PTV Expansion Research Articles

Evaluation of CTV to PTV expansion of prostate patients in Cork University Hospital

Evaluation of CTV to PTV expansion of prostate patients in Cork University Hospital

Prostate volume variation during 1.5T MR-guided adaptive stereotactic body radiotherapy (SBRT) and correlation with treatment toxicity

IntroductionTo evaluate prostate volume change during daily-adaptive prostate SBRT on 1.5 T MR-linac and to correlate it with treatment toxicity. Methodsa series of patients affected by low-to-intermediate risk prostate cancer was treated by 5-fraction SBRT within a prospective study (Prot. n° 23748). Total dose was 35 Gy and 36.25 Gy delivered every day or on alternate days. Treatment toxicity was recorded with the following patient reported outcomes (PROMs): IPSS, ICIQ-SF, and EPIC-26. Results254 patients were included in the analysis. Baseline median CTV volume was 55 cc (range 15.3–163.3). Mean prostate volume were 58.9 cc, and 62.7 cc at first and last fraction respectively (mean volume increase 6.4 %; p = <0.0001). We observed prostate swelling (mean 15.4 % increase) in 50 % of cases, stable volume (≤5% volume change) in 39 % of patients, and prostate shrinkage in 11 % of cases (mean 12.2 % reduction). Baseline CTV > 55 cc showed a trend towards higher CTV shrinkage (-10.5 % versus −14.5 %; p = 0.052). We found no correlation between CTV change and PROMs. Prostate swelling was generally compensated by the planned PTV expansion, even though the mean setup volume dropped from 47.4 cc to 38.9 cc at last fraction, with few cases not covered by initial setup margins. ConclusionThe present study reported a significant prostate volume change during prostate SBRT on 1.5T MR-linac. We observed both prostate swelling in half of cases and few cases of prostate shrinkage. No correlations were found with PROMs in this population treatment with daily-adaptive strategy.

CT-based online adaptive radiotherapy improves target coverage and organ at risk (OAR) avoidance in stereotactic body radiation therapy (SBRT) for prostate cancer

IntroductionStereotactic body radiation therapy (SBRT) is an emerging treatment modality for clinically localized prostate cancer (PCa). Online daily adaptive radiotherapy (ART) could potentially improve the therapeutic ratio of prostate SBRT by accounting for inter-fraction variation in target and OAR volumes. To our knowledge, no group has evaluated the clinical utility of a novel AI-augmented CT-based ART system for prostate SBRT. In this study we hypothesized that adaptive prostate SBRT plans would result in improved target coverage and lower dose to OARs in comparison to unadapted treatment plans. MethodsSeven patients with favorable intermediate to oligometastatic PCa treated with 5-fx prostate adaptive SBRT were retrospectively reviewed. Patients were treated with 3625 cGy to the prostate and seminal vesicles. 6 patients additionally received 2500 cGy to the pelvic nodes, 5 patients underwent a boost to 4000 cGy to the prostate. For each fraction, a CBCT was acquired and OARs (rectum, bladder, bowel, sigmoid, femurs) were segmented/deformed using AI. CTVs were rigidly registered. Volumes were adjusted manually and PTV expansions added. Adaptive treatment plans were developed based on the contoured targets and OARs and dose to these volumes for the adapted vs. initial plans were compared for each fraction. V100 and the D0.03 cc between scheduled and adapted treatment plans were compared using a Student’s t-test, with significance threshold of P < 0.05. ResultsSeven patients completed 35 Fx’s of adaptive RT. Daily adaptation resulted in a statistically significant mean improvement in PTV V100 for all targets: [21.4 % ± 4.3 % for PTV 4000 (p < 0.0001); 8.7 % ± 1.1 % for PTV 3625 (p < 0.0001); and 11.5 % ± 3.1 % for PTV 2500 (p = 0.0013)]. Mean rectal D0.03 was significantly reduced by 38.8 cGy ± 5.95 cGy (p < 0.0001) per fraction (194 cGy/5 fractions) compared to the initial plans. There was a modest increase in bladder dose of 10.9 cGy ± 4.93 cGy per fraction (p = 0.0424) for the adaptive plans. The adaptive plans met bladder constraints for every fraction. There were no statistically significant differences between sigmoid or bowel dose for adapted vs. initial plans. No patients experienced acute CTCAE grade ≥ 3 GI/GU adverse events (median F/U 9.5 months). All statistically significant differences were maintained in the presence and absence of rectal hydrogel spacer (p < 0.05). ConclusionsCT-based online adaptive SBRT resulted in statistically significant and clinically meaningful improvements in PTV coverage and D0.03 cc dose to the rectum. A trial evaluating CT adaptive whole-pelvis prostate SBRT is underway.

Reducing CTV to PTV Margins with Daily Adaptive Radiotherapy in the Postoperative Treatment of Endometrial and Cervical Cancer

