Faculty Bibliography

PURPOSE/OBJECTIVE:A 3D stereoscopic camera system developed by .decimal was commissioned and implemented into the clinic to improve the efficiency of clinical electron simulations. Capabilities of the camera allowed simulations to be moved from the treatment vault into any room with a flat surface that could accommodate patient positioning devices, eliminating the need for clinical patient setup timeslots on the treatment machine. This work describes the process used for these simulations and compares the treatment parameters determined by the system to those used in delivery. METHODS:The Decimal3D scanner workflow consisted of: scanning the patient surface; contouring the treatment area; determining gantry, couch, collimator, and source-to-surface distance (SSD) parameters for en face entry of the beam with sufficient clearance at the machine; and ordering custom electron cutouts when needed. Transparencies showing the projection of in-house library cutouts at various clinical SSDs were created to assist in choosing an appropriate library cutout. Data from 73 treatment sites were analyzed to evaluate the accuracy of the scanner-determined beam parameters for each treatment delivery. RESULTS:Clinical electron simulations for 73 treatment sites, predominately keloids, were transitioned out of the LINAC vault using the new workflow. For all patients, gantry, collimator, and couch parameters along with SSD and cone size were determined using the Decimal3D scanner with 57% of simulations using library cutouts. Tolerance tables for patient setup were updated to allow differences of 10, 20 and 5 degrees for gantry, collimator and couch, respectively. Approximately 7% of fractions (N=181 total fractions) were setup outside of the tolerance table based on physician-direction during treatment. This reflects physician preference to adjust the LINAC rather than patient position during treatment setup. No scanner-derived plan was untreatable due to cutout shape inaccuracy or clearance issues. CONCLUSION/CONCLUSIONS:Clinical electron simulations were successfully transitioned out of the LINAC vault using the Decimal3D scanner without loss of setup accuracy as measured through machine parameter determination and electron cutout shape.

PURPOSE/OBJECTIVE(S): TBI is a backbone of many conditioning regimens for hematopoietic stem cell transplants but can lead to both acute and late toxicity including radiation-induced interstitial pneumonitis. The incidence of idiopathic pneumonia syndrome (IPS) after TBI-based myeloablative conditioning regimens ranges from 7% to 35%. The purpose of this study is to implement image guided volumetrically modulated technique (VMAT) for TBI with the goal of lung sparing and improved target coverage. MATERIALS/METHODS: Nine patients have been treated using image-guided VMAT based TBI at our institution as part of a single-arm phase 2 clinical trial for patients undergoing myeloablative conditioning regimens. The trial was approved by our internal review board (IRB) in September 2020 and aims to accrue 15 patients within one year. All patients enrolled in the trial have signed informed consent. The primary endpoints of the study are the following dosimetric constraints: V100% >= 90%, D98% >= 85% of Rx dose for the planning target volume (PTV), and a mean lung dose < 9 Gy. PTV is defined as the body contour cropped 5 mm from the surface and excluding lungs and kidneys but extended 3 mm into these organs. Additional secondary dosimetric endpoints include mean dose to each individual kidney < 11 Gy, and maximum dose to 2cc of the entire body < 130% of Rx dose. Clinical endpoints include the occurrence of IPS in the first 100 days after transplant, occurrence of acute graft versus host disease (GVHD), transplant related mortality or mortality in the first 100 days following transplant.
RESULT(S): Patients were treated to 12 Gy in 8 BID fractions (n=6) or 13.2 Gy in 8 BID fractions (n=3) over four consecutive days. All patients were able to complete treatment to the prescribed dose as planned. All patient plans met dosimetric constraints of the study. The median PTV V100% was 93.2% of Rx dose (Max: 95.6%, Min: 92.1%), the median PTV D98% was 90.2% of Rx dose (Max: 94.3%, Min: 88.3%), and the median lung dose mean was 7.63 Gy (Max: 7.94 Gy, Min: 7.29 Gy). In addition, individual kidney mean doses were < 11 Gy, and body maximum dose (D2cc) was < 130% of Rx dose for all patients. At this time, only one patient (12 Gy treatment) has reached the 100 day post-transplant follow-up with the following findings: no relapse on bone marrow biopsy, no pneumonitis, resolved acute GVHD overall grade 1 (skin: 1, GI: 0, Liver: 0), resolved dermatitis (grade 1), resolved vomiting (grade 2), ongoing diarrhea and nausea (grade 1, previously grade 2).
CONCLUSION(S): Our initial results indicate that primary and secondary dosimetric endpoints were achievable for all protocol patients treated thus far. As the trial progresses, secondary clinical endpoints at 100 day follow-up will be analyzed to evaluate occurrence of IPS, survival, and treatment related toxicities.
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Purpose: Patients receiving myeloablative total body irradiation (TBI) doses >= 12Gy are at risk of developing interstitial pneumonitis. At our institution, TBI transitioned from extended distance opposed Laterals to image-guided VMAT, in an effort to improve coverage while sparing lungs and kidneys. This work presents a dosimetric comparison between 3D Laterals and VMAT.
Method(s): Nine patients were treated with VMAT as part of an ongoing phase II single-arm clinical trial. VMAT patients were CT-simulated supine, with thermoplastic masks for head/neck, chest/abdomen/pelvis and feet. VMAT planning (12Gy (n=6) or 13.2Gy (n=3) in 8-BID fractions) utilizes 6MV multi-isocentric arcs and AP/PA beams to treat the upper and lower body, respectively. Ten 3D Lateral patients were CT-simulated supine with arms positioned/immobilized for lung shielding, with rice compensation between legs/feet. Laterals plan (12Gy in 8-BID fractions) uses 15MV, beam spoiler, head compensation, and subfields to maintain coverage and mean-lungs dose <10.5Gy. Target (Body-5mm, extending 3mm into lungs and kidneys for VMAT; Body-2cm for Laterals) coverage was evaluated at V100%, and D98% (percentage of Rx). Absolute dose to lungs and kidneys were reportedResults: Median Target V100% and D98% for VMAT was 93.2% (Range: 95.6% to 92.1%) and 90.2% (94.3% to 88.3%), whereas for Laterals V100% and D98% was 57.3% (66.5% to 46.3%) and 80.6% (75.5% to 84%). The median Lung mean dose was 7.6Gy (7.3Gy to 7.9Gy) for VMAT. The median mean dose to kidney was 10.4Gy (10.1Gy to 10.7Gy) for VMAT, and 12.5Gy (11.9Gy to 13.5Gy) for Laterals.
Conclusion(s): We have established a VMAT-TBI program for patients requiring myeloablative irradiation. Improvement in target coverage is demonstrated by V100% and D98%, while reducing the mean dose to the lungs significantly from 10.5Gy to 8Gy

