Dosimetry Metrics Research Articles

A Novel Dose Rate Optimization Method to Maximize Ultrahigh-Dose-Rate Coverage of Critical Organs at Risk Without Compromising Dosimetry Metrics in Proton Pencil Beam Scanning FLASH Radiation Therapy

PurposeTo investigate a dose rate optimization framework based on the spot scanning patterns to improve ultra-high dose rate coverage of critical organs-at-risk (OARs) for proton PBS FLASH radiotherapy, and to present implementation of a genetic algorithm (GA) method for spot sequence optimization to achieve PBS FLASH dose rate optimization under relatively low nozzle beam currents. Materials and MethodsFirstly, a multi-field FLASH plan was developed to meet all the dosimetric goals and optimal FLASH dose rate coverage by considering the deliverable minimum MU (MMU) constraint. Then, a GA method was implemented into the in-house treatment platform to maximize the dose rate by exploring the best spot delivery sequence. A phantom study was performed to evaluate the effectiveness of the dose rate optimization. Then, 10 consecutive plans for lung cancer patients previously treated using PBS intensity-modulated proton therapy (IMPT) were optimized using 45 GyRBE in 3 fractions both transmission and Bragg peak FLASH-RT for further validation. The spot delivery sequence of each treatment field was optimized using this GA. The ultra-high dose rate volume histogram (DRVH) and dose rate coverage V40GyRBE/s were investigated to assess the efficacy of dose rate optimization quantitatively. ResultsUsing a relatively low MU/spot of 150, corresponding to nozzle beam current of 65 nA, the FLASH dose rate ratio V40GyRBE/s of the OAR contour of the core was increased from 0 to ∼60% in the phantom study. In the lung cancer patients, the ultra-high dose rate coverage V40GyRBE/s were improved from 15.2%, 15.5%, 17.6%, and 16.0% before the delivery sequence optimization, to 31.8%, 43.5%, 47.6%, and 30.5% after delivery sequence optimization, in the lungs-GTV, spinal cord, esophagus, and heart (p-values all<0.001). When beam current increased to 130 nA, V40GyRBE/s was improved from 45.1%, 47.1%, 51.2%, and 51.4%, to 65.3%, 83.5%, 88.1%, and 69.4% (p-values<0.05). The averaged V40GyRBE/s for the target and OARs was increased from 12.9% to 41.6%, and 46.3% to 77.5% for 65 and 130 nA, respectively, showing significant improvements based on a clinical proton system. After optimizing the dose rate for the Bragg peak FLASH technique with a beam current of 340 nA, the V40GyRBE/s for the lung-GTV, spinal cord, esophagus, and heart were increased by 8.9%, 15.8%, 22%, and 20.8%, respectively. ConclusionAn optimal plan quality can be maintained as the spot delivery sequence optimization is a separate independent process after the plan optimization. Both the phantom and patient results demonstrated that novel spot delivery sequence optimization can effectively improve the ultra-high dose rate coverage for critical OARs, which can potentially be applied in clinical practice for better OARs sparing efficacy.

Evaluating the relationship between contouring variability and modelled treatment outcome for prostate bed radiotherapy.

Objectives.Contouring similarity metrics are often used in studies of inter-observer variation and automatic segmentation but do not provide an assessment of clinical impact. This study focused on post-prostatectomy radiotherapy and aimed to (1) identify if there is a relationship between variations in commonly used contouring similarity metrics and resulting dosimetry and (2) identify the variation in clinical target volume (CTV) contouring that significantly impacts dosimetry.Approach.The study retrospectively analysed CT scans of 10 patients from the TROG 08.03 RAVES trial. The CTV, rectum, and bladder were contoured independently by three experienced observers. Using these contours reference simultaneous truth and performance level estimation (STAPLE) volumes were established. Additional CTVs were generated using an atlas algorithm based on a single benchmark case with 42 manual contours. Volumetric-modulated arc therapy (VMAT) treatment plans were generated for the observer, atlas, and reference volumes. The dosimetry was evaluated using radiobiological metrics. Correlations between contouring similarity and dosimetry metrics were calculated using Spearman coefficient (Γ). To access impact of variations in planning target volume (PTV) margin, the STAPLE PTV was uniformly contracted and expanded, with plans created for each PTV volume. STAPLE dose-volume histograms (DVHs) were exported for plans generated based on the contracted/expanded volumes, and dose-volume metrics assessed.Mainresults. The study found no strong correlations between the considered similarity metrics and modelled outcomes. Moderate correlations (0.5 <Γ< 0.7) were observed for Dice similarity coefficient, Jaccard, and mean distance to agreement metrics and rectum toxicities. The observations of this study indicate a tendency for variations in CTV contraction/expansion below 5 mm to result in minor dosimetric impacts.Significance. Contouring similarity metrics must be used with caution when interpreting them as indicators of treatment plan variation. For post-prostatectomy VMAT patients, this work showed variations in contours with an expansion/contraction of less than 5 mm did not lead to notable dosimetric differences, this should be explored in a larger dataset to assess generalisability.

