeThe aim of this study was to analyze a magnetic resonance imaging (MRI)-only radiotherapy workflow from an economic perspective in terms of reduced time, costs and systematic uncertainties. Material/MethodsA documented Swedish clinical implementation of MRI-only radiotherapy was used as template for cost assessments compared to a combined computed tomography (CT)/MRI workflow. The costs were taken from official regional price lists from 2021. MRI-only specific quality assurance (QA) was assumed necessary in an initial phase. Treatment plans for target volumes with margins of 5–10 mm were created for ten prostate cancer patients prescribed 78 Gy in 39 fractions. The risk of Grade ≥ 2 rectal toxicity or rectal bleeding was calculated using the QUANTEC recommended NTCP model and costs estimated based on subsequent diagnostic examinations. ResultsThe exclusion of the CT-examination and faster target delineation were the main contributors to cost reductions. Additional QA procedures limited the initial cost reduction to 14 EUR/patient. Long-term MRI-only reduced the costs by 209 EUR/patient. Reducing margins resulted in Grade ≥ 2 rectal toxicity or rectal bleeding probability of 9.7 % for 7 mm margin and 6.0 % for 5 mm margin. This margin reduction resulted in an additional cost reduction of 46 EUR/patient. ConclusionAn MRI-only workflow implementation is associated with reduced costs when the workflow tasks are more time efficient and side effects are reduced as a result of margin reduction. The short-term economic benefits are limited due to extra costs of QA procedures. The economic benefits of MRI-only will make impact first when the workflow is well established, and margin reduction has been included.

RADT-31. 68GA-DOTATATE PET VERSUS MRI-BASED TREATMENT PLANNING FOR POST-OPERATIVE AND INTACT MENINGIOMA: A DOSIMETRIC ANALYSIS

Abstract Background Nearly all meningiomas express somatostatin receptor 1/2 (SSTR1/SSTR2); therefore, SSTR ligands such as 68Ga-DOTATATE can be utilized for meningioma radiotherapy treatment planning. Incorporation of 68Ga-DOTATATE PET assists with target delineation and may reduce the dose to organs-at-risk (OARs). We hypothesize that 68Ga-DOTATATE PET-based treatment plans will reduce dose to OARs when compared to MRI-based plans. METHODS All treatment plans were rendered on computed tomography (CT) planning datasets, using RapidArc and 6MV photon energy. Two arcs were positioned on the coplanar axis with alternated collimator settings of 0 and 90 degrees to promote MLC sparing with a third unique vertex arc chosen independently to spare normal brain tissue and associated organs at risk (OAR). For MRI structures, a 1 cm expansion from the gross tumor volume (GTV) created a clinical treatment volume (CTV)60. A 2 cm expansion created a CTV54. For PET structures, a 1 cm expansion from the GTV created a CTV60. A 3 mm isotropic expansion created planning treatment volumes (PTVs). Plans were optimized such that Dmax to brainstem remained limited to 60Gy or less and Dmax to optic structures was limited to 54Gy or less. RESULTS 18 meningioma patients were included (9 post-operative, 9 intact). The mean PTV volume using MRI was 258 ccs (306 ccs for post-operative, 210 ccs for intact), and the mean PTV volume using PET was 60 ccs (97 ccs for post-operative, 91 ccs for intact). The mean radiation dose to both optic nerves, the optic chiasm, both hippocampi, and the brainstem was reduced favoring PET-based treatment plans for both post-operative and intact patients. CONCLUSION Our study shows that incorporation of 68Ga-DOTATATE PET can reduce dose to OARs for post-operative and intact patients, and this difference may translate to reduced toxicity. 68Ga-DOTATATE PET guided radiation treatments are worthy of future prospective investigation.

SURG-06. PREOPERATIVE NEOADJUVANT GAMMA KNIFE RADIOSURGERY IN THE TREATMENT OF LARGE BRAIN METASTASIS: AN INITIAL CASE SERIES OF SINGLE INSTITUTE

Abstract BACKGROUND Postoperative stereotactic radiosurgery (SRS) following the surgical resection of large brain metastasis is a promising therapeutic option to reduce local recurrence. However, there is an increased risk of radiation necrosis and leptomeningeal spreading and an issue of problematic target delineation. Neoadjuvant SRS, also called preoperative SRS, is an emerging treatment paradigm which delivers radiation in the preoperative setting. An initial series of single-institute cases treated with the neoadjuvant SRS followed by surgical resection of a large BM was reviewed. Method: Between 2019 and 2022, 5 patients underwent neoadjuvant SRS using Gamma Knife radiosurgery (GKS) unit, followed by surgical resection. Local tumor recurrence, distant tumor recurrence, leptomeningeal seeding, and overall survival were investigated. Pathologic findings and surgical gross findings were reviewed. RESULTS The mean age was 59 and 3 patients were female. All patients had multiple lesions, ranged 2 to 7, including a large lesion (mean tumor volume: 29.8 cc). The interval between GKS and surgery was varied between 1 to 14 days. The prescribed dose of plan-to-resect lesion was 12 Gy in four patients and 14 Gy in one patient. All patients underwent gross total resection without surgical complications and had no radiation-induced toxicity. 3 patients were died of extracranial progression during follow-up period, including 1 patient who died before the first follow-up image. Local tumor progression was noted in 1 patient 9 months after treatment. The mean overall survival was 4.5 months although two patients continued to require follow-up. CONCLUSIONS Neoadjuvant SRS may be safe and advantageous for the treatment of BM that require surgical intervention. Long-term investigation with larger patient cohorts and further randomized trials are essential to verify the efficacy of the approach.