In the postoperative treatment of endometrial and cervical cancer, a PTV expansion of 1.5cm or greater would be generally recommended to ensure adequate coverage of CTV with no or minimal image guidance. Online adaptive radiotherapy (oART) has demonstrated to be feasible to reduce inter-fractional radiotherapy errors as it re-optimizes treatment plan every fraction. In this study, we hypothesized using daily cone-beam computed tomography (CBCT)-guided oART would reduce CTV to PTV margins in the postoperative treatment of endometrial and cervical cancer. Seven patients from a single-center with postoperative endometrial and cervical cancer treated with daily CBCT-guided oART were retrospectively reviewed. A total dose of 45Gy is prescribed in CTV-vaginal cuff (CTV-V) and CTV- regional lymph nodes (CTV-N) with daily fractions of 1.8Gy for five fractions per week. A total of 175 radiotherapy fractions of daily pre-treatment CBCTs and post-treatment CBCTs scans were uploaded to oART emulator for CTV-V and CTV-N contouring by a single observer. CTVpre to PTVpre margins of 5, 7, 10, 12, 15mm was used in all directions. Each post-treatment CBCT was rigidly registered to the pre-treatment CBCT with respect to bony anatomy. The CTVpost was projected onto the PTVpre and assess required planning margins to encompass the CTVpost. The average total treatment time (post-treatment CBCTs - pre-treatment CBCTs) was 23 min 14s (range: 18min 53s ∼ 28min 53s). For all fractions, the adapted plan was selected, and the volume of the PTV-V and PTV-N receiving 100% of the prescribed dose or more (V100%) was more than required 95%. A uniform three-dimensional CTV-Npre to PTV-Npre margin of 5mm could encompass CTV-Npost in all fractions (100%; 175/175f). A uniform three-dimensional CTV-Vpre to PTV-Vpre margin of 10mm could encompass CTV-Vpost in all fractions (100%; 175/175f) and 5mm could encompass 98% fractions (172/175f). In the fractions of 5mm margins that failed to encompass CTV-Vpost, the volume of CTV missed three times in anterior-posterior directions and once in lateral direction. Compared with scheduled plan, adapted plan with 5mm margins significantly reduced the mean dose to bladder (Dmean: 103.97 cGy vs 107.20 cGy, P<0.05) and rectum (Dmean: 117.82 cGy vs 123.67 cGy, P<0.05), and achieved better PTV-N (V100%: 97.8% vs 90.0%, P<0.05) and PTV-V (V100%:96.8% vs 85.4%, P<0.05) dose coverage in 175 fractions. In this study, 5mm CTV to PTV margins was adequate to encompass 100% fractions of CTV-N and 98% fractions of the CTV-V. Adapted plan with 5mm margins significantly reduced the dose to critical organ at risks while improving target coverage.

Impact of Stomach Deformability on PTV Coverage in Patients with Gastric MALTs Treated with Deep Inspiration Breath Hold (DIBH) and Daily CT-based Image-guided Radiation Therapy (IGRT)

Patients with extranodal marginal zone lymphomas of the stomach (gastric MALTs) have excellent prognoses. DIBH with CT-based IGRT is used to minimize unwanted dose to heart and lungs. However, the stomach presents unique challenges for RT given its deformability, which may not be adequately compensated by standard 6-dimensional shifts. We conducted a dosimetry study to evaluate our hypothesis that DIBH, CT-based IGRT, and fasting after midnight may not be sufficient to compensate for stomach deformability. Patients included were treated with DIBH and had daily IGRT with cone-beam CTs (CBCTs). All patients were simulated in alpha cradles with arms up. The CTV consisted of stomach with a 1.0 to 1.5cm CTV to PTV margin. Patients were instructed to fast after midnight. CBCTs used for daily image guidance prior to RT were collected. The stomach was contoured on each CBCT and then overlayed on the simulation scan. The stomach volume outside the PTV was calculated both in absolute volume and as a percentage of daily stomach volume. The relative location of the volume of stomach outside the PTV was also recorded. Five patients with biopsy-proven gastric MALTs who received definitive dose RT were included. Patients were followed for at least 11 months (range: 11 - 20 mo.), and all achieved a complete pathological response. Seventy daily CBCTs were used in the analysis. Across all images, the daily stomach volume was smaller than the CTV by a mean 99.3cc (range: <305.3cc - >108.9cc). The mean volume of stomach outside the PTV was 7.8cc (range: 0 - 75.5cc). This represented 3.1% of the daily stomach volume (range: 0 - 18.7%). Per patient, the mean percent volume outside the PTV ranged from 0.4% to 6.9%. In 3 of the 5 patients, the percent volume outside the PTV was <2.5%. The stomach was most often anterior to the PTV (31.4% of images). Medial and posterior extensions were the next most frequent, representing 21.4% and 14.3% of images, respectively. Of all the daily stomach contours, 15.7% remained wholly within the PTV. DIBH with daily IGRT in patients with gastric MALTs may result in moderate underdosing of the stomach due to its deformability. Daily stomach volumes were different in size from the CTV, and patients had an average of 0.4% to 6.9% of stomach volume outside the PTV. Regions of decreased coverage were most frequently seen anteriorly. Future dosimetry studies may suggest non-isometric PTV expansions to better cover these areas.

Abstract PD3-06: Robotic Stereotactic APBI for Early-Stage Breast Cancer: 2-year Outcomes of a Prospective Multi-Institutional Trial

Abstract Purpose: Outcomes following adjuvant accelerated partial breast irradiation (APBI) in select women with early-stage breast cancer are comparable to whole breast irradiation. Robotic stereotactic accelerated partial breast irradiation (RSAPBI) with fiducial tracking is an attractive treatment option, but limited outcomes data are available for this approach. We report 2-year outcomes for a prospective multi-institutional trial treating select women with RSAPBI. Materials and Methods: Post-menopausal women with DCIS and Stage IA breast cancer were treated over a five-year period extending from November 2015 to November 2020 and were followed for a minimum of 18 months. Treatments were delivered with a robotic radiosurgery system. Four gold fiducials were implanted around the lumpectomy cavity prior to the start of treatment for tumor bed delineation and target tracking. The CTV was defined as the lumpectomy cavity with a uniform 5-15 mm expansion confined to the breast tissue and the PTV was defined as the CTV with a 0-5 mm uniform expansion. The PTV was prescribed 30 Gy in 5 fractions. Disease status assessments were completed at 4 weeks, 3 months, 6 months, 12 months, 18 months, 24 months and yearly intervals thereafter for five years. Results: Eighty-one patients (median age 68 years) with hormone receptor-positive tumors were treated over a median 9 days (range, 5-15). Sixty-eight women had invasive ductal carcinoma (84%) and thirteen had DCIS (16%). The median treated PTV was 108 cm3 (IQR 66-156) and the median prescription isodose line was 81% (IQR 79-83). The median CTV expansion was 10 mm (range 5-10) and the median PTV expansion was 3 mm (range 0-5). At a median follow up of 2 years there was one new ipsilateral breast tumor diagnosed. There were no local, regional, or distant treatment failures. Conclusions: Two-year results suggest that RSAPBI with fiducial tracking is an effective technique for the adjuvant treatment of post-menopausal women with hormone receptor-positive early-stage breast cancer. Additional follow-up is planned to confirm this preliminary finding. Citation Format: Jonathan M. Cantalino, David D’Ambrosio, Arica Hirsch, Brian Collins, Malika Danner, Dawn Matsanka, Sonali Rudra, Simeng Suy, Sean Collins, Monica Pernia Marin, Michael Good, Jing Feng, John Lamond, Deborah Markiewicz, Rachelle Lanciano, Olusola Obayomi-Davies. Robotic Stereotactic APBI for Early-Stage Breast Cancer: 2-year Outcomes of a Prospective Multi-Institutional Trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD3-06.