90Y post-radioembolization clinical assessment with whole-body Biograph Vision Quadra PET/CT: image quality, tumor, liver and lung dosimetry

PurposeEvaluation of 90Y liver radioembolization post-treatment clinical data using a whole-body Biograph Vision Quadra PET/CT to investigate the potential of protocol optimization in terms of scan time and dosimetry.Methods17 patients with hepatocellular carcinoma with median (IQR) injected activity 2393 (1348–3298) MBq were included. Pre-treatment dosimetry plan was based on 99mTc-MAA SPECT/CT with Simplicit90Y™ and post-treatment validation with Quadra using Simplicit90Y™ and HERMIA independently. Regarding the image analysis, mean and peak SNR, the coefficient of variation (COV) and lesion-to-background ratio (LBR) were evaluated. For the post-treatment dosimetry validation, the mean tumor, whole liver and lung absorbed dose evaluation was performed using Simplicit90Y and HERMES. Images were reconstructed with 20-, 15-, 10-, 5- and 1- min sinograms with 2, 4, 6 and 8 iterations. Wilcoxon signed rank test was used to show statistical significance (p < 0.05).ResultsThere was no difference of statistical significance between 20- and 5- min reconstructed times for the peak SNR, COV and LBR. In addition, there was no difference of statistical significance between 20- and 1- min reconstructed times for all dosimetry metrics. Lung dosimetry showed consistently lower values than the expected. Tumor absorbed dose based on Simplicit90Y™ was similar to the expected while HERMES consistently underestimated significantly the measured tumor absorbed dose. Finally, there was no difference of statistical significance between expected and measured tumor, whole liver and lung dose for all reconstruction times.ConclusionIn this study we evaluated, in terms of image quality and dosimetry, whole-body PET clinical images of patients after having been treated with 90Y microspheres radioembolization for liver cancer. Compared to the 20-min standard scan, the simulated 5-min reconstructed images provided equal image peak SNR and noise behavior, while performing also similarly for post-treatment dosimetry of tumor, whole liver and lung absorbed doses.

Deep learning for automatic organ and tumor segmentation in nanomedicine pharmacokinetics.

Rationale: Multimodal imaging provides important pharmacokinetic and dosimetry information during nanomedicine development and optimization. However, accurate quantitation is time-consuming, resource intensive, and requires anatomical expertise. Methods: We present NanoMASK: a 3D U-Net adapted deep learning tool capable of rapid, automatic organ segmentation of multimodal imaging data that can output key clinical dosimetry metrics without manual intervention. This model was trained on 355 manually-contoured PET/CT data volumes of mice injected with a variety of nanomaterials and imaged over 48 hours. Results: NanoMASK produced 3-dimensional contours of the heart, lungs, liver, spleen, kidneys, and tumor with high volumetric accuracy (pan-organ average %DSC of 92.5). Pharmacokinetic metrics including %ID/cc, %ID, and SUVmax achieved correlation coefficients exceeding R = 0.987 and relative mean errors below 0.2%. NanoMASK was applied to novel datasets of lipid nanoparticles and antibody-drug conjugates with a minimal drop in accuracy, illustrating its generalizability to different classes of nanomedicines. Furthermore, 20 additional auto-segmentation models were developed using training data subsets based on image modality, experimental imaging timepoint, and tumor status. These were used to explore the fundamental biases and dependencies of auto-segmentation models built on a 3D U-Net architecture, revealing significant differential impacts on organ segmentation accuracy. Conclusions: NanoMASK is an easy-to-use, adaptable tool for improving accuracy and throughput in imaging-based pharmacokinetic studies of nanomedicine. It has been made publicly available to all readers for automatic segmentation and pharmacokinetic analysis across a diverse array of nanoparticles, expediting agent development.

A novel lung SBRT treatment planning: Inverse VMAT plan with leaf motion limitation to ensure the irradiation reproducibility of a moving target

This study exposed the implementation of a novel technique (VMATLSL) for the planning of moving targets in lung stereotactic body radiation therapy (SBRT). This new technique has been compared to static conformal radiotherapy (3D-CRT), volumetric-modulated arc therapy (VMAT), and dynamic conformal arc (DCA). The rationale of this study was to lower geometric complexity (54.9% lower than full VMAT) and hence ensure the reproducibility of the treatment delivery by reducing the risk for interplay errors induced by respiratory motion. Dosimetry metrics were studied with a cohort of 30 patients. Our results showed that leaf speed limitation provided conformal number (CN) close to the VMAT (median CN of VMATLSL is 0.78 vs 0.82 for full VMAT) and was a significant improvement on 3D-CRT and DCA with segment-weight optimized (respectively 0.55 and 0.57). This novel technique is an alternative to VMAT or DCA for lung SBRT treatments, combining independence from the patient's breathing pattern, from the size and amplitude of the lesion, free from interplay effect, and with dosimetry metrics close to the best that could be achieved with full VMAT.