RADT-12. PRE- AND POST-OPERATIVE RADIATION TREATMENT PLANNING FOR PATIENTS RECEIVING STEREOTACTIC RADIOSURGERY FOR BRAIN METASTASES: A VOLUMETRIC ANALYSIS

Abstract BACKGROUND Pre-operative stereotactic radiosurgery (SRS) has emerged as a recent treatment option to treat large or symptomatic brain metastases. Compared to post-operative SRS, pre-operative treatment may reduce rates of radiation necrosis (RN) and meningeal disease through treating a smaller treatment volume and by preventing post-operative tumor seeding. We hypothesize that pre-operative radiation volumes will be smaller than post-operative volumes, which in turn may lead to a decreased treatment morbidity. METHODS A retrospective analysis was conducted and patients who had surgical resection and post-operative SRS or fractionated stereotactic radiosurgery (FSRT) for a posterior fossa brain metastasis were eligible for inclusion. Both pre-operative and post-operative MRIs were required to allow for accurate radiation target delineation. A pre-operative tumor volume was added for each patient, and the post-operative clinical treatment volume (CTV) used for radiation treatment was included. Pre-operative tumor and post-operative cavity volumes were compared using Wilcoxon signed rank test. RESULTS 28 patients who received post-operative SRS or FSRT from 1/1/2016-12/31/2020 were included in this analysis. The mean pre-operative tumor volume was 14.9 ccs, and the mean post-operative CTV was 21.0 ccs (p < 0.01). 75% of patients had a smaller initial tumor size compared to the post-operative CTV used for radiation treatment planning. For patients with at least 4 follow up MRIs (n = 8), the size of the post-operative cavity progressively decreased with a mean initial cavity volume of 18.9 ccs and mean follow up volumes of 8.1, 7.1, 6.9, and 6.2 ccs. CONCLUSIONS In this study evaluating patients who received post-operative SRS, the pre-operative tumor volume was lower than the post-operative CTV for most patients. Previous studies including PROPS-BM have shown how pre-operative treatment may reduce the risk of RN because smaller radiation volumes are used. Pre-operative radiosurgery for patients with brain metastases requires prospective validation.

What is appropriate target delineation for MRI-based brachytherapy for medically inoperable endometrial cancer?

PURPOSEFor medically inoperable endometrial cancer (MIEC), the volumetric target of image-guided brachytherapy (IGBT) techniques is not well established. We propose a high-risk CTV (HRCTV) concept and report associated rates of local control and toxicity. METHODS AND MATERIALSFor all MIEC patients receiving definitive external beam radiotherapy (EBRT) followed by MRI-based IGBT at a single institution, BT dose was prescribed to HRCTV defined as GTV plus endometrial cavity with a planning goal of a summed EQD2 D90 of ≥85 Gy. Freedom from local progression (FFLP) and overall survival (OS) were estimated via Kaplan Meier method. RESULTSThirty two MIEC patients received EBRT followed by MRI-based IGBT between December 2015 and August 2020. Median follow up was 19.8 months. A total of 75% of patients had FIGO stage I/II disease, 56% endometrioid histology, and 50% grade 3 disease. OS was 73.6% (95% CI 57.8%–89.3%) at 12 months and 65.8% (95% CI 48.4%–83.2%) at 24 months. FFLP was 93.8% (95% CI 85.3%–100%) at 12 months and 88.8% (95% CI 86.6%–91.0%) at 24 months. 23 (72%) patients experienced no RT-related toxicity, while 2 of 32 patients (6%) experienced late grade 3+ toxicities (grade 3 refractory vomiting; grade 5 GI bleed secondary to RT-induced proctitis). CONCLUSIONSPatients with MIEC receiving definitive EBRT followed by MRI-based IGBT prescribed to the MRI-defined HRCTV demonstrated favorable long-term local control with an acceptable toxicity profile.

PP11 Presentation Time: 11:10 AM: Assessment of Brachytherapy-Induced Prostate Edema on Post-Implant Dosimetric Analysis in MRI-Assisted Radiosurgery

