Dose Planning Research Articles

Dosimetric impact of a robust optimization approach to mitigate effects from rotational uncertainty in prostate intensity modulated brachytherapy.

Intensity modulated brachytherapy (IMBT) is an emerging technology for cancer treatment, in which radiation sources are shielded to shape the dose distribution. The rotatable shields provide an additional degree of freedom, but also introduce an additional, directional, type of uncertainty, compared to conventional high dose-rate brachytherapy (HDR BT). We propose and evaluate a robust optimization approach to mitigate the effects of rotational uncertainty in the shields with respect to planningcriteria. A previously suggested prototype for platinum-shielded prostate 169 Yb-based dynamic IMBT is considered. We study a retrospective patient data set (anatomical contours and catheter placement) from two clinics, consisting of six patients that had previously undergone conventional 192 Ir high dose-rate brachytherapy treatment. The Monte Carlo based treatment planning software RapidBrachyMCTPS is used for dose calculations. In our computational experiments, we investigate systematic rotational shield errors of ±10 and ±20degrees, and the same systematic error is applied to all dwell positions in each scenario. This gives us three scenarios, one nominal and two with errors. The robust optimization approach finds a compromise between the average and worst-case scenariooutcomes. We compare dose plans obtained from standard models and their robust counterparts. With dwell times obtained from a linear penalty model, for 10degrees errors, the dose to urethra (D0.1cc ) and rectum (D0.1cc and D1cc ) increase with up to 5% and 7%, respectively, in the worst-case scenario, while with the robust counterpart, the corresponding increases were 3% and 3%. For all patients and all evaluated criteria, the worst-case scenario outcome with the robust approach had lower deviation compared to the standard model, without compromising target coverage. We also evaluated shield errors up to 20degrees and while the deviations increased to a large extent with the standard models, the robust models were capable of handling even such largeerrors. We conclude that robust optimization can be used to mitigate the effects from rotational uncertainty and to ensure the treatment plan quality ofIMBT. This article is protected by copyright. All rights reserved.

Effectiveness of robust optimization against geometric uncertainties in TomoHelical planning for prostate cancer.

Geometrical uncertainties in patients can severely affect the quality of radiotherapy. We evaluated the dosimetric efficacy of robust optimization for helical intensity-modulated radiotherapy (IMRT) planning in the presence of patient setup uncertainty and anatomical changes. Two helical IMRT plans for 10 patients with localized prostate cancer were created using either minimax robust optimization (robust plan) or a conventional planning target volume (PTV) margin approach (PTV plan). Plan robustness was evaluated by creating perturbed dose plans with setup uncertainty from isocenter shifts and anatomical changes due to organ variation. The magnitudes of the geometrical uncertainties were based on the patient setup uncertainty considered during robust optimization, which was identical to the PTV margin. The homogeneity index, and target coverage (TC, defined as the V100% of the clinical target volume), and organs at risk (OAR; rectum and bladder) doses were analyzed for all nominal and perturbed plans. A statistical t-test was performed to evaluate the differences between the robust and PTV plans. Comparison of the nominal plans showed that the robust plans had lower OAR doses and a worse homogeneity index and TC than the PTV plans. The evaluations of robustness that considered setup errors more than the PTV margin demonstrated that the worst-case perturbed scenarios for robust plans had significantly higher TC while maintaining lower OAR doses. However, when anatomical changes were considered, improvement in TC from robust optimization was not observed in the worst-case perturbed plans. For helical IMRT planning in localized prostate cancer, robust optimization provides benefits over PTV margin-based planning, including better OAR sparing, and increased robustness against systematic patient-setup errors.

Evaluation of a workflow for cone-beam CT-guided online adaptive palliative radiotherapy planned using diagnostic CT scans.

Single-visit radiotherapy (RT) is beneficial for patients requiring pain control and can limit interruptions to systemic treatments. However, the requirement for a dedicated planning CT (pCT)-scan can result in treatment delays. We developed a workflow involving preplanning on available diagnostic CT (dCT) imaging, followed by online plan adaption using a cone-beam CT (CBCT)-scan prior to RT-delivery, in order to account for any changes in anatomy and target position. Patients previously treated with palliative RT for bone metastases were selected from our hospital database. Patient dCT-images were deformed to treatment CBCTs in the Ethos platform (Varian Medical Systems) and a synthetic CT (sCT) generated. Treatment quality was analyzed by comparing a coverage of the V95% of the planning/clinical target volume and different organ-at-risk (OAR) doses between adapted and initial clinical treatment plans. Doses were recalculated on the CBCT and sCT in a separate treatment planning system. Adapted plan doses were measured on-couch using an anthropomorphic phantom with a Gafchromic EBT3 dosimetric film and compared to dose calculations. All adapted treatment plans met the clinical goals for target and OARs and outperformed the original treatment plans calculated on the (daily) sCT. Differences in V95% of the target volume coverage between the initial and adapted treatments were <0.2%. Dose recalculations on CBCT and sCT were comparable, and the average gamma pass rate (3%/2mm) of dosimetric measurements was 98.8%. Online daily adaptive RT using dCTs instead of a dedicated pCT is feasible using the Ethos platform. This workflow has now been implemented clinically.