Strategies for safety thresholds of phonation for performers via dosimetry

Singers are among a category of professionals, along with many other professionals, who rely on their voice for work [1]. Phonotrauma can be caused by many factors, among which the amount and intensity of vocal use are at the forefront [2]. Dosimetry metrics can assist voice professionals aim for safe amounts of voice use, but a threshold of voice use has not yet been established and recommended by researchers [3]. Understanding what dosimetry research can offer is one important step towards that goal. Furthermore, once dosimetry devices become mainstream, singers can apply the understanding of dosimetry measurements to pace vocal use and minimize vocal fatigue, vocal effort, and consequently preventing vocal injury.

Risk-Adapted SABR for Ultra-Central Lung Tumors Attaining High BED10 to Targets in a Steep Dose-Response Window of 72-105 Gy under Tumor Tracking: A Planning Study

Higher rate of durable local control in stereotactic ablative radiotherapy (SABR) for ultra-central lung tumors (UCLTs) strongly correlates to a BED10 ≥ 100 Gy to the tumors. However, higher rates of grade 5 toxicities are reported to be directly associated with higher dose to the organs at risk (OARs) abutting or in close distance to the UCLTs, including trachea, proximal bronchial tree (PBT) and esophagus. This study evaluated two risk-adapted SABR schemes in a steep dose-response window of BED10 at 72 -105 Gy with isotoxic optimization and dynamic tumor tracking to safely attain high BED10 to the tumors while keeping dose to the OARs within evidence-based tolerances. A total of 12-16 patients are included for planning under two risk-adapted-SABR schemes of 60 Gy in 8 (scheme A) and15 (scheme B) fractions to attain high BED10 of ≥100 Gy to the distal sections of the PTV and yet potent BED10 at 72-84 Gy to the proximal sections of the PTV. Such inhomogeneous dose plans use a 3-5 mm GTV to PTV margins under a scenario that patients have at least one fiducial marker placed in or near the GTV for real-time tumor tracking on a robotic SRS-SABR system. All plans used fixed cone collimators and a planning tool's dose calculation to reach highest BED10 dose coverage achievable to the target volumes while respecting the OAR dose tolerances, including V105 Gy, V100 Gy, V84 Gy and V72 Gy in BED10 for target volumes and the maximum EQD2 dose tolerances (α/β = 3 Gy) in D0.03cc for the trachea/PBT ≤80.5-82.5 Gy and esophagus ≤64.0-77.6 Gy in scheme-A; and trachea/PBT≤97.7 Gy and esophagus ≤64.3 Gy in scheme-B. Median and range of plan dosimetry metrics are compared between the two schemes. For scheme-A, mean BED10 to PTV are 118.0 Gy (median) in105.5-133.5 Gy (range), and mean BED10 dose to GTV are 132.2 (118.3-149.4 Gy). EQD2 dose to trachea are 80.5 Gy (68.0-81.7 Gy), PBT 80.5 Gy (49.1-82.2 Gy), esophagus 67.2 Gy (51.6-77.3 Gy). PTV coverage by BED10 of 72 Gy, 84 Gy, 100 Gy and 105 Gy are 98.9% (87.5-100%), 97.1% (81.8-99.8%), 91.9% (72.5-98.6%) and 85.3% (68.5-97.9%). For scheme-B, mean BED10 to PTV are 102.2 Gy (93.4-106.0 Gy), and mean BED10 dose to GTV are 115.4 (103.4-117.3 Gy). EQD2 dose to trachea are 73.3 Gy (67.5-96.3 Gy), PBT 90.9 Gy (74.4-97.2 Gy), esophagus 61.4 Gy (41.9-64.1 Gy). PTV coverage by BED10 of 72 Gy, 84 Gy, 100 Gy and 105 Gy are 100% (94.9-100%), 95.5% (83.9-99.0%), 54.8% (31.2-65.4%) and 42.8% (17.9-53.2%). Two risk-adapted SABR of 7.5 Gyx8 and 4 Gyx15 are implemented for treating ultra-central lung tumors with BED10 ≥100 Gy to the distal sections of PTV and yet attain potent BED10 ≥72-84 Gy to the proximal sections of PTV that abut or overlap with trachea, proximal bronchial tree or esophagus. Accurate dose calculation by a planning tool and real-time tumor tracking are essential for safe and accurate delivery of such high-risk SABR treatments.