A dose escalated fiducial marker-based image guided radical radiotherapy in locally advanced prostate cancers: A single institute experience from India

Image guided radiotherapy (IGRT) is one of the most commonly used treatment in LAPC. Dose escalation >74 Gy has shown to improve the biochemical control and freedom from failure rate in LAPC.We started treating LAPC patients with dose escalated IGRT in our institute since 2008. We did a retrospective analysis to see the biochemical relapse-free survival, cancer-specific survival, and bladder and rectal toxicity. A total of 50 consecutive prostate cancer patients were treated with dose escalated IGRT between January 2008 to Dec 2013. Out of these, 37 patients of LAPC were analyzed and their medical records were retrieved. All were biopsy proven adenocarcinoma of prostate with D'Amico high risk category (PSA >20 ng/mL or Gleason score (GS) >7 or T2c-T4). Three gold fiducial markers were placed in the prostate. Patients were immobilized in supine position with either ankle or knee rest. Partial bladder filling and rectum emptying protocol was followed. Clinical target volume (CTV) segmentation was done according to EORTC recommendation. Population based PTV expansion from CTV of 10 mm (cranio-caudal), 10 mm (medio-lateral), 10 mm (anterior) and 5 mm (posterior) was given. In patients with radiologically enlarged pelvic lymph node, whole pelvis intensity modulated radiation therapy (IMRT) to a dose of 50.4 Gy/28# followed by prostatic boost 26Gy/13# by IMRT using image guidance. Rest of the patients received prostate only RT to a dose of 76Gy/38# by IGRT. Daily On board KV images were taken and 2D-2D fiducial marker matching was done and shifts were applied on machine before treatment. Biochemical relapse was defined as per Phoenix definition (nadir + 2 ng/mL). Radiation Therapy Oncology Group (RTOG) toxicity grading system was used to document acute and late toxicity. Median age of patients was 66 years. Median pre-treatment PSA was 22 ng/mL. Thirty patients (81%) had T3/T4 lesions and nodal metastasis was seen in 11 (30%). Median GS was 8. Median radiotherapy dose was 76 Gy. Imaging before radiation delivery was done in 19(51%) patients and 100% in 14 (38%) patients. With a median follow up of 6.5 years, 5-year biochemical relapse-free survival (bRFS) and cancer-specific survival (CSS) was 66% and 79% respectively. Mean bRFS and CSS were 71 months and 83 months however Median bRFS and CSS were not reached. Distant metastasis was seen in 8 (22%). RTOG grade III bladder and rectal toxicity was seen in 2 (6%) and 2 (6%) patients respectively. Dose escalated IGRT with fiducial marker positional verification for LAPC is doable in Indian setup provided more emphasis given on daily on-board imaging with rigorous bladder filling and rectal emptying protocol. Long term follow up is needed to assess the effect on distant disease-free survival and CSS.

A Dosimetric Analysis of Single Fraction Lattice Radiotherapy as a Boost for the Palliative Treatment of Bulky Head and Neck Tumors

<h3>Purpose/Objective(s)</h3> This study investigates the dosimetry achieved in VMAT/IMRT-planned lattice radiotherapy (LRT) for bulky, palliative head and neck tumors. LRT allows for the delivery of higher radiation doses without dramatically increasing toxicity by using high-dose spheres within the tumor while directing high dose away from OAR's. Our objective was to analyze LRT using 3 dose levels as a boost for palliative treatment of bulky head and neck tumors. The clinical goal of LRT boost would be to increase the effectiveness of treatment while minimizing the burden of care for patients requiring a quick palliation of symptoms associated with bulky tumors. <h3>Materials/Methods</h3> Six bulky palliative head and neck cases were used for this study. A 3D lattice of 0.75 cm spheres spaced 0.85 cm apart was applied to the planning scan and restricted to the area inside the lattice inward contour (GTV was contracted by 0.5 mm). Each case was planned with three different dose prescriptions (15 Gy, 20 Gy and 25 Gy in a single fraction) to the lattice of spheres. In addition, a standard IMRT plan was also created to deliver a uniform 20 Gy in 5 fractions to a defined PTV. The margins (CTV + PTV expansions) used were between 0.5 to 1 cm. A total of 7 plans were created for each case, which included the three single fraction doses, the five fractions of standard treatment, and a combination of the single and five fraction plans. <h3>Results</h3> The average planned tumor volume was 209.77 mL (range 137.80 – 411.01). The number of spheres used in the LRT plan ranged from 13 to 41. The median maximum dose (Dmax) and dose statistics achieved are listed in Table 1. The median ipsilateral carotid dose was high for the 25 Gy (single fraction) plan, so our results suggest that a neoadjuvant LRT boost with of a single fraction of 20 Gy would likely be safe. <h3>Conclusion</h3> Lattice radiotherapy is a viable treatment option for bulky head and neck cases treated with palliative intent. We were able to achieve acceptable dosimetric plans in both the 15 and 20 Gy boosts (single fraction) followed by 20 Gy in 5 fractions, a combination that would be expected to provide more favorable tumor control and symptom relief than traditional palliative regimens of 20-30 Gy. A clinical application with prospective dose escalation trial is under consideration.