Relationships between uptake of [68Ga]Ga-DOTA-TATE and absorbed dose in [177Lu]Lu-DOTA-TATE therapy

BackgroundSomatostatin receptor 68Ga PET imaging is standard for evaluation of a patient’s suitability for 177Lu peptide receptor radionuclide therapy of neuroendocrine tumours (NETs). The 68Ga PET serves to ensure sufficient somatostatin receptor expression, commonly evaluated qualitatively. The aim of this study is to investigate the quantitative relationships between uptake in 68Ga PET and absorbed doses in 177Lu therapy.MethodEighteen patients underwent [68Ga]Ga-DOTA-TATE PET imaging within 20 weeks prior to their first cycle of [177Lu]Lu-DOTA-TATE. Absorbed doses for therapy were estimated for tumours, kidney, spleen, and normal liver parenchyma using a hybrid SPECT/CT–planar method. Gallium-68 activity concentrations were retrieved from PET images and also used to calculate SUVs and normalized SUVs, using blood and tissue for normalization. The 68Ga activity concentrations per injected activity, SUVs, and normalized SUVs were compared with 177Lu activity concentrations 1 d post-injection and 177Lu absorbed doses. For tumours, for which there was a variable number per patient, both inter- and intra-patient correlations were analysed. Furthermore, the prediction of 177Lu tumour absorbed doses based on a combination of tumour-specific 68Ga activity concentrations and group-based estimates of the effective half-lives for grade 1 and 2 NETs was explored.ResultsFor normal organs, only spleen showed a significant correlation between the 68Ga activity concentration and 177Lu absorbed dose (r = 0.6). For tumours, significant, but moderate, correlations were obtained, with respect to both inter-patient (r = 0.7) and intra-patient (r = 0.45) analyses. The correlations to absorbed doses did not improve when using 68Ga SUVs or normalized SUVs. The relationship between activity uptakes for 68Ga PET and 177Lu SPECT was stronger, with correlation coefficients r = 0.8 for both inter- and intra-patient analyses. The 177Lu absorbed dose to tumour could be predicted from the 68Ga activity concentrations with a 95% coverage interval of − 65% to 248%.ConclusionsOn a group level, a high uptake of [68Ga]Ga-DOTA-TATE is associated with high absorbed doses at 177Lu-DOTA-TATE therapy, but the relationship has a limited potential with respect to individual absorbed dose planning. Using SUV or SUV normalized to reference tissues do not improve correlations compared with using activity concentration per injected activity.

The effect of tumor shape irregularity on Gamma Knife treatment plan quality and treatment outcome: an analysis of 234 vestibular schwannomas

The primary aim of Gamma Knife (GK) radiosurgery is to deliver high-dose radiation precisely to a target while conforming to the target shape. In this study, the effects of tumor shape irregularity (TSI) on GK dose-plan quality and treatment outcomes were analyzed in 234 vestibular schwannomas. TSI was quantified using seven different metrics including volumetric index of sphericity (VioS). GK treatment plans were created on a single GK-Perfexion/ICON platform. The plan quality was measured using selectivity index (SI), gradient index (GI), Paddick’s conformity index (PCI), and efficiency index (EI). Correlation and linear regression analyses were conducted between shape irregularity features and dose plan indices. Machine learning was employed to identify the shape feature that predicted dose plan quality most effectively. The treatment outcome analysis including tumor growth control and serviceable hearing preservation at 2 years, were conducted using Cox regression analyses. All TSI features correlated significantly with the dose plan indices (P < 0.0012). With increasing tumor volume, vestibular schwannomas became more spherical (P < 0.05) and the dose plan indices varied significantly between tumor volume subgroups (P < 0.001 and P < 0.01). VioS was the most effective predictor of GK indices (P < 0.001) and we obtained 89.36% accuracy (79.17% sensitivity and 100% specificity) for predicting PCI. Our results indicated that TSI had significant effects on the plan quality however did not adversely affect treatment outcomes.