Exploring beamline momentum acceptance for tracking respiratory variability in lung cancer proton therapy: a simulation study

Objective. Investigating the aspects of proton beam delivery to track organ motion with pencil beam scanning therapy. Considering current systems as a reference, specify requirements for next-generation units aiming at real-time image-guided treatments. Approach. Proton treatments for six non-small cell lung cancer (NSCLC) patients were simulated using repeated 4DCTs to model respiratory motion variability. Energy corrections required for this treatment site were evaluated for different approaches to tumour tracking, focusing on the potential for energy adjustment within beamline momentum acceptance (dp/p). A respiration-synchronised tracking, taking into account realistic machine delivery limits, was compared to ideal tracking scenarios, in which unconstrained energy corrections are possible. Rescanning and the use of multiple fields to mitigate residual interplay effects and dose degradation have also been investigated. Main results. Energy correction requirements increased with motion amplitudes, for all patients and tracking scenarios. Higher dose degradation was found for larger motion amplitudes, rescanning has beneficial effects and helped to improve dosimetry metrics for the investigated limited dp/p of 1.2% (realistic) and 2.4%. The median differences between ideal and respiratory-synchronised tracking show minimal discrepancies, 1% and 5% respectively for dose coverage (CTV V95) and homogeneity (D5-D95). Multiple-field planning improves D5-D95 up to 50% in the most extreme cases while it does not show a significant effect on V95. Significance. This work shows the potential of implementing tumour tracking in current proton therapy units and outlines design requirements for future developments. Energy regulation within momentum acceptance was investigated to tracking tumour motion with respiratory-synchronisation, achieving results in line with the performance of ideal tracking scenarios. ±5% Δp/p would allow to compensate for all range offsets in our NSCLC patient cohort, including breathing variability. However, the realistic momentum of 1.2% dp/p representative of existing medical units limitations, has been shown to preserve plan quality.

Dosimetry Sensitivity in a Lower Dimensional Model of Patient-Specific Asthma Subjects.

Experimental uncertainty will impact in silico model calculations of aerosol delivery and deposition. Patient-specific dosimetry models are often parameterized based on medical imaging data, which contain inherent experimental variability. Here, we created and parameterized 1D models of three subject-specific asthmatic subjects and randomly assigned perturbations of up to 15 % on airway diameter, segmental volume, and defected volume. Sensitivity of imaging data experimental variability on dosimetry metrics were quantified. Lobar particle delivery primarily depended on the distal segmental volumes; 15 % range of noise resulted in delivery to the upper right lobe to vary at most from 15.2 and 18.2 % for one of the severe subjects. Particle deposition was most sensitive to airway diameter; 95 % confidence intervals spanned from 8 to 10.6 % in the mild/moderate subject for 15 % variation on input metrics for 5 [Formula: see text] diameter particles. While these results provide possible ranges of dosimetry calculations for a specific subject, the perturbations were not sufficient to model the large observed inter-subject variability (8.9, 19, and 14.5 % deposition, subjects 1--3, respectively, 5 [Formula: see text] diameter particles). This study highlights that in silico model predictions are robust in the presence of experimental uncertainty and that it continues to be necessary to perform subject-specific simulations, especially within the presence of heterogeneous airway disease. Sensitivity analysis provides confidence in calculating deposition in the airways of asthmatic subjects within the presence of experimental uncertainty.

Evaluation of Dosimetry Metrics in Single Isocenter Multiple Targets and Single Isocenter Single Target in the Treatment of Brain Metastases

Purpose: This study compares two LINAC-based techniques for treating multiple brain lesions using quantitative evaluation of plan dosimetric indices. Material and Methods: Twenty-one patients with a total of 168 targets were selected. All plans were designed with five non-coplanar arcs in the RayStation treatment planning system (TPS). The composite dose of Multiple Isocenter Multiple Targets (MIMT) was compared to that of single isocenter multiple targets (SIMT) by evaluating conformity indices, Paddick gradient-index (PGI), V8, V12, and Delivery Time (DT).

Adaptive Positron Emission Tomography Radiation Therapy in Patients With Locally Advanced Vulvar Cancer: A Prospective Study