Dosimetric Impact of Simulated Daily Adaptive Radiotherapy with Significantly Reduced Setup Margins in the Definitive Treatment of Head and Neck Cancer

<h3>Purpose/Objective(s)</h3> Even when treating head and neck cancer (HNC) with daily image-guided radiation therapy, setup margins of 3-5 mm are required to ensure adequate coverage of disease due to setup error. These margins lead to additional normal tissue dose that may compromise acute and long-term quality-of-life. The use of daily adaptive radiotherapy (ART) using an online adaptive system significantly reduces the need for a PTV margin, since the targets and organs-at-risk (OAR) are recontoured each fraction. In this study, we hypothesized using daily ART (simulated) with 1 mm margins based on our involved nodal radiotherapy (INRT) paradigm would improve coverage and reduce OAR dose. <h3>Materials/Methods</h3> Ten patients with HNC treated on our institutional phase II INRT trial were selected for this study. Cone-beam CT scans were uploaded to the ETHOS emulator for daily adaptive replanning which was simulated using fractions 5, 10, 15, 20, 25, 30, and 35. Two plans were generated with a 1 mm (reduced) and a 5 mm (conventional) PTV expansion. The dose grid from the delivered, conventional margin plan was transposed onto the CBCT and adapted contours. Separately, a reduced target margin plan optimized on the online adaptive system was created. PTV coverage (70 Gy-gross/primary disease, 66.5 Gy-suspicious nodes, 63 Gy-extended volume surrounding the primary disease) and D1 were calculated for target structures along with the dose to OARs. Comparisons performed via paired two tailed t-test. <h3>Results</h3> Six patients with oropharynx cancer and four patients with larynx cancer were included. Median V100% coverage for 70 Gy (96% vs 95.6%), 66.5 Gy (98.5% vs 76.5%), and 63 Gy (98.9% vs 74.9%) were improved with the adaptive planning (all p-values<0.03). Plan homogeneity was improved in the ART plan (e.g., PTV70 median D1% 72.8 Gy vs 74.8 Gy, all p-values<0.02). The total median dose reduction was 10.8 Gy (interquartile range, IQR 8.0-12.6 Gy) for the ipsilateral submandibular gland (SMG), 12.2 Gy (IQR 7.3-23.5 Gy) for the contralateral SMG, 8.3 Gy (IQR 3.8-12.6 Gy) for the ipsilateral parotid, 8.1 Gy (IQR 4.4-11.0 Gy) for the contralateral parotid, 7.0 Gy (IQR 4.5-9.1 Gy) for larynx, 6.6 Gy (IQR 2.7-8.4 Gy) for oral cavity, and 8.6 Gy (IQR 5.9-12.1 Gy) for constrictors. <h3>Conclusion</h3> Simulated daily ART using a 1 mm PTV margin provided significant improvement over conventional PTV margins in coverage and critical OAR dosimetry. This small proof-of-concept study suggests this approach may lead to improved quality-of-life while preserving or improving tumor coverage. Further evaluation is needed to assess the resources needed to implement daily adaptations. A phase II randomized trial of daily ART is underway to determine if dosimetric improvements lead to similar gains in patient-reported outcomes.

Effects of Expansion Size from Gross Tumor Volume to Planning Target Volume on Control Rates and Patterns of Recurrence in Conventionally Fractionated Radiotherapy for WHO Grade I Meningiomas

<h3>Purpose/Objective(s)</h3> In single fraction stereotactic radiosurgery (SRS) for WHO grade I meningiomas, no- or minimal GTV to PTV margin is accepted practice with good outcomes. For complex tumors not eligible for SRS due to size or proximity to critical structures, there has been historical heterogeneity in target volume expansions practices, ranging from margins of a few mm to up to 2 cm. Additional margin is typically added for fractionated treatments to account for setup variability and less precise imaging and setup technique. Larger expansions result in treatment of uninvolved brain parenchyma and organs at risk, which is a concern in patients with expected long survivals. We sought to evaluate whether there is a control difference based on GTV to PTV expansion size in fractionated radiotherapy with either stereotactic or conventional conformal technique. <h3>Materials/Methods</h3> With IRB approval, we identified 87 patients from an institutional database with imaging-defined or WHO grade I meningioma treated with either 3D or IMRT radiation with 5 to 15 mm expansions from GTV (with or without intermediate CTV) or fractionated stereotactic radiotherapy (fSRT) using a system with ≤ 3mm GTV to PTV expansions, treated between 1999 and 2018 with at least 2 years of follow-up. All treatments were delivered using 1.8 Gray (Gy) fractions. Kaplan-Meier estimators were used for local failure-free survival (LFFS), marginal-failure free survival (MFFS) and distant failure-free survival (DFFS) analysis, with censoring occurring at failure event or most recent MRI. Local failure was defined as within the treatment field, marginal within 2 cm, and distant as beyond 2 cm. Patient clinical and demographic characteristics analyzed with χ² testing. <h3>Results</h3> With a median follow-up of 9.0 years, 25 patients (29%) received 3D or IMRT and 62 patients (71%) received fSRT. Doses delivered varied from 50.4-54 Gy, with the majority (71%) receiving 54 Gy. Thirty-two (37%) underwent surgery; 24 were sub-total resections. Total events were low, with 4 local failures (5%), 1 marginal failure (1%) and 1 distant failure (1%). The fSRT and 3D/IMRT groups each had two local failures; 3/4 local failures occurred in areas near critical organs at risk (two in suprasellar region, one in cavernous sinus). Extent of surgical resection was not correlated with risk for local failure. For 3D/IMRT vs fSRT, 5- and 10-year LFFS were 100% vs 98% (p=0.46) and 94% vs 96% (p=0.34), 5- and 10-year MFFS were 100% vs 100% and 100% vs 92% (p=0.004), and 5- and 10-year DFFS were 100% vs 98% at both time points (p=0.65 and p=0.67, respectively). <h3>Conclusion</h3> In this patient cohort, there was no local control benefit to larger GTV to PTV expansions. For patients with tumors not eligible for SRS, fractionated treatment using a rigorous stereotactic workflow with ≤ 3 mm PTV expansions is an effective approach for WHO grade I meningiomas.