A dosimetric comparison for SBRT plans of localized prostate cancer between Cyberknife and Varian Truebeam STX device

As the Stereotactic Body Radiotherapy (SBRT) approach began to increase in treating patients with localized prostate cancer, it became necessary to investigate which methods used in practice were better. The aim of this study is to perform a dosimetric comparison of the advantages and disadvantages of SBRT treatments for localized prostate cancer delivered by CyberKnife (CK) and Varian Truebeam STX (FF and FFF). Seventeen intermediate and high-risk patients with localized prostate cancer were included in the study. SBRT plans for the CK system and Varian Truebeam STX systems with and without Flattening Filters (Tru-FF and Tru-FFF) were prepared for each patient. Plans prepared for each patient were planned at a fraction dose of 6.7 Gy at 6 MV energy and a target dose of 33.5 Gy in 5 fractions. For all plans, cumulative dose-volume histograms (DVHs) were generated for target volumes and organs at risk (OAR). The maximum doses of PTV (41 Gy) in CK plans are higher than the maximum doses (35 Gy) in VMAT plans prepared with Tru-FF or Tru-FFF beams. The mean dose of the rectal wall (10.06 ± 2.40Gy for CK) is still relatively low compared to other plans (13.46 ± 2.16 Gy for Tru-FF and 13.61 ± 2.32 Gy for Tru-FFF). The bladder wall (14 Gy for CK, 26 Gy for Tru-FF and Tru-FFF) and femoral head (6.8 Gy for CK, 9 Gy for Tru-FF and 9.4 Gy Tru-FFF) doses were also lower for CK plans. The CK plans provide better tumour control due to low doses in critical organs and high target doses than the Tru-FF or Tru-FFF plans. It was observed that CK and VMAT plans for SBRT with 6 MV photon beams provided acceptable results in term of treatment planning criteria such as Conformity Index and Homogeneity Index. It is recommended to use a target tracking system to provide an accurate and reliable SBRT treatment with VMAT and CK techniques.

Automated Robust Planning for IMPT in Oropharyngeal Cancer Patients Using Machine Learning

The aim of this study was to evaluate an automated treatment planning method for robustly optimized intensity modulated proton therapy (IMPT) plans for oropharyngeal carcinoma patients and to compare the results with manually optimized robust IMPT plans. An atlas regression forest based machine learning (ML) model for dose prediction was trained on CT scans, contours and dose distributions of robust IMPT plans of 88 oropharyngeal cancer patients (OCP). The ML model was combined with a robust voxel and dose volume histogram based dose mimicking optimization algorithm for 21 perturbed scenarios to generate a machine deliverable plan from the predicted dose distribution. Machine learning optimization (MLO) configuration was performed using a cross-validation approach with 3 × 8 tuning patients and comprised of adjustments to the mimicking optimization, to generate higher quality MLO plans. Independent testing of the MLO algorithm was performed with another 25 patients. Plan quality of clinical and MLO plans was evaluated by the clinical target volume (D98% voxel-wise minimum dose >94%), organ at risk (OAR) doses and the normal tissue complication probability (NTCP) (sum (Σ) of grade-2 and grade-3 dysphagia and xerostomia). Adequate robust target coverage was achieved in 24/25 clinical plans and in 23/25 MLO plans in the primary clinical target volume (CTV). In the elective CTV, 22/25 clinical plans and 24/25 MLO plans passed the robust target coverage evaluation threshold. The MLO average Σgrade 2 and Σgrade 3 NTCPs were comparable to the clinical plans (Σgrade 2 NTCPs: clinical 47.5% vs MLO 48.4%, Σgrade 3 NTCPs: clinical 11.9% vs MLO 12.3%). Significant increases in OAR average doses in MLO plans were found in the pharynx constrictor muscles (163 cGy, p=0.002) and cervical esophagus (265 cGy, p=0.002). The MLO plans were created within 45 min. This study showed that automated MLO planning is able to generate robustly optimized MLO plans with quality comparable to clinical plans in OCP.

Craniospinal Irradiation: A Dosimetric Comparison Between O-Ring Linac and Conventional C-arm Linac

PurposeThe aim of this study was to perform a dosimetric evaluation between craniospinal irradiation volumetric modulated arc therapy plans designed for an O-Ring and a conventional C-arm Linac. Methods and MaterialsTwo adult patients were selected for this study. Two plans were designed one for a TrueBeam Edge and one for Halcyon O-ring Linac for each patient. The evaluation of the plans was conducted in terms of dose volume histogram analysis of the target volume and organs at risk (OARs) along with total plan monitor units and beam-on time. Paired sample t test was performed to compare Dmax and Dmean of OARs for each plan's comparison. The delivery of volumetric modulated arc therapy plans was evaluated using Octavius 4D phantom. ResultsAll plans demonstrated dose distributions with sufficient planned target volume conformity and homogeneity. The Homogeneity Index and Conformity Index for all plans were found comparable. The paired sample t test did not demonstrate significant difference in terms of Dmax and Dmean of OARs between plans for both patients. All plans met the threshold of 90%, with Halcyon plans having higher gamma passing rates. ConclusionsCraniospinal irradiation plans generated for Halcyon and Edge are equivalent in terms of plan quality and dose sparing at OARs. The small variations may have originated from the differences in beam profile or mean energy, the degree of the modulation for each plan and characteristics of multi leaf collimators for each treatment unit. Halcyon is able to deliver a distinctly faster treatment, but Edge provides an automatic rotational correction for patient positioning.