In this prospective trial, we aim to determine whether fluorodeoxyglucose positron emission tomography and computed tomography (PET/CT)-based adaptive radiation therapy (ART) improves dosimetry outcomes for patients treated with definitive radiation for locally advanced vulvar cancer. Patients were enrolled in 2 sequential institutional review board-approved prospective protocols for PET/CT ART from 2012 to 2020. Patients were planned with pretreatment PET/CT to 45 to 56 Gy in 1.8 Gy/fraction, followed by a boost to gross disease (nodal and/or primary) to a total of 64 to 66 Gy. Intratreatment PET/CT was obtained at 30 to 36 Gy, and all patients were replanned to the same dose goals with revised organ at risk (OAR), gross tumor volume, and planned target volume contours. Radiation therapy consisted of either intensity modulated radiation therapy or volumetric modulated arc therapy. Toxicity was graded by Common Terminology Criteria for Adverse Events, version 5.0. Local control, disease-free survival, overall survival, and time to toxicity were estimated using the Kaplan-Meier method. Dosimetry metrics for OARs were compared using the Wilcoxon signed rank test. Twenty patients were eligible for analysis. Median follow-up among surviving patients was 5.5 years. Local control, disease-free survival, and overall survival at 2 years were 63%, 43%, and 68%, respectively. ART significantly reduced the following OAR doses: bladder, maximum dose (Dmax; median reduction [MR], 1.1 Gy; interquartile range [IQR], 0.48-2.3 Gy; P < .001) and D2cc (MR, 1.5 Gy; IQR, 0.51-2.1 Gy; P < .001); bowel, Dmax (MR, 1.0 Gy; IQR, 0.11-2.9 Gy; P < .001), D2cc (MR, 0.39 Gy; IQR, 0.023-1.7 Gy; P < .001), and D15cc (MR, 0.19 Gy; IQR, 0.026-0.47 Gy; P=.002); and rectal, mean dose (MR, 0.66 Gy; IQR, 0.17-1.7 Gy; P=.006) and D2cc (MR, 0.46 Gy; IQR, 0.17-0.80 Gy; P=.006). No patients experienced any grade ≥3 acute toxicities. There were no reported late grade ≥2 vaginal toxicities. Lymphedema at 2 years was 17% (95% confidence interval, 0%-34%). Doses to bladder, bowel, and rectum were significantly improved with ART, though the median magnitudes were modest. Which patients benefit most from adaptive treatment is a matter for future investigation.

Mid-treatment adaptive planning during thoracic radiation using 68 Ventilation-Perfusion Positron emission tomography

Four-Dimensional Gallium 68 Ventilation-Perfusion Positron Emission Tomography (68Ga-4D-V/Q PET/CT) allows for dynamic imaging of lung function. To date there has been no assessment of the feasibility of adapting radiation therapy plans to changes in lung function imaged at mid-treatment function using 68Ga-4D-V/Q PET/CT. This study assessed the potential reductions of dose to the functional lung when radiation therapy plans were adapted to avoid functional lung at the mid-treatment timepoint using volumetric arc radiotherapy (VMAT). MethodsA prospective clinical trial (U1111-1138–4421) was performed in patients undergoing conventionally fractionated radiation therapy for non-small cell lung cancer (NSCLC). A 68Ga-4D-V/Q PET/CT was acquired at baseline and in the 4th week of treatment. Functional lung target volumes using the ventilated and perfused lung were created. Baseline functional volumes were compared to the week 4 V/Q functional volumes to describe the change in function over time. For each patient, 3 VMAT plans were created and optimised to spare ventilated, perfused or anatomical lung. All key dosimetry metrics were then compared including dose to target volumes, dose to organs at risk and dose to the anatomical and functional sub-units of lung. Results25 patients had both baseline and 4 week mid treatment 68Ga-4D-V/Q PET/CT imaging. This resulted in a total of 75 adapted VMAT plans. The HPLung volume decreased in 16/25 patients with a mean of the change in volume (cc) −28 ± 515 cc [±SD, range −996 cc to 1496 cc]. The HVLung volume increased in 13/25 patients with mean of the change in volume (cc) + 112 ± 590 cc. [±SD, range −1424 cc to 950 cc]. The functional lung sparing technique was found to be feasible with no significant differences in dose to anatomically defined organs at risk. Most patients did derive a benefit with a reduction in functional volume receiving 20 Gy (fV20) and/or functional mean lung dose (fMLD) in either perfusion and/or ventilation. Patients with the most reduction in fV20 and fMLD were those with stage III NSCLC. ConclusionFunctional lung volumes change during treatment. Some patients benefit from using 68Ga-4D-V/Q PET/CT in the 4th week of radiation therapy to adapt radiation plans. In these patients, the role of mid-treatment adaptation requires further prospective investigation.

Uncertainty Considerations that Impact the Use of Dosimetry Metrics in Modern Semiconductors

The use of calculated dosimetry metrics is a crucial element in predicting neutron damage to modern semiconductors under various conditions, e.g., control electronics in avionic systems, satellite sensors, or power output from solar panels. These dosimetry metrics have little value unless they are accompanied by a quantified uncertainty. This paper outlines a mathematical framework that captures the response models for most semiconductor damage metrics and addresses some of the challenges faced in quantifying the relevant sources of uncertainty. The energy-dependent correlations in the damage functions are a critical underpinning in propagating the uncertainty back to a measured quantity. Significant issues are associated with “model defect” in some of the models used, i.e., assumptions in the model form that are not easily considered in the uncertainty estimate. Other issues relate to fundamental differences between the experimental measured quantity and the calculated metric used to represent the damage mode.