Dosimetric Analysis of Radiation Treatment Plans Based on a Deep Learning Auto Contouring Model for Patients with Localized Prostate Cancer

<h3>Purpose/Objective(s)</h3> To evaluate the feasibility and safety of creating radiation treatment plans based on a convolutional neural network auto-contouring (AC) model for patients receiving radiation for localized prostate cancer with a radio-opaque rectal spacer hydrogel. <h3>Materials/Methods</h3> The deep learning model produces ACs for target volumes (prostate, proximal seminal vesicle), OARs (rectum, bladder, penile bulb, femoral heads), and rectal hydrogel spacers. ACs were compared to manual contours (MC) by MDs for clinical use. Individual volumes and overall performance (composite score) were previously evaluated and reported, qualitatively using a 1 (minor discrepancy), 2 (moderate discrepancy), 3 (significant discrepancy), and 4 (rejected, gross error) scale and quantitatively using Dice Similarity Coefficient (DSC) and mean distance to agreement. VMAT treatment plans were created on unedited AC volumes from five composite 1 (C1) cases and five composite 2 (C2) cases and dose volume metrics were computed for the final MCs. Dosimetry comparison was performed to assess the safety of delivering a treatment plan created from unedited ACs. Cases were planned to 50.4 Gy in 28 fractions to the SVs, with a SIB of 70 Gy the prostate. PTV expansions were CTV+5 mm (CTV+4 mm posteriorly). <h3>Results</h3> Of 62 initial cases evaluated, 11 were scored as a C1, 28 were scored as a C2, and 23 received a C3. 10 cases were randomly selected for treatment planning. For prostate (CTV P) in selected C1 and C2 cases, mean score was 1 and 1.8 and mean DSC was 0.919 and 0.876, respectively. Clinical goals for planning and dosimetric data from treatment plans are shown in Table 1. The mean V70 Gy for prostate CTV/PTV was 99.83%/90.05% for C1 cases and 99.46%/89.15% for C2 cases. All clinical goals for OARs were met in both cohorts. <h3>Conclusion</h3> 63% of all cases evaluated received a C1 or C2 score. Our results indicate the treatment plans generated from unedited ACs on these cases respected all clinical goals for OARs. Although C1 plans produced better PTV coverage than C2, almost all plans had sub-optimal PTV coverage, indicating the need for MDs to edit target volumes before treatment planning.

CT-Based Online Adaptive Prostate SBRT Improves Target Coverage and Reduces Rectal Dose

<h3>Purpose/Objective(s)</h3> Stereotactic body radiation therapy (SBRT) is an emerging treatment modality for clinically localized prostate cancer (PCa). Online daily adaptive radiotherapy (ART) could potentially improve the therapeutic ratio of prostate SBRT by accounting for inter-fraction variation in target and OAR volumes. To-date no group has evaluated the clinical utility of a novel AI-augmented CT-based ART system for prostate SBRT. In this study we hypothesized that adaptive prostate SBRT plans would result in improved target coverage and lower dose to OARs versus the unadapted treatment plan. <h3>Materials/Methods</h3> Eight patients with unfavorable intermediate or high-risk PCa treated with 5-fx whole-pelvis prostate adaptive SBRT were reviewed. Patients were treated to the elective nodes (2500 cGy) with simultaneous boosts to the prostate/seminal vesicles (3625 cGy) and prostate (4000 cGy). 4 of 8 had rectal hydrogel spacer. For each Fx, CBCT was acquired and OARs (rectum, bladder, bowel, sigmoid, femurs) were segmented/deformed using AI. CTVs were rigidly registered. Volumes were adjusted manually and PTV expansions added. Adaptive treatment plans were developed based on the contoured targets and OARs and dose to these volumes for the adapted vs. initial plans were compared. The V100 (% PTV receiving prescription dose) and the D0.03 cc between scheduled and adapted treatment plans were compared using a Student's t-test, with significance threshold of <i>P<0.05</i>. <h3>Results</h3> Eight patients successfully completed 40 Fx's of adaptive RT as scheduled. Daily adaptation resulted in a statistically significant mean improvement in PTV V100 for all targets: [11.9% ± 2.1% for PTV 4000 (p <0.0001); 7.9% ± 1.5% for PTV 3625 (p <0.0001); and 6.1% ± 1.6% for PTV 2500 (p= 0.0007)]. Mean rectal D0.03 was significantly reduced by 45.4 cGy ± 10.8 cGy (p= 0.0002) per fraction (227 cGy/5 fractions) compared to the initial plans. There were no statistically significant differences between scheduled and adapted D0.03 to the bladder, sigmoid, or bowel (Table). No patients experienced acute CTCAE grade ≥3 GI/GU adverse events (median F/U 9.5 months). All statistically significant differences were maintained in the presence and absence of rectal hydrogel spacer. <h3>Conclusion</h3> CT-based online adaptive SBRT resulted in statistically significant and clinically meaningful improvements in PTV coverage and D0.03cc dose to the rectum. A trial evaluating CT adaptive prostate SBRT is underway.