Role of biological effective dose for prediction of endocrine remission in acromegaly patients treated with stereotactic radiosurgery.

Stereotactic radiosurgery (SRS) can be used in acromegaly patients to achieve endocrine remission. In this study we evaluate the biological effective dose (BED) as a predictor of SRS outcomes for acromegaly. This retrospective, single-center study included patients treated with single-fraction SRS with growth hormone secreting pituitary adenomas and available endocrine follow-up. Kaplan-Meier analysis was used to study endocrine remission, new pituitary deficit, and tumor control. Cox analyses were performed using two models [margin dose (model 2) versus BED (model 1)]. Sixty-seven patients (53.7% male) with a median age of 46.8 years (IQR 21.2) were treated using a median dose of 25Gy (IQR 5), and a median BED of 171.9Gy2.47 (IQR 66.0). Five (7.5%) were treated without stopping antisecretory medication. The cumulative probability of maintained endocrine remission off suppressive medications was 62.5% [47.9-73.0] at 3 years and 76.5% [61.0-85.9] at 5 years. IGF1i > 1.5 was a predictor of treatment failure [Hazard ratio (HR) 0.40 (0.21-0.79) in model 1, p = 0.00783]. Margin dose > 22Gy [HR 2.33 (1.06-5.13), p = 0.03593] or a BED > 170Gy2.47 [HR 2.02 (1.06-3.86), p = 0.03370] were associated with endocrine remission. The cumulative probability of new hypopituitarism after SRS was 36.8% (CI 95% 22.4-45.9) at 3 years and 53.2% (CI 95% 35.6-66) at 5 years. BED or margin dose were not associated with new hypopituitarism. BED is a strong predictor of endocrine remission in patients treated with SRS. Dose planning and optimization of the BED to > 170Gy2.47 give a greater probability of endocrine remission in acromegalic patients.

A novel approach for dose painting radiotherapy of brain metastases guided by mr perfusion images.

To investigate the feasibility and dosimetric index features of dose painting guided by perfusion heterogeneity for brain metastasis (BMs) patients. A total of 50 patients with single BMs were selected for this study. CT and MR simulation images were obtained, including contrast-enhanced T1-weighted images (T1WI+C) and cerebral blood flow (CBF) maps from 3D-arterial spin labeling (ASL). The gross tumor volume (GTV) was determined by fusion of CT and T1WI+C images. Hypoperfused subvolumes (GTVH) with less than 25% of the maximum CBF value were defined as the dose escalation region. The planning target volume (PTV) and PTVH were calculated from GTV and GTVH respectively. The PTVN was obtained by subtracting PTVH from PTV, and conventional dose was given. Three kinds of radiotherapy plans were designed based on the CBF values. Plan 1 was defined as the conventional plan with an arbitrary prescription dose of 60 Gy for PTV. For dose painting, Plan 2 and Plan 3 escalated the prescription dose for PTVH to 72 Gy based on Plan 1, but Plan 3 removed the maximum dose constraint. Dosimetric indices were compared among the three plans. The mean GTV volume was 34.5 (8.4-118.0) cm3, and mean GTVH volume was 17.0 (4.5-58.3) cm3, accounting for 49.3% of GTV. Both conventional plan and dose painting plans achieved 98% target coverage. The conformity index of PTVH were 0.44 (Plan1), 0.64 and 0.72 (Plan 2 and Plan 3, P<0.05). Compared to Plan 1, the D2%, D98% and Dmean values of the PTVH escalated by 20.50%, 19.32%, and 19.60% in Plan 2 and by 24.88%, 17.22% and 19.22% in Plan 3 respectively (P<0.05). In the three plans, the index of achievement value for PTVH was between 1.01 and 1.03 (P<0.05). The dose increment rates of Plan 2 and Plan 3 for each organs at risk (OARs) was controlled at 2.19% - 5.61% compared with Plan 1. The doses received by OARs did not significantly differ among the three plans (P >0.05). BMs are associated with significant heterogeneity, and effective escalation of the dose delivered to target subvolumes can be achieved with dose painting guided by 3D-ASL without extra doses to OARs.