<h3>Purpose</h3> Prior studies have shown that brachytherapy-induced prostate edema (PE) results in altered target dose coverage on post-implant CT dosimetry in prostate cancer patients treated with low dose rate brachytherapy (LDR). The pre-plan target volume (PTV) accounts for PE by adding 1-5mm margins to the prostate for calculation of the goal dose coverage on the day of the implant (day zero, D0). However, visualization of PE on CT is limited, diminishing PE measurement accuracy. MRI-based LDR (MRI-assisted radiosurgery, MARS) improves accuracy through better delineation of target, normal structures, and seeds compared to CT. Thus, MARS may improve the accuracy of PE measurements. We investigated PE using MARS to analyze its effect on D0 dosimetry and determine if current pre-plan PTV margins are adequate in all isotopes. <h3>Materials and Methods</h3> We identified 346 patients with low to intermediate-risk prostate cancer treated with Cesium-131 (Cs-131, 15 [4.3%] patients, prescription dose (PD)=115 Gy), Iodine-125 (I-125, 83 [23.9%] patients, PD=144 Gy), or Palladium-103 (Pd-103, 248 [71.7%] patients, PD=125 Gy) MARS as definitive LDR from 2016-2021. Patients treated with hormone therapy, as a boost with external-beam RT, or for salvage were excluded. Implants were pre-planned using MRI (MIM Symphony treatment planning system) and our published nomograms verified the activity per volume within 5%. PE was measured as the change in prostate volume from the pre-plan to D0 based on physician contours. Post-implant dosimetry was performed using MRI on D0. Treatment characteristics, including pre-plan and D0 prostate volumes, prostate volumes receiving 100%, 150%, and 200% of the PD (V₁₀₀, V_150, V₂₀₀), percent of the PD going to 90% of the prostate volume (D₉₀), rectum volume receiving 100% of the PD (V₁₀₀), and external urethral sphincter (EUS) volume receiving 200% of the PD (V₂₀₀), were extracted from MIM. Simple linear regression with Pearson's correlation analysis was used to assess the effect of PE on post-implant D0 prostate V₁₀₀, V₁₅₀, V₂₀₀, and D₉₀, rectum V₁₀₀, and EUS V₂₀₀. Ordinary one-way ANOVA analysis was used to compare PE between categorical variables. <h3>Results</h3> Median age at diagnosis was 63 years. Median PSA at diagnosis was 6. 24 (6.9%) patients had GS 6 and 322 (93.1%) had GS 7. On the pre-plan, 55 (15.9%) patients had a prostate size of 0-20 ccs, 222 (64.2%) had 20-40 ccs, 63 (18.2%) had 40-60 ccs, 6 (1.7%) had 60+ ccs. On D0 analysis, the median [IQR] change in prostate volume was +28% [15-42%] as compared to the pre-plan. There was no significant difference in magnitude of PE by isotope (median change in prostate volume: +22.6% for Cs-131, +27.3% for I-125, +29.5% for Pd-103, p=0.33). There was no difference in PE by increasing number of needles (R²=0, p=0.70). PE decreased with increasing pre-plan prostate size (R²=0.05, p<0.001). Greater PE was associated with decreased post-implant D0 V₁₀₀ (<b>Graphic</b>, R²=0.08, p<0.001), V₁₅₀ (R²=0.03, p<0.001), and D₉₀(R²=0.12, p<0.001); there was no association with D0 V_{200 (}R²=0.01, p=0.06). Greater PE was associated with decreased D0 rectum V₁₀₀ (R²=0.02, p=0.02) and trended toward decreased EUS V₂₀₀ (R²=0.01, p=0.05). <h3>Conclusions</h3> MARS confirmed that patients undergoing LDR experienced PE. PE was uniform between isotopes, supporting the use of the same pre-plan PTV margins among isotopes. There was decreased D0 prostate V₁₀₀, V₁₅₀, and D₉₀ in patients with greater PE. We suggest pre-plan PTV margins as follows: 3mm laterally, 2mm anteriorly/cranially/caudally, and 0mm posteriorly. These PTV margins can help ensure adequate D0 dosimetry. <b>Graphic:</b> Correlation of post-implant D0 prostate V₁₀₀ with the magnitude of prostate edema (PE). PE was calculated as the percent change in prostate volume from the pre-plan to post-implant D0.

GSOR6 Presentation Time: 9:25 AM: Integrating a Novel GYN Brachytherapy Workflow Using In-Room CBCT Images with MR-LINAC Scans

<h3>Purpose</h3> Imaging capabilities within Radiation Oncology departments are increasing. Departments now have access to MRI imaging with MR-Linacs and designated brachytherapy mobile CBCT scanners. Utilization of these within-department imaging capabilities have the potential to improve clinical brachytherapy workflow. The purpose of this report is to investigate the feasibility of utilizing an MR-LINAC and a mobile CBCT scanner for patient imaging during HDR GYN brachytherapy. <h3>Materials and Methods</h3> To compare the spatial integrity of these imaging modalities, an HDR ring and tandem applicator was placed in a water phantom and imaged using a CT simulator (SOMATOM go.Open Pro, Siemens), 0.35T MR-LINAC (MRIdian, ViewRay) and a mobile CBCT scanner (ImagingRing, Elekta). The images acquired included a MR T2 weighted scan, a pelvis image guided brachytherapy CBCT acquisition, and a CT simulation. The images from the MR-LINAC and mobile CBCT scanner were each rigidly registered to CT-SIM images focusing on the applicator. Deviations in ring diameter, angle, and the position of the tip of the applicator was measured on each fusion. Additionally, a patient was imaged with the applicator inserted using the CT-Sim and the MR-LINAC systems. Physician drawn contours using these two modalities were compared to one another to investigate the benefits of incorporating MR-LINAC scans in the clinical HDR treatment planning process. <h3>Results</h3> The applicator was visible on each of the imaging modalities investigated. The deviation in the position of the applicator dimensions were less than 1mm on the MR-CT and CBCT-CT fusions. The analysis of the multimodality image fusions for the applicator revealed no large geometric distortions or loss of image quality when comparing the MR-CBCT image sets to MR-CT image sets. In patient scans, The MR planning image provided superior soft tissue contrast compared to the image acquired using the CT simulator. In this particular case, the superior soft tissue contrast provided by the MR scan provided more information for the physician for target delineation and resulted in a clinical decision to reduce the volume of the GTV. <h3>Conclusion</h3> This work presents a novel clinical workflow for HDR GYN brachytherapy using a mobile CBCT scanner and MR-LINAC for patient imaging. Incorporating the MR T2 scans into the clinical HDR brachytherapy workflow may aide in GYN brachytherapy target and OAR delineation. Additionally, the mobile CBCT scanner can reduce the challenges associated with patient setup and transportation between imaging and treatment locations. We found that the image quality and spatial resolution of MR and mobile CBCT based applicator images were clinically acceptable. A potential clinical workflow could involve using the MRIdian to obtain a planning image and contours the first fraction of treatment. Then on subsequent fractions the mobile CBCT scan could be fused to MR scans and used for daily applicator verification. For future work we will investigate a modified T2 sequence for MR guided brachytherapy to improve catheter reconstruction. Our work validates the clinical feasibility of using new technology for an MR-CBCT clinical workflow for HDR brachytherapy treatment planning.