HyperArc Planning for Benign Intracranial Pathologies: A Prospective Analysis on Plan Quality, Delivery, Control and Toxicity

<h3>Purpose/Objective(s)</h3> This study reviews a treatment planning system (TPS) that automates both treatment planning and delivery of single-isocenter VMAT radiosurgery (SRS). The TPS was intended for complex multi-metastasis cases, for which it generates high quality, rapidly-deliverable plans. The effectiveness of treating benign intracranial pathologies (BIP) with TPS was unknown. Under an IRB-approved prospective registry, we collected data on the treatment planning and delivery as well as clinical outcomes of BIP managed with SRS since deployment of TPS. <h3>Materials/Methods</h3> Patients included received SRS between October 2017 and November 2021 using TPS. Patients with less than 3 months follow-up (FU) were censored. For all targets, full prescription dose was normalized to ≥ 99% of target volume without additional PTV expansion. All treatments were delivered on a linear accelerator with 10MV flattening-filter free beam at 2400 MU/min with high-definition (2.5mm) multi-leaf collimator. Descriptive statistics related to SRS treatment, pathology, and prior therapy determined at the time of or prior to treatment were analyzed. Post-treatment imaging, toxicities, and standard pathology-specific outcomes were assessed during FU visits. Significant toxicity was defined as ≥ Grade 3 by CTCAE. <h3>Results</h3> 245 targets (min=0.1cc, max=58.9cc) in 235 patients (53.5% female, median age=59) were treated with TPS. The most common pathologies were meningiomas (134), AVMs (43), pituitary adenomas (30), and acoustic schwannomas (23). 45% of patients were treated due to recurrent, residual, or persistent disease after prior treatment with 15% having prior radiation. A majority (53.5%) were treated in a single fraction (12-22Gy), and 46.5% were treated with fractionated SRS (24-35Gy). A majority (74%) were treated with 3 arcs with median (MED) total treatment and beam-on time lasting 10.4 and 2.3 minutes, respectively. MED RTOG CI and Paddick GI were 1.12 and 3.28, respectively. MED FU was 1.1 years. Excluding AVMs, 11 of 202 (5.4%) benign tumors progressed on FU imaging. Among those that progressed, MED time to failure was 1.14 years. Of those, 9 were WHO grade 2+ meningiomas with prior surgery. Of all patients, significant CNS toxicity was reported in 13 (5.6%, G3=9, G4=4), most of whom had persistent cerebral edema after steroids. Of those with BIP near cochlea (> 3 Gy dose max) without severe hearing loss prior to SRS (n=65), 14% subjectively reported decreased hearing after SRS and 3.5% did not preserve hearing. Of those with BIP near the optics (> 3 Gy dose max, n=81), visual preservation was 100%. <h3>Conclusion</h3> Although developed for multi-metastasis SRS, TPS is capable of generating and efficiently delivering high quality plans characterized by excellent conformality and rapid fall-off. Early clinical outcomes appear congruent with historical controls.

Robustness of Biology-Guided Radiotherapy Delivery to PET Biodistribution Changes within Target

<h3>Purpose/Objective(s)</h3> Biology-guided radiotherapy (BgRT) is a novel tracked dose delivery modality that uses positron emission tomography (PET) emissions to deliver hypofractionated planned fluence to the target. This study evaluated PET biodistribution changes within target during BgRT delivery. <h3>Materials/Methods</h3> Two phantom experiments were designed to evaluate the FDG signal variations and dosimetric effects under stationary target and moving target on a non-clinical investigational radiotherapy system. For the stationary case, a custom built cylindrical 4-D detector phantom insert with a spherical target of 22 mm in diameter containing 4 quadrants that can be filled independently was used to mimic homogenous and non-homogenous FDG uptake conditions. A C-shape insert was used as a PET avid OAR. For simulating PET signal loss within tumor: BgRT plan was created for the spherical target with all quadrants filled and the plan was delivered when ¾ volume of the target was PET-avid. For the PET signal gain within tumor scenario: BgRT plan was created for the same spherical CTV structure where ¾ volume of the target was filled with FDG, mimicking non-homogeneous uptake, and delivered to a full volume PET-avid target. The dosimetric accuracy of BgRT delivery was evaluated using 3%/3 mm Gamma criterion using 4-D detector measurements. Target biodistribution changes under motion was evaluated by creating a BgRT plan with an FDG filled 26 mm diameter spherical target as a CTV and C-shape OAR moving independently (target:3D respiratory, OAR: sinusoidal motions) within a large water filled phantom. During BgRT delivery the PET avid target insert was switched to a smaller 22 mm spherical insert, simulating 40% uniform PET distribution reduction within 26mm CTV. For the moving target experiment, BgRT delivered dose was evaluated using the radiographic film under the condition of CTV coverage ≥97% and maximum dose ≤130%. For all the experiments, the target and the C-shaped OAR were filled with FDG at a ratio of 8:1 with respect to the water background mimicking normal tissue uptake, and a 5 mm CTV to PTV expansion to be targeted with a prescription of 50Gy in 5 fractions. <h3>Results</h3> For the stationary scenarios, BgRT delivery at remissive and progressive conditions achieved gamma pass-rate of 93.2 and 97.7%, respectively. The CTV coverage goals for the motion experiment were met where the minimum CTV dose was 103.3% and the maximum dose was 123.8%. <h3>Conclusion</h3> This study evaluated the biodistribution changes within CTV from planning to delivery suggesting the BgRT delivery is robust with respect to daily variations of PET signal within the PTV under these experimental conditions. Clinical feasibility of BgRT needs further evaluation.