Establishment of CT diagnostic reference levels (DRLs) for a Singapore healthcare cluster

IntroductionThe use of computed tomography (CT) in healthcare institutions has increased rapidly in recent years. The Singapore Health Services (SingHealth) cluster of healthcare institutions has taken the first step in establishing a local cluster-wide CT Diagnostic Reference Levels (DRL) in Singapore. CT dose data from each institution were collected through two primary dosimetry metrics: volume CT dose index (CTDIvol measured in mGy) and dose-length product (DLP measured in mGy.cm). MethodsData from 19 CT scanners in seven institutions under one of Singapore healthcare cluster were retrospectively collected and analysed. The five common adult CT examinations analysed were CT Brain (non-contrast enhanced), CT Chest (IV contrast enhanced), CT Kidney-Ureter-Bladder (CT KUB, non-contrast enhanced), CT Pulmonary Angiogram (CT PA, IV contrast enhanced) and CT Abdomen-Pelvis (CT AP, IV contrast enhanced, single phase). Median CTDIvol and DLP values for the five CT examinations from each institution were derived, with the cluster DRLs determined as the 75th percentile of the distribution of the institution median dose values. ResultsA total of 2413 dose data points were collected over a six-month period from June to November 2020. The cluster CT DRLs for the five CT examinations were determined to be 47 mGy and 820 mGy.cm for CT Brain, 5.4 mGy and 225 mGy.cm for CT Chest, 6.7 mGy and 248 mGy.cm for CT PA, 4.6 mGy and 190 mGy.cm for CT KUB and 6.9 mGy and 349 mGy.cm for CT AP. ConclusionThe establishment of the cluster CT DRLs provided individual institutions with a better understanding if their CT doses are unusually high or low, while emphasising that these DRLs are not meant as hard dose limits or constraints to follow strictly.

GSOR7 Presentation Time: 9:30 AM: Shortening the Learning Curve: Pre-Planning with a Digital Library of 3D Printed Custom Templates for Improved Dosimetry on Day One in Combined Tandem and Ovoid + Interstitial Cases

<h3>Background</h3> In cervical brachytherapy the addition of interstitial (IS) needles to intra-cavitary (IC) applicators can significantly enhance dosimetry by improving target coverage without increasing dose to normal tissues when needles are optimally placed. It usually takes more than one implant to learn and adapt to the patient specific challenges to provide the best possible needle distribution, resulting in improved dosimetry metrics over the course of multi-fraction treatments. <h3>Material and Methods</h3> We developed a digital library of customized supplemental templates that attach to the tandem applicator and guide needles to optimized positions for different tumor topologies. The library consists of CT scans of tandem and ovoid (T&O) applicators with needles guided through these 3D printed templates that can be used for treatment planning (Figure 1A). Following a pre-procedure MRI, CTV contours were drawn and a grid overlaid. Then, an appropriate template can be selected based on tumor dimensions and extent from the uterine canal and a treatment plan generated to best prepare for the first implant (Figure 1B). An appropriate strategy could thus be defined for the first implant, including the specific custom applicator, number of needles and depth of needles. We started using this preplanning library approach in March 2021. We compared the dosimetry of hybrid implants with library preplanning to those that we were not pre-planned from the preceding year and compared the CTV V100%. <h3>Results</h3> Thirteen patients were pre-planned (PP) using the digital library and compared to the previous year's hybrid cases without preplan (n=16). The mean CTV V100% for the no pre-plan (NPP) group was 86.2%, 90.8%, 91.5% and 93.0% for fractions 1-2-3-4 respectively whereas the pre-planning group achieved a V100% of 90.8%, 94.7%, 94.3% and 92.9%. The difference in the first fraction's V100% is statistically significant (p=0.015). Similar dosimetry was achieved in the second fraction of the NPP cohort as compared to the first fraction of the PP cohort, showing the benefit of detailed preplanning (Figure 1C). By taking into account all the available pre-procedure anatomic information and potential implant geometry options in preplanning, many of the challenges of the first applicator insertion can be overcome and thereby shortening the learning curve to optimized dosimetry right from the first implant. <h3>Conclusions</h3> By developing a digital library of customized hybrid applicators and then utilizing it for planning on pre-procedure MRI, we were able to execute dosimetrically on the first fraction what was historically achieved on the second fraction. The pre-planning steps outlined above significantly improved the CTV coverage of the first fraction, showing that many of the patient specific learning steps can be surmounted before the first implant.