GPP04 Presentation Time: 8:30 AM: What is Appropriate Target Delineation for MRI-Based Brachytherapy for Medically Inoperable Endometrial Cancer?

<h3>Purpose</h3> While the incidence of medically inoperable endometrial cancer (MIEC) is anticipated to rise within the growing elderly population, practice patterns demonstrate persistent underutilization of brachytherapy (BT). Contemporary Image-guided BT (IGBT) offers superior target dose escalation while sparing adjacent organs at risk. However, the volumetric target of IGBT techniques is not well established. We propose a high-risk CTV (HRCTV) concept and report associated rates of local control and toxicity. <h3>Materials and Methods</h3> A retrospective review identified all patients with MIEC receiving definitive external beam radiotherapy (EBRT) followed by MRI-based IGBT at a single institution. BT dose was prescribed to a HRCTV defined as gross tumor volume plus endometrial cavity with a planning goal of a summed equivalent dose at 2Gy/fraction (EQD2) D90 of 85Gy or greater. An additional intermediate-risk volume (IRCTV) was developed by global 5mm expansion upon this HRCTV, removing any volume entering into organs at risk (OAR). The myometrium and uninvolved uterine body was not defined as target volume. OARs including rectum, bladder, sigmoid and bowel were limited to EQD2 constraints directly adapted from cervical cancer. Late gastrointestinal (GI), genitourinary (GU), and vaginal toxicity events were graded using CTCAE, version 5.0. Clinical endpoints including local progression-free survival (LPFS), progression-free survival (PFS), and overall survival (OS) were estimated via the Kaplan Meier method. <h3>Results</h3> 32 patients received MRI-based HDR-BT for MIEC between 12/2015 and 8/2020, with a median follow up of 19.8 months (IQR 8.9 - 27.6 months). Patients were a median 71.5 years at diagnosis (range, 43 - 91 years), with a median ECOG 1 performance status (IQR 1 - 2). FIGO stages included 1A (56%), IB (16%), II (3%), III (22%), and IV (3%). Common histologies included endometrioid (56%), serous adenocarcinoma (22%), and carcinosarcoma (13%). All patients completed EBRT (45 Gy/25 fractions, 69%; 50.4 Gy/28 fractions, 16%; other, 16%) followed by MRI-guided HDR-BT (27.5 Gy/5 fractions, 66%; 28 Gy/4 fractions, 28%; other, 6%). Composite dosimetry included the following median EQD2 values: D90 HRCTV 91.3 Gy (IQR 88.2 - 96.4 Gy), D98 HRCTV 78.2 Gy (73.7 - 82.6 Gy), D90 IRCTV 73.4 Gy (69.9 - 76.0 Gy), D2cc Bladder 72.9 Gy (66.7 - 76.4 Gy), D2cc Rectum 57.0 Gy (52.1 - 66.0 Gy), D2cc Sigmoid 65.0 Gy (63.3 - 70.3 Gy), D2cc Bowel 60.2 Gy (53.2 - 66.0 Gy), and D2cc Vagina 82.9 Gy (79.9 - 90.6 Gy). 16% of patients received hormonal therapy, and 3% of patients received chemotherapy. Figure 1 provides LPFS, PFS, and OS. There were 14 (44%) total deaths, 3 local recurrences (9%), 3 nodal recurrences (9%), and 5 distant recurrences (16%). 2-year LPFS was 88.8%, and 2-year OS was 65.8%, with no locoregional or distant progression observed beyond 24 months post-BT. For the 3 patients with local progression, median D90 HRCTV was 99.2 Gy (range 77.9 - 103.5 Gy), D98 HRCTV 80.2 Gy (68.6 - 89.4 Gy), and D90 IRCTV 73.3 Gy (66.0 - 83.5 Gy). For patients with stage I/II disease (n=24), LPFS was 100% at 12 months and 94% (95% CI 81.9 - 100%) at 24 months. 23 (72%) patients experienced no RT-related toxicity. Two grade 3+ toxicities (6%) were observed: one grade 3 vomiting event prompting hospitalization and one grade 5 GI bleed presumed secondary to RT-induced proctitis in the setting of longstanding clopidogrel use. <h3>Conclusions</h3> Patients with MIEC receiving definitive EBRT followed by MRI-based IGBT demonstrated high rates of long-term local control with a favorable toxicity profile, when dose was prescribed to the MRI-defined GTV and endometrial cavity. These data compare favorably to historical controls despite a significant proportion of patients with locally advanced disease and/or aggressive histology.

Neoadjuvant stereotactic radiosurgery for brain metastases: a new paradigm.