A Dosimetric Analysis of Modified Volumetric Modulated Arc Therapy for Bone Marrow Sparing Radiotherapy in Cervical Cancer-An alternative Approach to Conventional VMAT

External beam radiotherapy remains the primary treatment modality in cervical cancer. Nowadays Intensity Modulated Radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) are increasingly being used to reduce normal tissue toxicity. The drawback of conventional VMAT is that a considerable volume of pelvic bone marrow receives a low dose. We analyzed whether there was a way to reduce the volume of the low dose regions of bone marrow, and assessed the potential benefit of conventional-4Arc (C-4Arc VMAT), and Modified-4Arc (M-4Arc VMAT) over the conventional 2 ARC VMAT. Twelve clinically proven locally advanced cervical cancer patients treated with concurrent chemo-radiotherapy by Conventional VMAT (RapidArc) in dual rotation mode (C-2Arc VMAT) were selected for this study.C-4Arc VMAT and M-4Arc VMAT dose plans were generated for these twelve patients and these three different types of plans were evaluated for the quality and compared dosimetrically. M-4Arc VMAT designs exhibited a greater bone marrow sparing when compared with conventional VMATs with respect to volume receiving 5Gy to 35Gy without compromising PTV dose coverage. M-4Arc VMAT plans, the bone marrow volume receiving 30 Gy (V30Gy),40Gy (V40Gy), and mean doses were lower than the C- 4 Arc plan and a similar result was observed for V50(Gy) also when comparing with the standard 2 Arc plan. In modified VMAT plans, the rectum and bladder dose volumes were lower than standard VMAT. Similarly, the bowel bag V35(Gy), V40(Gy), V50(Gy), mean doses. The right and left femoral head doses were reduced significantly when compared to conventional VMAT plans. The M-4Arc VMAT plans are better than the C-2Arc and C-4Arc VMAT plans for reducing the dose to bone marrow by limiting the MLC field width travel.

Factors Associated With Cosmetic Outcomes After Treatment With a Novel Form of Breast Intraoperative Radiation Therapy

IntroductionPrecision breast intraoperative radiation therapy (PB-IORT) incorporates computed tomography–guided treatment planning and high dose rate brachytherapy to deliver a single dose of highly conformal radiational therapy. The purpose of this study is to determine factors associated with poor cosmetic outcomes after treatment with PB-IORT. MethodsThe study included all consecutive participants enrolled in an ongoing phase II clinical trial that had completed a minimum of 12 mo of follow-up. A poor cosmetic outcome was defined as scoring “fair” or “poor” on the Harvard Cosmesis evaluation, or “some” or “very much” on any of the three general cosmesis categories. Statistical analysis was performed utilizing R. ResultsThe final cohort included 201 participants, of which 181 (90%) had an overall good/excellent cosmetic outcome. Group 1 consisted of 162 (81%) participants who reported only excellent/good cosmetic outcomes. Group 2 consisted of 39 (19%) participants who reported some aspect of a poor cosmetic outcome. On multivariable analysis, participants with ductal carcinoma in situ were significantly more likely to experience a poor cosmetic outcome (odds ratio 2.45, 95% confidence interval 1.03-5.82, P = 0.04), and those who received subsequent whole breast irradiation were also more likely to have a poor cosmetic outcome (odds ratio 10.20, 95% confidence interval CI 1.04-99.95, P = 0.04). ConclusionsPatients with need for further radiation after PB-IORT are at increased risk for a poor cosmetic outcome. Larger balloon volume and distance between the skin do not have deleterious effects on cosmetic outcomes.

Validation of pencil beam scanning proton therapy with multi-leaf collimator calculated by a commercial Monte Carlo dose engine.

This study aimed to evaluate the clinical beam commissioning results and lateral penumbra characteristics of our new pencil beam scanning (PBS) proton therapy using a multi-leaf collimator (MLC) calculated by use of a commercial Monte Carlo dose engine. Eighteen collimated uniform dose plans for cubic targets were optimized by the RayStation 9A treatment planning system (TPS), varying scan area, modulation widths, measurement depths, and collimator angles. To test the patient-specific measurements, we also created and verified five clinically realistic PBS plans with the MLC, such as the liver, prostate, base-of-skull, C-shape, and head-and-neck. The verification measurements consist of the depth dose (DD), lateral profile (LP), and absolute dose (AD). We compared the LPs and ADs between the calculation and measurements. For the cubic plans, the gamma index pass rates (γ-passing) were on average 96.5%±4.0% at 3%/3mm for the DD and 95.2%±7.6% at 2%/2mm for the LP. In several LP measurements less than 75mm depths, the γ-passing deteriorated (increased the measured doses) by less than 90% with the scattering such as the MLC edge and range shifter. The deteriorated γ-passing was satisfied by more than 90% at 2%/2mm using uncollimated beams instead of collimated beams except for three planes. The AD differences and the lateral penumbra width (80%-20% distance) were within±1.9% and±1.1mm, respectively. For the clinical plan measurements, the γ-passing of LP at 2%/2mm and the AD differences were 97.7%±4.2% on average and within±1.8%, respectively. The measurements were in good agreement with the calculations of both the cubic and clinical plans inserted in the MLC except for LPs less than 75mm regions of some cubic and clinical plans. The calculation errors in collimated beams can be mitigated by substituting uncollimated beams.

Cross-engine transformation-based fast dose calculation for MRI-Linac online treatment planning.