Assessment of IMPT versus VMAT plans using different uncertainty scenarios for prostate cancer

BackgroundTo assess the impact of systematic setup and range uncertainties for robustly optimized (RO) intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT) plans in patients with localized prostate cancer.MethodsTwenty-six localized prostate patients previously treated with VMAT (CTV to PTV expansion of 3-5 mm) were re-planned with RO-IMPT with 3 mm and 5 mm geometrical uncertainties coupled with 3% range uncertainties. Robust evaluations (RE) accounting for the geometrical uncertainties of 3 and 5 mm were evaluated for the IMPT and VMAT plans. Clinical target volume (CTV), anorectum, and bladder dose metrics were analyzed between the nominal plans and their uncertainty perturbations.ResultsWith geometric uncertainties of 5 mm and accounting for potential inter-fractional perturbations, RO-IMPT provided statistically significant (p < 0.05) sparing at intermediate doses (V4000cGy) to the anorectum and bladder and high dose sparring (V8000cGy) to the bladder compared to VMAT. Decreasing the RO and RE parameters to 3 mm improved IMPT sparing over VMAT at all OAR dose levels investigated while maintaining equivalent coverage to the CTV.ConclusionsFor localized prostate treatments, if geometric uncertainties can be maintained at or below 3 mm, RO-IMPT provides clear dosimetric advantages in anorectum and bladder sparing compared to VMAT. This advantage remains even under uncertainty scenarios. As geometric uncertainties increase to 5 mm, RO-IMPT still provides dosimetric advantages, but to a smaller magnitude.

A Study on Set Up Variations During Treatment, and Assessment of Adequacy of Current CTV-PTV Margins in Head and Neck Radiotherapy

Introduction: 3DCRT and IMRT demands high accuracy in patient positioning and accurate and faithful reproducibility of the treatment position right from the day of acquisition of planning scans and throughout the entire duration of radiation treatment delivery. Inadequacies in the accurate reproduction of the treatment positions during each fraction can lead to setup variations which can significantly compromise the ultimate precision of idealized treatment delivery. Materials and methodology: We retrospectively analysed the daily setup variations in patients with head and neck malignancies who received radical or adjuvant radiotherapy from January 2018 to June 2018. A CTV-PTV margin of 0.5 cm is used at our centre. The average displacement from the reference treatment position in lateral, longitudinal and vertical directions were calculated based on CBCT shifts recorded during the entire course of treatment. Results: 101 patients were included in the study (45.54% radical radiotherapy and 54.45% postoperative radiotherapy). The mean shift in any direction was between 0.13 cm and 0.19 cm in the study population as a whole, in radically treated patients and postoperative patients. The shift in any direction of more than 0.5 cm occurred only once or twice during the entire treatment period per patient, except one postoperative patient. The frequency of shift was more in postoperative patients. The mean shifts in the lateral and longitudinal directions were significantly more for postoperative patients (p 0.008 and 0.014 respectively). Conclusions: The CTV to PTV expansion margin used at our institute is adequate for radically treated patients with head and neck cancers both in definitive and postoperative settings. The smaller mean shifts (&lt;2mm) and low frequency of shifts points to the potential for reducing the current CTV to PTV expansion, which needs to be validated in larger studies.

Assessment of intra-fraction motion during automated linac-based SRS treatment delivery with an open face mask system.

To evaluate intra-fraction target shift during automated mono-isocentric linac-based stereotactic radiosurgery with open-face mask system and optical real-time tracking. Ninety-five patients were treated using automated linac-based stereotactic radiosurgery in 1-5 fractions with single isocenter for a total of 195 fractions. During treatment, patient positioning was tracked real-time with optical surface guidance and immobilized with a rigid open-face mask. Patients were re-positioned if optical surface guidance error exceeded 1mm magnitude or 1°. Translational and rotational intra-fractional changes were determined by post-treatment CBCT matched to the planning CT. Target specific error was calculated by translation and rotation matrices applied to isocenter and target spatial coordinates. For 132 fractions with isocenter within a single target, the median shift magnitude was 0.40mm with a maximum shift of 1.17mm. A total of 398 targets treated for plans having multiple or single targets that lied outside isocenter, resulted in a median shift magnitude of 0.46mm, with median translational shifts of 0.20mm and 0.20° rotational shifts. A 1mm PTV margin was insufficient in 18% of targets at a distance greater than 6cm away from isocenter, but sufficient for 96% of targets within 6cm. The findings of this study support 1mm PTV expansion due to intra-fraction motion to ensure target coverage for plans with isocenter placement less than 6cm away from the targets.

CT-guided versus MR-guided radiotherapy: Impact on gastrointestinal sparing in adrenal stereotactic body radiotherapy

Background and purposeTo quantify the indication for adaptive, gated breath-hold (BH) MR-guided radiotherapy (MRgRTBH) versus BH or free-breathing (FB) CT-based image-guided radiotherapy (CT-IGRT) for the ablative treatment of adrenal malignancies. Materials and methodsTwenty adrenal patients underwent adaptive IMRT MRgRTBH to a median dose of 50 Gy/5 fractions. Each patient was replanned for VMAT CT-IGRTBH and CT-IGRTFB on a c-arm linac. Only CT-IGRTFB used an ITV, summed from GTVs of all phases of the 4DCT respiratory evaluation. All used the same 5 mm GTV/ITV to PTV expansion. Metrics evaluated included: target volume and coverage, conformality, mean ipsilateral kidney and 0.5 cc gastrointestinal organ-at-risk (OAR) doses (D0.5cc). Adaptive dose for MRgRTBH and predicted dose (i.e., initial plan re-calculated on anatomy of the day) was performed for CT-IGRTBH and MRgRTBH to assess frequency of OAR violations and coverage reductions for each fraction. ResultsThe more common VMAT CT-IGRTFB, with its significantly larger target volumes, proved inferior to MRgRTBH in mean PTV and ITV/GTV coverage, as well as small bowel D0.5cc. Conversely, VMAT CT-IGRTBH delivered a dosimetrically superior initial plan in terms of statistically significant (p ≤ 0.02) improvements in target coverage, conformality and D0.5cc to the large bowel, duodenum and mean ipsilateral kidney compared to IMRT MRgRTBH. However, non-adaptive CT-IGRTBH had a 71.8% frequency of predicted indications for adaptation and was 2.8 times more likely to experience a coverage reduction in PTV D95% than predicted for MRgRTBH. ConclusionBreath-hold VMAT radiotherapy provides superior target coverage and conformality over MRgRTBH, but the ability of MRgRTBH to safely provide ablative doses to adrenal lesions near mobile luminal OAR through adaptation and direct, real-time motion tracking is unmatched.