For patients with surgically accessible solitary metastases or oligometastatic disease, treatment often involves resection followed by postoperative stereotactic radiosurgery (SRS). This strategy has several potential drawbacks, including irregular target delineation for SRS and potential tumor "seeding" away from the resection cavity during surgery. A neoadjuvant (preoperative) approach to radiation therapy avoids these limitations and offers improved patient convenience. This study assessed the efficacy of neoadjuvant SRS as a new treatment paradigm for patients with brain metastases. A retrospective review was performed at a single institution to identify patients who had undergone neoadjuvant SRS (specifically, Gamma Knife radiosurgery) followed by resection of a brain metastasis. Kaplan-Meier survival and log-rank analyses were used to evaluate risks of progression and death. Assessments were made of local recurrence and leptomeningeal spread. Additionally, an analysis of the contemporary literature of postoperative and neoadjuvant SRS for metastatic disease was performed. Twenty-four patients who had undergone neoadjuvant SRS followed by resection of a brain metastasis were identified in the single-institution cohort. The median age was 64 years (range 32-84 years), and the median follow-up time was 16.5 months (range 1 month to 5.7 years). The median radiation dose was 17 Gy prescribed to the 50% isodose. Rates of local disease control were 100% at 6 months, 87.6% at 12 months, and 73.5% at 24 months. In 4 patients who had local treatment failure, salvage therapy included repeat resection, laser interstitial thermal therapy, or repeat SRS. One hundred thirty patients (including the current cohort) were identified in the literature who had been treated with neoadjuvant SRS prior to resection. Overall rates of local control at 1 year after neoadjuvant SRS treatment ranged from 49% to 91%, and rates of leptomeningeal dissemination from 0% to 16%. In comparison, rates of local control 1 year after postoperative SRS ranged from 27% to 91%, with 7% to 28% developing leptomeningeal disease. Neoadjuvant SRS for the treatment of brain metastases is a novel approach that mitigates the shortcomings of postoperative SRS. While additional prospective studies are needed, the current study of 130 patients including the summary of 106 previously published cases supports the safety and potential efficacy of preoperative SRS with potential for improved outcomes compared with postoperative SRS.

PO04: Clinical and Dosimetric Outcomes with Image-Guided High Dose Rate (HDR) Endoluminal Brachytherapy in Endobronchial Tumours: A Single Institute Experience

<h3>Purpose</h3> Endoluminal brachytherapy is an effective treatment modality for endobronchial tumours. With the advent of image-guidance, target coverage has improved and volume-based target delineation has helped in conformal treatment. In this study, we report our experience with high dose rate endobronchial brachytherapy (HDR-EBBT) and to assess its efficacy and tolerability. <h3>Materials and Methods</h3> Between January 2015 to February 2022, a retrospective review of patients with endobronchial tumours treated at our institution with HDR-EBBT was retrospectively assessed. Analysis of disease characteristics and thereafter, the subjective response and treatment related toxicities were extracted from patient records. The follow-up was undertaken as per the institutional protocol. Local control (LC) and overall survival (OS) were analysed. During each procedure of EBBT, a source-centralizing applicator was used which kept the source-dwelling catheter in the centre of bronchial lumen and reference dose point was placed 5 mm from source axis as per the bronchial diameter. <h3>Results</h3> With a median follow up of 18 months, a total of 8 patients with histopathologically proven endobronchial tumours were identified. The median external beam radiation dose was 45Gy in 25 fractions followed by brachytherapy dose of 14Gy in 2 fractions with treatment a week apart. The commonest symptoms to palliate included dyspnea (5/8), cough (3/8), and hemoptysis (4/8). The median disease-free survival and overall survival was 18 months and 21 months respectively. No grade 3-5 acute or late toxicities were reported and only one patient required further palliation measures to control the symptoms. On dosimetric analysis the PTV V95% and V98% were 95.12% and 93.45% respectively. On assessing the D20 and D5 for lungs, the median dose was 1.32Gy and 0.73Gy respectively. Similarly, the D2cc, 1cc and 0.1cc of oesophagus and heart were 1.63, 1.89 and 2.67Gy and 1.15, 1.37 and 1.69Gy respectively. <h3>Conclusion</h3> HDR-EBBT is a promising treatment with minimal complications and improved outcomes and thus provide curative options. If performed properly, desirable dosimetry can be achieved to improve target coverage and reduced normal tissue doses.

Effects of metal implants and validation of four treatment planning methods used for radiotherapy dose calculation.

The radiotherapy treatment planning process involves target delineation and dose calculation, both of which directly depend on image quality and Hounsfield unit (HU) accuracy of computed tomography (CT) images. CT images of patients having metal implants undergo image quality deterioration and show inaccurate HU values due to various artifacts. Metal artifact reduction (MAR) is used to improve the image quality. In this study, four treatment planning methods with and without MAR, in combination with actual and assigned HU values, were analyzed for dose calculation accuracy. The aim was to study the effects of metal implants on planning CT and to evaluate the dose calculation accuracy of four treatment planning methods for radiotherapy. Two phantoms with six different metal inserts were scanned in the extended HU mode, with and without MAR. Geometry verification and HU analysis of the metals and the surrounding region were carried out. Water equivalent distance (WED) measurements and dose calculation for each metal insert were done in the treatment planning system (TPS) using the anisotropic analytical algorithm (AAA). Point dose and two-dimensional dose distribution were studied. Percentage variation analysis between calculated and measured doses and gamma evaluation were conducted to determine the most suitable method for treatment planning. This study concludes that an MARCT image with an assigned HU similar to that of the metal implant is better for contouring and high dose calculation accuracy. If MAR is not available, the actual HU value from the extended HU CT for the metal should be used for dose calculation.