Adaptive PET Radiation Therapy in Patients with Locally Advanced Vulvar Cancer: A Prospective Study

Adaptive PET Radiation Therapy in Patients with Locally Advanced Vulvar Cancer: A Prospective Study

A Method to Simplify Patient-Specific QA of Single Isocenter Multiple Met SRS Plans

<h3>Purpose/Objective(s)</h3> Pre-treatment quality assurance of single isocenter multi-met stereotactic radiosurgery (SRS) plans is standardly performed by acquiring a dose measurement using a detector array or ion chamber and film. The major efficiency issue with QA of these plans is how many, and which of the multiple metastases (mets) should be measured. The purpose of this study was to develop a workflow using an independent 3D dose calculation (DC) and trajectory log (TL) QA software to efficiently determine which of the brain mets should be further evaluated by measurement. <h3>Materials/Methods</h3> 30 single isocenter multiple metastasis patient SRS plans with a total of 82 mets were retrospectively studied. The clinical SRS plans were generated with an automated dynamic conformal arc SRS planning system (TPS). All plans were sent to the DC software for calculation. The gamma analysis (3%, 1 mm), percent difference between TPS and DC calculated mean and max dose to PTV, and the dose difference of the centroid point of each PTV was recorded. All plans were delivered to a high-resolution diode array designed for SRS and measurements were used to perform a gamma analysis (3%, 1 mm). In addition, the trajectory logs of the QA delivery were used to calculate 3D dose distributions and the same metrics acquired from the DC software were calculated and recorded. Pearson correlation between DC and TL calculated metrics was calculated. <h3>Results</h3> The results of the dosimetric metrics calculated by the DC and TL software are presented in Table 1. Overall agreement between DC and TPS was excellent, with all mean percent differences within 1.1% and mean gamma pass rates greater than 99%. Furthermore, dosimetric metrics calculated by DC and TL were highly correlated, with the correlation coefficients for all metrics at least 0.96, which suggests DC results can be used to predict TL QA results. To use DC to screen which PTVs should be further evaluated, an action level was set such that if any metric fell outside the 95% confidence interval (mean + 1.96 × SD), a QA measurement would be performed. For all metrics, a ±5% action level was calculated to cover the 95% confidence interval. DC results for 6 PTVs exceeded the action level. Of these plans, the measured patient specific QA passed for all plans. While all measured QA passed, DC and TL QA appeared to be more sensitive than the diode array measurement in identifying potential plan quality issues. <h3>Conclusion</h3> An efficient workflow for patient specific QA of single isocenter multiple met SRS plans was developed. An independent 3D dose calculation is used to calculate agreement of dosimetry metrics compared to the TPS. If the percent difference of a metric for a PTV exceeds a 5% action level, it is selected for further QA using a dose measurement.

Stereotactic Ablative Radiotherapy for Early-Stage Glottic Larynx Carcinoma: Results From the Phase II LT-SABR Study

<h3>Purpose/Objective(s)</h3> Traditional radiotherapy for early-stage larynx cancer irradiates the whole larynx over 5.5-6 weeks. Phase I data suggest that stereotactic ablative radiotherapy (SAbR) is a viable strategy in this population to reduce the irradiated volume and compress the treatment time. This phase II study evaluated the efficacy of gLoTtic larynx SAbR (LT-SABR), utilizing a 5 or 16 fraction regimen depending on treatment volume and smoking status. <h3>Materials/Methods</h3> Eligibility required stage 0-II biopsy-proven carcinoma of the true glottic larynx, including squamous cell variants. The arytenoid cartilage could not be involved beyond the vocal process, and patients smoking more than one pack-per-day were excluded. The planning target volume (PTV) consisted of the gross tumor volume, including respiratory motion assessed on a 4D-CT, with a 3 mm AP/LR margin and 5 mm SI margin. Patients without active smoking and PTV volume < 10 cc receive 42.5 Gy in 5 fractions, two fractions per week; all other patients received 58.08 Gy in 16 daily fractions. The routine beam arrangement included 2 coplanar VMAT arcs and 2 non-coplanar arcs. Patients were treated with swallow-gating using surface imaging. The study required 25 patients to show that the local control probability is non-inferior to 75% at 2 years. <h3>Results</h3> A total of 25 patients were accrued to this study, with 21 and 4 treated with 5 and 16 factions, respectively. The stage distribution was CIS (n=1), T1a (n=16), T1b (n=5), and T2 (n=3). The median age was 72 years, with a prior smoking history in 16 and active smoking in one. Dosimetry metrics (mean values) are summarized in the Table. At a median follow-up of 21.1 months (IQR 14-31.8) in surviving patients, there have been two in-field recurrences (one in each dose cohort). One was salvaged with partial laryngectomy and the other died from local and regional disease. The cumulative risks of local failure were 4.2% and 9.3% at 1 and 2 years, respectively. There have been no acute or late grade 3+ toxicities. The baseline, 6 and 12 month total Voice Handicap Index (VHI) scores were: 53.0 (n=25), 9.8 (n=18), and 18.4 (n=15). The improvements in VHI between 6/12 months and baseline were both statistically significant (p<0.001). <h3>Conclusion</h3> Highly conformal stereotactic radiotherapy is safe and efficacious for early-stage glottic larynx cancer, with encouraging patient-reported outcomes. Additional follow-up and ultimately comparative studies are necessary to validate this paradigm.