To propose a novel magnetic field dose calculation method based on transformation from pencil beam (PB) to Monte Carlo (MC) distribution for MRI-Linac online treatment planning. The novel magnetic field dose calculation algorithm was established by a PB dose engine and a magnetic field with tissue inhomogeneity influence correction network. The correction network was constructed with a Res-UNet framework, including residual modules and an encoding-decoding path, by inputting three-dimensional PB dose and patient electron density map, and outputting transformed dose distribution. The influences of magnetic fields and tissue heterogeneity were considered and corrected simultaneously in the correction model. A total of 110 clinically treated static beam IMRT plans were collected, including plans for brain, head-and-neck, lung, and rectum cases. A total of 90 cases were used and enhanced to train and validate the model, and the other 20 cases were for test. By comparing the proposed pipeline-generated dose distribution with original input PB dose and corresponding MC dose, the feasibility and effectiveness of the method was evaluated. Results on both beam dose and plan dose accuracy comparisons on all investigated four tumor sites show great consistency between the cross-dose-engine transformation generations and the MC results, with averaged plan mean absolute error of 0.90%±0.13% for the voxel-wise dose difference and 98.33%±1.07% gamma passing rate at the 2%/2mm criteria. The whole PB calculation and transformation process can be completed within second. We have successfully developed a fast novel magnetic field dose calculation pipeline based on transformation from PB distribution to MC distribution for MRI-Linac online treatment planning.

RADT-05. VOLUMETRIC MODULATED ARC THERAPY CRANIOSPINAL IRRADIATION UTILIZING A VERTEBRAL BODY SPARING APPROACH: A DOSIMETRIC ANALYSIS

Abstract BACKGROUND Craniospinal irradiation (CSI) is indicated for adult patients diagnosed with leptomeningeal disease (LMD). Proton-based vertebral-body-sparing (VBS) CSI has been explored with pediatric patients to minimize hematologic toxicity; however, utilization of VBS in an adult population is limited. We hypothesize that VBS-CSI utilizing volumetric modulated arc therapy (VMAT) will effectively reduce vertebral body dose while minimizing dose to surrounding organs-at-risk (OAR) for adult patients with LMD. METHODS Ten adult patients with LMD received CSI, 30 Gy in 10 fractions. These patients were initially treated with a variety of VMAT and IMRT treatment techniques. These patients were then retrospectively replanned using VMAT VBS-CSI. For their VBS-CSI replan, full VMAT arcs for the brain fields matched to two spine isocenters for the upper and lower spine were created utilizing limited posterior arcs. The PTV was created with margins of 3mm uniformly around the brain contour and 7mm around the spinal canal. To further decrease the vertebral body dose, an avoid entry and exit contour was created. This structure was a margin on the PTV anteriorly designed to carve dose out of the vertebral bodies while still maintaining coverage to the PTV. RESULTS All 10 patients had a reduction in vertebral body dose with VMAT VBS-CSI when compared to their initial plan. The mean vertebral dose for the VBS-CSI plans had an average reduction of 33% compared to the initial plans. The mean dose to the small bowel and esophagus were reduced by 46% and 67%, respectively. CONCLUSIONS VBS-CSI may be utilized to minimize dose to surrounding OARs and vertebral bodies. In this study, VMAT VBS-CSI led to significant reductions in vertebral body dose while also minimizing dose to other OARs. VMAT VBS-CSI may be an acceptable alternative to proton-based VBS-CSI in an adult population with LMD. Prospective evaluation is warranted.

NIMG-01. PREDICTING POST-STEREOTACTIC RADIOTHERAPY MAGNETIC RESONANCE IMAGE OUTCOMES OF BREAST CANCER METASTASES TO THE BRAIN

Abstract BACKGROUND Stereotactic radiosurgery (SRS) is a cornerstone in the management of Breast Cancer Metastases to the Brain (BCMB). While control rates are high following SRS, radiation necrosis is a rare but potentially devastating long-term toxicity. There is a clinical need for automated/semi-automated methods to assess tumor response and optimize the RT plans for local control with minimal long-term toxicity. Multiparametric MRI (mpMRI), particularly Apparent Diffusion Coefficient of water (ADC) maps, contain information that is mechanistically relatable to voxel-level tumor response to RT. We report a deep learning-based approach to predict post-SRS ADC maps, FLAIR, T2-weighted (T2W), T1-weighted unenhanced (T1W) and contrast-enhanced (T1WCE) images, from pre-SRS T1W, T1WCE, T2W and FLAIR images, ADC maps, and the delivered RT dose map. These “forward models” will enable the radiation oncologist to simulate radiologic outcomes and iteratively optimize RT plans for local control with minimal toxicity. METHODS We trained a variant of the pix2pix Generative Adversarial Network (GAN) on MRI and RT dose map data from 18 BCMB patients treated with stereotactic radiation with confirmed controlled and locally recurrent metastases. Patients were treated with stereotactic radiation dose of 1-40 Gy between 2013-2019. RESULTS On test data from 6 BCMB patients, the trained forward model predicted post-SRS ADC values within the Gross Tumor Volume (GTV) that were broadly in agreement with ground truth post-SRS ADC maps. In agreement with expectations, the forward model also predicts increasing post-RT ADC within the GTV with increasing simulated RT doses in the range of 1-71 Gy. We have also explored an inverse model to predict the RT dose map required to produce “prescribed” post-SRS ADC values within the GTV. CONCLUSIONS We envision that the forward models will assist the radiation oncologist in initial RT dose plan optimization, while the inverse model may be useful for daily RT plan optimization.