Impact of fiducial markers placement on the delineation of target volumes in radiation therapy for oesophageal cancer: FIDUCOR study.

This prospective monocentric phase II study (FIDUCOR-study, NCT02526134) aimed to assess the impact of fiducial markers (FMs) implantation on conformal chemo-radiation therapy (CRT) planning in oesophageal carcinoma (EC) patients. Fifteen EC patients were enrolled in the study. Each patient underwent two simulation CT-scans before (CT1) and after (CT2) FMs implantation, in the same position. FMs (3 mm length gold markers, preloaded in a 22G needle) were implanted after sedation, under endoscopic ultrasound (EUS) and X-Ray guidance, and were placed at the tumor's extremities, and in the visible lymph nodes. Target delineation and treatment plan were both performed first on CT1 with the assistance of diagnosis CT, gastroscopy and EUS details, and second on CT2 using FMs and CT-data. The value of FMs implantation was assessed by the difference of growth-tumor-volume (GTV) and clinical-target-volume (CTV) between CT1-based and CT2-based delineation. A significant difference was defined as a ≥5 mm-difference on axial(x) or coronal(y) slices, a ≥10mm-difference on sagittal slices, or a ≥20%-difference in GTV. The impact on dose distribution in organs at risk (OAR) (lung, heart, liver) was also studied. Between 09/2014 and 12/2015, 15 patients could achieve fiducial procedures, without any complication. One FM migration occurred. We observed a significant modification of the GTV-dimension in 100% of the cases (15/15, 95%CI: [78.2;100.0]), mainly due to a difference in sagittal dimension with a mean variation of 11.2 mm and a difference&gt; 10 mm for 8/15 patients (53.3%). One patient had a significant isocenter displacement as high as 20 mm. The oesophagus tumor was not seen on the CT-scan in one patient due to its small size. One patient had a distant lymph node metastasis not visible on CT-scan. We observed no significant impact on OAR distribution. In our study, FMs-implantation under EUS had a positive impact on accurate volume definition in EC-patients (modification of GTV in 15/15 patients). Close cooperation between gastroenterologist and radiation oncologist has the potential to improve local treatment of oesophagealcarcinoma.

A Randomized Trial of Comparative Dosimetric Study of Radiotherapy in High Grade Gliomas Based on the Guidelines of 2019-NRG and EORTC Target Delineation

<h3>Purpose/Objective(s)</h3> Postoperative radiotherapy is important for high-grade glioma (WHO Grade 3-4), but the optimal way for contouring the target area for radiotherapy is still unclear. The purpose of this randomized trial was to compare the dosimetric differences in radiation treatment plans (TPS) generated based on the European Organization for Research and Treatment of Cancer (EORTC) and NRG consensus (NRG-2019) guidelines to provide evidence for optimal target delineation for high-grade glioma. <h3>Materials/Methods</h3> Patients with diagnosis of high-grade glioma were randomly assigned (1:1) to receive postoperative radiotherapy based on either EORTC guideline or NRG-2019 guideline. We report the result of dosimetric comparison first. In 13 high grade glioma patients, both EORTC and NRG-2019 radiation treatment plans were generated for dosimetric comparison. For EORTC group, the prescription dose was 95% PTV 60 Gy in 30 fractions. The gross tumor volume (GTV) including resection cavity and residual tumor encompassed the T1-weighted images (T1WI) and T2 FLAIR contrast-enhancing lesion. For NRG-2019group, GTV involved the T1-weighted images (T1WI) and T2 FLAIR contrast-enhancing foci and peritumoral edema on the preoperative CT or MRI. The prescription dose of PTV1 was 46 Gy in 23 fractions. The areas were then diminished to contain merely the contrast enhancing foci with a 2.5 cm margin as CTV2, and PTV2 was equal the CTV2 plus 0.3 cm margin for additional prescription dose was 14 Gy in 7 fractions. It should be noted that CTV1 and CTV2 were trimmed according to the anatomical barriers, falx, ventricles, etc. This trial is registered with chictr.org.cn, number ChiCTR2100046667. <h3>Results</h3> The median volume for PTV of EORTC plans, PTV1 of NRG-2019 plans, PTV2 of NRG-2019 plans were 336.6 cm³ (range 161.1-511.5 cm³), 365.3 cm³ (range 123.4-535.0 cm³), and 263.2 cm³ (range 116.8-497.7 cm³), respectively. Both treatment plans were found to have similar efficiency and evaluated as acceptable for patient treatment. Both treatment plans showed well conformal index and homogeneity index and were not statistically significantly different. There was no significant difference in the volume percent of brain irradiated to 30 Gy, 46 Gy, and 60 Gy according to different target delineations (P = 0.397, P = 0.590 and P = 0.739, respectively). These two plans also showed no significant differences in the doses to the brainstem, optic chiasm, optic nerve, lens, eyes, pituitary, and temporal lobes (P = 0.858, P = 0.858, 0.701, P = 0.794, P = 0.701, P = 0.427, P = 0.489, P = 0.898, P = 0.626, P = 0.942 and P = 0.161, respectively). <h3>Conclusion</h3> Compared with EORTC guidelines for postoperative radiotherapy for high-grade glioma, the NRG-2019 target area delineation guidelines were able to include peritumor edema in the CTV without increasing the radiation dose to organs at risk (OARs). This study further provides a foundation for the application of the NRG-2019 consensus in radiotherapy for patients with high-grade gliomas.