Prospective Study of Surface Guided Radiation Therapy (SGRT) for Breath Hold SBRT Treatments of the Lung: Analysis of Reliability of Surface Guidance Alone for Internal Tumor Position During Breath Hold

<h3>Purpose/Objective(s)</h3> Respiratory motion management with breath hold (BH) is one technique to limit normal lung and other organ dose during SBRT treatments of the lung. Surface guidance (SGRT) has been utilized for BH in larger targets such as breast cancer, but its reliability in BH treatments of small targets has not been investigated. We report the initial results of the reliability of utilizing SGRT alone as a technique for BH during a prospective study of treatment of primary and metastatic lung tumors. <h3>Materials/Methods</h3> An IRB approved prospective study was conducted in patients (pts) with primary NSCLC or lung metastasis undergoing SBRT. Eligible pts had at least 1 cm of respiratory associated motion on free breathing 4DCT. Pts underwent planning CT with BH either in end inspiration or expiration per investigator choice. The GTV was contoured on the BH planning CT and a 5mm margin added for PTV expansion. During each SBRT treatment, pts performed BH and were aligned within tolerance based on the reference BH surface capture created from the BH planning CT. Pts underwent short-arc CBCT during BH for volumetric match of the target. Before shifts were made for target match, pts performed a BH within tolerance, then shifts were made prior to a new reference surface capture being immediately obtained during the same BH. Pts were then treated in BH based on the new reference surface with a tolerance of 3mm translations and 2 degree rotations. Treatment was stopped if pts fell outside of the predefined tolerances. Midway through treatment, pts underwent a second short arc CBCT during BH to verify the reliability of target position during BH treatment utilizing SGRT. Any subsequent shifts on target volumetric match after the second CBCT were recorded. Linear mixed models for repeated measures were used to analyze rotational and translational shift measurements. <h3>Results</h3> 13 pts and 39 SBRT fractions were treated utilizing SGRT alone for BH. All pts were treated with 54 Gy in 3 fractions. Median free breathing range of motion (4D ROM) was 1.3 cm. Mean displacements after mid-treatment CBCT were minimal in all directions as shown in Table 1. Only one observed fraction had a displacement that exceeded the 5mm PTV expansion (longitudinal shift of 0.59 cm with resulting vector shift of 0.68cm). Factors evaluated for association with shift magnitude included age, BMI, gender, tumor location, 4D ROM, and GTV and PTV size. Higher 4D ROM was associated with increased longitudinal and vector shifts (<i>P</i> = 0.019 and 0.046). <h3>Conclusion</h3> SGRT provides a reliable technique for BH SBRT treatments of the lung. Minimal target displacements on mid treatment BH CBCT were observed. Higher 4D ROM on free breathing scans correlated with larger displacements in tumor position utilizing this technique.

Prostate-Centric vs. Bony-Centric Registration in the Definitive Treatment of Node-Positive Prostate Cancer With Simultaneous Integrated Boost: A Dosimetric Comparison

<h3>Purpose/Objective(s)</h3> To determine the impact of daily shifts based on rigid registration to intraprostatic markers (IPMs) on coverage of boost doses delivered to gross nodal disease for prostate cancer. <h3>Materials/Methods</h3> Seventy-five cone-beam CTs (CBCTs) from 15 patients treated with definitive radiation for clinically node-positive prostate cancer underwent fiducial-based and pelvic bony-based registration to the initial planning scans. Gross tumor volumes of nodal boost targets were contoured directly on each CBCT registration. The nodal displacement (3-dimensional translation from the node centroid on planning CT to node centroid on registered CBCT) and dose coverage (minimum dose [Dmin], mean dose [Dmean], dose delivered to 95% of the GTV [D95]) were calculated for each registration on all nodal targets. All doses for each node were normalized to its prescription dose (dose covering 95% of a 3 mm PTV expansion). <h3>Results</h3> Forty-one gross nodal targets were analyzed. Most boosted nodes (80.5%, 33/41) were treated with volumetric-arc radiation therapy (VMAT), while 19.5% (8/41) underwent stereotactic body radiation therapy (SBRT). Dmin, Dmean, and D95 were all significantly lower with fiducial-based registration compared to bony-based registration (<b>P</b> < 0.0001). Nodal displacement was significantly higher for fiducial-based registrations (<b>P</b> < 0.0001). The 3-dimensional translation between the fiducial-based and bony-based registrations (bony-to-fiducial vector) was the most significant predictor of nodal displacement (<b>P</b> < 0.0001). On fiducial-based registrations, a 3-5 mm gross nodal PTV margin is sufficient in most directions; however, superior and posterior margins of 8-9 mm are required as a result of asymmetric prostatic motion. <h3>Conclusion</h3> Large and anisotropic PTV margins are likely needed to adequately dose gross nodal targets when patient setup is based on rigid registration to IPMs. Alternative approaches such as adaptive replanning may be required to overcome these limitations.