Clinical outcomes of conventional HDR intracavitary brachytherapy combined with complementary applicator-guided intensity modulated radiotherapy boost in patients with bulky cervical tumour.

To investigate the clinical outcomes and toxicity in patients with locally advanced cervical cancer treated with supplementary applicator guided-intensity modulated radiation therapy (IMRT) based on conventional intracavitary brachytherapy (IC/IMRT). A retrospective cohort study. Sichuan Cancer Hospital & Institute, Sichuan Cancer Centre, China. Large high-risk clinical target volume (HR-CTV) volume (>40 ml) at the time of brachytherapy cervical cancer patients were recruited. This study is a retrospective analysis of 76 patients with locally advanced cervical cancer (FIGO IIB-IVA) treated with concurrent chemoradiotherapy followed by IC/IMRT between June 2010 and October 2016. External radiotherapy (45 Gy in 25 fractions) was adminstered with cisplatin chemotherapy treatment before IC/IMRT. The IMRT plan was optimised using the ICBT plan base dose plan by an inverse dose optimisation tool which allows the use of DVH constraints on the total dose of ICBT. A seven-field gantry angle IMRT plan was devised to avoid hotspots when optimising the boost plan. The prescription dose for HR-CTV and IR-CTV were 6and 5Gy per fraction for five fractions, respectively. Mean HR-CTV was 65.8 ± 23.6ml at the time of brachytherapy. D90 for HR-CTV and IR-CTV were 88.7 ± 3.6Gy and 78.1 ± 2.5Gy. D2cc for bladder, rectum, sigmoid and small intestine were 71.8 ± 3.8, 64.6 ± 4.9, 63.9 ± 5.3and 56.7 ± 8.7Gy, respectively. Median follow-up was 85 months (47.9-124.2months). Five-year local recurrence-free survival rate, metastasis recurrence-free survival rate, disease-free survival rate and cancer-special survival rate were 87.6, 82.4, 70.9 and 76.3%, respectively. The grade 1 + 2 gastrointestinal and urinary late toxicities were 15.8 and 21.1%, and grade 3 late toxicities were 3.9 and 5.2%, respectively. Neither acute nor late grade 4 gastrointestinal or urinary toxicities were seen. The combination of ICBT with an applicator-guided supplementary IMRT boost achieved excellent local control and overall survival with low toxicity for bulky residual cervical tumour.

Local Recurrence in Choroidal Melanomas following Robotic-Assisted Radiosurgery (CyberKnife)

Introduction: Tumor recurrence in choroidal melanoma has been associated with decreased overall survival due to metastatic spreading. To detect risk factors of local recurrence and side effects, we analyzed tumor planning and treatment parameters in patients with recurrence of choroidal melanoma after treatment with robotic-assisted radiosurgery (CyberKnife). Methods: Six hundred ninety-four patients treated with CyberKnife between 2005 and 2019 were retrospectively reviewed. Age, gender, best-corrected visual acuity, tumor height, and diameter were recorded. Treatment planning and radiation doses were reviewed. Salvage therapy, overall survival, metastasis, and complications were recorded. Results: Seventy-four patients showed local recurrence. Local recurrence occurred after 42.1 months post CyberKnife treatment (mean; range: 5–100 months). Fourteen out of 74 patients (18.9%) died during follow-up. Recurrence treatment included enucleation in 51 patients (68.9%) and radiosurgery in 19 patients (25.7%). Treatment planning without contrast medium MRI, radiation dose of less than 21 Gy, and insufficient margin delineation were identified as risk factors incrementing local control. Discussion: Robotic-assisted radiosurgery (CyberKnife) is a suitable treatment option for large choroidal melanoma up to 12 mm. Patients with significantly better visual acuity received repeat CyberKnife treatment as salvage therapy and showed an eye retention rate of 81%.

4DCT ventilation function image-based functional lung protection for esophageal cancer radiotherapy.