Novel Simulation for Dosimetry Impact of Diaphragm Respiratory Motion in 4D VMAT for Esophageal Cancer

<h3>Purpose/Objective(s)</h3> Respiratory motion affects the robustness of the dose distribution. For esophageal cancer (EC) near the esophagogastric junction, respiratory motion causes unintentional over- or under- dose distributions for the target. Although four-dimensional computed tomography (4DCT) is useful for robust evaluation, it increases the exposure dose and acquisition time. The objective of the current study is to propose the framework of the virtual 4DCT based robust evaluation method. Moreover, the robust evaluation with the virtual 4DCT was applied to EC patients. <h3>Materials/Methods</h3> We retrospectively evaluated 10 superficial (T1a - T1b) EC patients, had real 4DCT scans (real-4DCT). Target delineation was as follows; GTV: the primary lesion, CTV: GTV + 10mm (horizontal direction (H)), and 30mm (cranial and caudal direction (CC)). PTV: CTV + 8mm (H), and 15mm (CC). Volumetric modulated arc therapy (VMAT) was planned using double arcs which rotated 360° including two 60° lateral avoidance sectors during delivery. The virtual 4DCT defined as simulated 4DCT images (sim-4DCT) was generated with the simulated-organ motion function in commercial treatment planning system. Each sim-4DCT of 10 phases was generated from the end-inhalation CT image (reference image) by moving the diaphragm with mathematical equation proposed by Lujan et al (Med Phys 2003). First, we validated the accumulated CTV dose (D_99%, D_mean and D_1%) of 4D dose distribution in the sim-4DCT by comparing with the real-4DCT. The diaphragm motions (max, min and 60-90% tile of value) in the real 4DCT were obtained from an in-house program and adapted to sim-4DCTs. Then, the robustness of the CTV (D_99%, D_mean and D_1%) was evaluated by moving diaphragm from 5 to 20 mm. The percentage dose difference (ΔDx) between sim 4D-CT and static 3D-CT images which were used for planning was investigated. <h3>Results</h3> The accumulated CTV dose difference between sim-4DCT and real-4DCT was within 1% at 70% tile of the diaphragm motion. The mean CTV value of ΔD_99%, ΔD_mean and ΔD_1%, increased from 0.2% to 2.3%, 0.1% to 0.7 % and 0.1% to 0.6%, respectively, along with an increase in the diaphragm amplitude. For the diaphragm amplitude up to 15 mm, the mean ΔD_99%, ΔD_mean and ΔD_1% of CTV were within 3%. The largest percentage change was ΔD_99%, which was 2.2%. For the diaphragm amplitude of 20 mm, ΔD_99% for one patient was over 5%. The ΔD_99% of CTV for the other patients was within 3.9%. <h3>Conclusion</h3> This study proposed a novel method to evaluate the robust by generating virtual 4DCT. For EC case near the esophagogastric junction, the robust was enough if the diaphragm motion was within the PTV margin. The treatment planning with virtual 4DCT based robust evaluation method could propose an appropriate margin for PTV.

Developing a Translational Platform for Therapeutic Delivery of Image-Guided Ultra-Hypofractionated Prostate-Directed Radiotherapy in Orthotopic Prostate Cancer Models

<h3>Purpose/Objective(s)</h3> Prostate stereotactic body radiation therapy (SBRT) is rapidly evolving as a standard-of-care option for definitive management of localized prostate cancer. To our knowledge, there are currently no preclinical models that deliver SBRT-like treatment to orthotopic prostate tumors that reflect the precision and accuracy of clinical treatment. The objective of this study is to develop and credential a small animal radiation research platform (SAARP)-based therapeutic model of image-guided ultra-hypofractionated prostate-directed radiotherapy (RT) that is dosimetrically accurate with clinically relevant radiation doses. <h3>Materials/Methods</h3> Orthotopic prostate tumors are established following intraprostatic injection of 2 × 10⁵ PC3 tumor cells (stably expressing luciferase following lentiviral transduction) into the dorsal aspect of the prostate gland of nude mice. Serial orthotopic tumor growth is monitored with weekly bioluminescence (BLI) and Q2week MRI. Reverse-contrast cone-beam CT imaging (CBCT) is performed with intraperitoneal injection of 2mL of 60% omnipaque immediately prior to CBCT. Pre-treatment MRI is fused to CBCT to facilitate delineation of organs-at-risk (rectum, bladder, bowel bag) and target (prostate) for contouring. Individual plans (generally 4 or 5-field coplanar beam arrangement) were generated to target the prostate (SV not intentionally targeted) and avoid OAR. The SARRP was used to deliver 36.25Gy in 5 fractions of prostate-directed RT [day 30] and weekly serial BLI + MRI used to assess treatment response. <h3>Results</h3> Reverse contrast CBCT imaging +/- MR fusion was sufficient for OAR and target delineation and facilitated daily image-guidance whereas CBCT alone was insufficient. Radiation dosimetry noted a positive association between pH2AX mean fluorescence intensity (MFI) with escalating radiation doses of 0Gy, 2Gy, 8Gy and 20Gy. Preliminary DVH analysis of therapeutically irradiated orthotopic prostate tumors and adjacent OARs noted CTV prostate coverage>90%, rectumD50%<4Gy and bladderD50<4Gy in 3 of 5 serial fractions delivered. Surgically implanted intraprostatic TLD were harvested post-RT to confirm and quantify intraprostatic radiation dose received. Therapeutic response was observed with significantly lower luminescence counts in irradiated (n=13) vs non-irradiated controls (n=11) [p=0.049] and no treatment-related deaths were observed in irradiated mice. <h3>Conclusion</h3> We have developed a preclinical workflow for image-guided delivery of ultra-hypofractioned radiotherapy of orthotopic prostate tumors with ongoing optimization goals to achieve clinically relevant dose constraints and target coverage of NRG GU-005. Ongoing translational work involves DNA damage response assessment in irradiated prostate and adjacent organs-at-risks. This platform is now being explored in immunocompetent settings to better understand intraprostatic immune responses induced by prostate-directed RT.