Dosimetric Comparison of Various Spot Placement Techniques in Proton Pencil Beam Scanning.

PurposeTo present quantitative dosimetric evaluations of five proton pencil beam spot placement techniques.Materials and MethodsThe spot placement techniques that were investigated include two grid-based (rectilinear grid and hexagonal grid, both commonly available in commercial planning systems) and three boundary-contoured (concentric contours, hybrid, and optimized) techniques. Treatment plans were created for two different target volumes, one spherical and one conical. An optimal set of planning parameters was defined for all treatment plans and the impact of spot placement techniques on the plan quality was evaluated in terms of lateral/distal dose falloff, normal tissue sparing, conformity and homogeneity of dose distributions, as well as total number of spots used.ResultsThe results of this work highlight that for grid-based spot placement techniques, the dose conformity is dependent on target cross-sectional shape perpendicular to beam direction, which changes for each energy layer. This variable conformity problem is mitigated by using boundary contoured spot placement techniques. However, in the case of concentric contours, the conformity is improved but at the cost of decreased homogeneity inside the target. Hybrid and optimized spot placement techniques, which use contoured spots at the boundary and gridlike interior spot patterns, provide more uniform dose distributions inside the target volume while maintaining the improved dose conformity. The optimized spot placement technique improved target coverage, homogeneity of dose, and minimal number of spots. The dependence of these results on spot size is also presented for both target shapes.ConclusionThis work illustrates that boundary-contoured spot placement techniques offer marked improvement in dosimetry metrics when compared to commercially available grid-based techniques for a range of proton scanned beam spot sizes.

A Novel Proton Pencil Beam Scanning FLASH RT Delivery Method Enables Optimal OAR Sparing and Ultra-High Dose Rate Delivery: A Comprehensive Dosimetry Study for Lung Tumors.

Simple SummaryThis study attempts to answer a novel and clinically relevant question of the value of Bragg-peak-based FLASH planning for lung tumors. Most existing studies and literature are limited to using transmission proton beams at ultra-high dose rates, resulting in unnecessary irradiation exposure to normal tissues beyond the target volume. By combining a new hardware design (universal range shifter and range compensator) and an inverse planning system, the novel Bragg peak method makes the Bragg-peak-based FLASH planning possible. The treatment planning study and dosimetry comparison between single-energy proton Bragg peak beams and transmission proton beams demonstrated superior performances in OAR sparing and comparable FLASH dose rate of the Bragg peak FLASH. Beam angle optimization can further improve Bragg peak FLASH dosimetry performance while maintaining the similar 3D FLASH dose rate coverage for OARs.Purpose: While transmission proton beams have been demonstrated to achieve ultra-high dose rate FLASH therapy delivery, they are unable to spare normal tissues distal to the target. This study aims to compare FLASH treatment planning using single energy Bragg peak proton beams versus transmission proton beams in lung tumors and to evaluate Bragg peak plan optimization, characterize plan quality, and quantify organ-at-risk (OAR) sparing. Materials and Methods: Both Bragg peak and transmission plans were optimized using an in-house platform for 10 consecutive lung patients previously treated with proton stereotactic body radiation therapy (SBRT). To bring the dose rate up to the FLASH-RT threshold, Bragg peak plans with a minimum MU/spot of 1200 and transmission plans with a minimum MU/spot of 400 were developed. Two common prescriptions, 34 Gy in 1 fraction and 54 Gy in 3 fractions, were studied with the same beam arrangement for both Bragg peak and transmission plans (n = 40 plans). RTOG 0915 dosimetry metrics and dose rate metrics based on different dose rate calculations, including average dose rate (ADR), dose-averaged dose rate (DADR), and dose threshold dose rate (DTDR), were investigated. We then evaluated the effect of beam angular optimization on the Bragg peak plans to explore the potential for superior OAR sparing. Results: Bragg peak plans significantly reduced doses to several OAR dose parameters, including lung V7.4Gy and V7Gy by 32.0% (p < 0.01) and 30.4% (p < 0.01) for 34Gy/fx plans, respectively; and by 40.8% (p < 0.01) and 41.2% (p < 0.01) for 18Gy/fx plans, respectively, compared with transmission plans. Bragg peak plans have ~3% less in DADR and ~10% differences in mean OARs in DTDR and DADR relative to transmission plans due to the larger portion of lower dose regions of Bragg peak plans. With angular optimization, optimized Bragg peak plans can further reduce the lung V7Gy by 20.7% (p < 0.01) and V7.4Gy by 19.7% (p < 0.01) compared with Bragg peak plans without angular optimization while achieving a similar 3D dose rate distribution. Conclusion: The single-energy Bragg peak plans achieve superior dosimetry performances in OARs to transmission plans with comparable dose rate performances for lung cancer FLASH therapy. Beam angle optimization can further improve the OAR dosimetry parameters with similar 3D FLASH dose rate coverage.