4DCT (four-dimensional computed tomography) can effectively obtain functional lung ventilation images for patients and integrate them into radiotherapy treatment planning. Studies have not been performed on esophageal cancer, and there is no clear consensus on the optimal functional lung threshold for functional lung. Functional lung images were generated for 11patients with esophageal cancer. The correlation between the dose-volume parameters of functional lung (FL) as defined by different thresholds and the change of PFT/PDFT (pulmonary [diffusion] function test) metrics before and after radiotherapy were evaluated. FL-sparing planning was generated for each patient to preserve the functional lung and compared to conventional anatomical CT (non-sparing) planning. There was asignificant positive correlation between the FL0.8 (defined Jacobian value ≤ 0.8), FL0.84, and FL0.9 dose-volume parameters and ΔFEV1/FVC (reduction before and after radiotherapy), and the FL0.8‑V30 correlation was the strongest (r = 0.819, P < 0.01). The FL-sparing planning had atarget area conformity index and homogeneity index comparable to the non-sparing planning (P > 0.05). For FL, the FL-sparing planning achieved lower FL-MLD (6.30 ± 2.14 Gy vs. 7.83 ± 2.70 Gy), V10 (17.13 ± 7.70% vs. 27.40 ± 9.48%), and V20 (6.96 ± 3.85% vs. 11.63 ± 7.19%) compared to the non-sparing planning (P < 0.05), while heart and spinal cord doses were not significantly different between the two planning groups. The 4DCT-based FL irradiation dose for esophageal cancer was significantly associated with adecrease in FEV1/FVC. The optimal FL defined as a Jacobian value ≤ 0.8 or about 21% of the whole lung volume may be a good choice. FL-sparing planning significantly reduced the FL dose without compromising target area coverage.

PO13: Development of a Risk-Based Workflow to Identify Dosimetric Relevance of Implanted Cardiac Devices (ICDs) for Brachytherapy Patients

<h3>Objective</h3> To develop a risk-based workflow for assessing dosimetric relevance of ICDs (i.e pacemaker/defibrillators) in patients undergoing brachytherapy in our department. <h3>Materials and Methods</h3> In order to conservatively assess possible dose to ICD in clinical brachytherapy situations, worst-case scenarios were generated for brachytherapy procedures performed at our institution. Cases were organized by site and technique: Pelvic HDR [prostate HDR, gynecologic HDR, rectum HDR, pelvic intraoperative (IORT)], pelvic permanent implantations (prostate and other pelvic seed implantations with I-125 and Pd-103), upper abdomen/chestwall/H&N HDR (Esophagus HDR, upper body HDR IORT). Twenty patients were randomly selected among recently treated cases; prescription doses, treatment images (when available) and treatment plan dose distributions (TG43) were used to develop worst-case scenario geometries for each application. Patients did not have ICDs, and theoretical ICD locations were estimated assuming closest reasonable placement to the treatment site. We evaluated the data to set a policy to identify, in advance of treatment, situations in which ICD dose could exceed threshold of 2Gy, and we developed a risk-based workflow. <h3>Results</h3> For pelvis HDR cases, maximal total dose prescription was 28Gy and distance from treatment site to 2Gy threshold was 7cm. For pelvic permanent implantations, maximal total dose prescription was 144Gy using I-125, and distance from treatment site to 2Gy threshold was 5cm. Closest location of ICD to treatment site was estimated to be >>7cm in all pelvic cases, therefore ICD dose for pelvic cases was considered negligible. For upper abdomen/chest wall/H&N HDR cases, maximal total dose prescription was 15Gy and distance from treatment site to 2Gy threshold was 5cm. Worst-case scenario geometries for Esophagus HDR treatments, where applicator location was simulated to be close to the chest wall, predicts a distance to the ICD >5cm, with doses to ICD of 0.5-1Gy. For upper body IORT cases, due to clinical variability, applicator placement can be <5cm from the ICD, thus requiring physical measurements to confirm safety of treatment. Therefore, we proposed a risk-based workflow, taking into account (i) negligible radiation ICD risk for all pelvic cases, (ii) geometric/dosimetric assessment at time of planning for esophagus HDR cases and (iii) intra-operative assessment for IORT cases at or above upper abdomen. For cases (ii) and (iii) the physicist 2^nd checking the plan would review the electronic medical record prior to treatment to identify presence of ICD. For image-guided cases, the ICD should be contoured to evaluate distance from implant site, and dose calculated. For IORT cases, an intraoperative distance measurement from ICD to treatment area would be performed. If distance is ≤5cm, possibility of 2Gy to ICD must be acknowledged. The dose estimate should be recorded in specified form and signed by MD. Sterile lead shielding may be used, if feasible, internally to limit ICD dose. This risk-based workflow is represented in the figure. <h3>Conclusion</h3> We developed a risk-based workflow to evaluate ICD dose for brachytherapy patients. Development of this workflow could help identify patients at clinically relevant risk to ICD perturbation and conversely, reduce unnecessary interventions for negligible risk scenarios. This policy will need to be re-evaluated if new treatment modalities or fractionations are introduced. Dose rate effects were not considered (<<8Gy/min in all situations). Differences between TG43 and physical doses were not systematically explored.