Analysis of Resident/Faculty Contour Concordance: A Potential Tool for Quantitative Assessment of Residents’ Performance in Target Volume Delineation

<h3>Purpose/Objective(s)</h3> Accurate target delineation/contouring is essential for radiation treatment planning and the clinical efficacy of radiation therapy. As a result, improving the quality of target delineation is an important goal in the education of radiation oncology residents. However, there is limited quantitative data on the quality of residents' contours. Therefore, it would be beneficial to track performance and improvement in resident target delineation during residency. The purpose of this study was to track the concordance of radiation oncology residents' contours with faculty physicians' contours over the course of one year to assess for patterns. <h3>Materials/Methods</h3> Residents were asked to contour target volumes (GTV, CTV, ITV, PTV, etc.); these contours were saved as separate structures and compared to the finalized, faculty physician-approved contours. Concordance between resident and faculty physician contours was determined by calculating the Jaccard Concordance Index (JCI). The JCI was calculated by dividing the intersection volume of the resident/faculty contours by the union volume of the resident/faculty contours. A score of 1 indicates complete agreement while a score of 0 indicates no agreement. Resident contours were stratified by PGY level (i.e., PGY-2, PGY-3, PGY-4). Data was collected from the 2020-2021 academic year. Two-tailed, unpaired t testing and one-way ANOVA testing was performed to compare mean values in JCI. A P-value of < 0.05 was considered statistically significant. <h3>Results</h3> In total, 904 structures were available for analysis. The mean JCI was 0.766, 0.749, and 0.618 for the PGY-4 level resident, PGY-3 level resident, and PGY-2 level resident, respectively. There was statistically significant difference in mean JCI between PGY-levels (P < 0.001). On subset analysis of disease type, there was statistically significant difference in mean JCI between PGY-levels for brain metastases, primary gastrointestinal cancers, primary genitourinary cancers, primary gynecologic cancers, primary head/neck cancers, primary thoracic cancers, and sarcomas (P = 0.020, P < 0.001, P = 0.012, P < 0.001, P = 0.018, P < 0.001, respectively). There was no significant difference in mean JCI between PGY-levels for primary brain tumors, primary breast cancers, and lymphomas; however, there were fewer than 30 structures per resident for these disease types available for analysis. <h3>Conclusion</h3> Tracking and comparing the concordance of resident contours and faculty physician contours is an intriguing method of assessing resident performance in target volume delineation and could potentially serve as a quantitative metric in radiation oncology resident evaluation, which is lacking currently. However, additional study is necessary before this technique can be incorporated into residency assessments.

Evaluating Accuracy, Completion Time and Usability of Everyday Touch Devices for Contouring

<h3>Purpose/Objective(s)</h3> Adaptive radiation therapy aims to account for patients' continuous anatomical changes and update the treatment plans on a daily basis. Despite the potential benefits, technological limitations hinder clinical adoption: daily re-evaluation of contours associated with adaptive planning is time-intensive and difficult to access, and often requires direct provider input which classically involves visualizing and editing contours on immobile desktop computers. This project evaluated the efficacy of everyday touch devices (i.e., tablet and phone) for contouring using a novel cross-platform interface. <h3>Materials/Methods</h3> Guided by user-centered and iterative design principles, we developed a cross-platform web-based contouring application which facilitates CT image navigation and target delineation. In a within-subject study design, we invited 8 radiation oncology residents to contour a liver using 4 different conditions: Desktop & Mouse, Tablet & Stylus, Tablet & Finger, and Phone & Finger. We compared the time spent contouring and assessed accuracy with Dice Similarity Coefficients (DSC). With each interface we administered a System Usability Scale (SUS) questionnaire which evaluates perceived ease of use. We incorporated repeated-measure ANOVA to assess differences between groups. <h3>Results</h3> Among the participants, using a Tablet & Finger and Phone & Finger produced the shortest time spent contouring per CT slice (<i>p</i> < .001), while Desktop & Mouse led to the longest time per slice (see Table). When comparing participant contours to a baseline gold standard, results were similar between all devices, though the Desktop & Mouse yielded a slightly lower contouring accuracy, and the most accurate was the Tablet & Stylus (<i>p</i> < .0001). Usability assessment after contouring found that participants scored Tablet & Stylus the highest, followed by Tablet & Finger, Desktop & Mouse, and Phone & Finger (<i>p</i> < .01). <h3>Conclusion</h3> Everyday touch devices (i.e., tablet and phone) not only met the standard contouring efficiency with a Desktop & Mouse, but also exceeded expectations in terms of both time spent and accuracy, especially contouring with a Tablet & Stylus which also received significantly better usability scores. Given the improved mobility with touch devices, these results suggest promising implications in addressing technological challenges associated with adaptive radiation therapy.