Purpose: In-vivo dosimetry for conventional total body irradiation (TBI) utilize point detectors placed along the patient surface to confirm the delivered dose matches prescription dose. However, in the volumetrically modulated arc therapy (VMAT) approach to TBI, the electronic portal imager device (EPID) can be utilized to acquire a 2-dimensional transmission fluence plane. This work explores the relationship between patient setup accuracy with transit in-vivo dosimetry.
Method(s): A total of 192 fields were investigated. Each VMAT plan consisted of four isocenters: head, chest, abdomen, and pelvis. Prior to treatment, the patient was imaged at the head, pelvis, and chest. Optimal couch shifts were determined for each isocenter under image guidance. The optimal IGRT shifts were determined using an inhouse application that minimized dose deviation using criteria established through plan uncertainty analysis performed in Eclipse. Translational couch residuals were recorded and defined as the difference in the global shift calculated and the optimal couch position with shifts. Transit dosimetry was measured per arc, and analyzed using SNC PerFRACTION with a gamma criteria of 10%/5mm, 5%/5mm, and 5%/7mm.
Result(s): Based on plan uncertainty analysis, clinical threshold for couch residuals were set to 7 mm (5 mm for chest isocenter) as there would be minimal impact on target coverage and organ sparing at those levels. Transit dosimetry showed that the average pass rate across all fields was 99.6%, 97.0%, and 99.2% for 10%/5mm, 5%/5mm, and 5%/7mm gamma criteria, respectively. Pearson correlation tests showed that there was weak correlation between gamma criteria and couch residuals. At stringent 3%/5mm gamma criteria, moderate correlation was found between lateral couch residuals for the head and chest and the head and chest arc analysis.
Conclusion(s): Transit dosimetry showed high pass rates using our couch residual tolerances, which confirmed the plan uncertainty analysis performed during treatment planning

The purpose of this work is to establish an automated approach for a multiple isocenter volumetric arc therapy (VMAT)-based TBI treatment planning approach. Five anonymized full-body CT imaging sets were used. A script was developed to automate and standardize the treatment planning process using the Varian Eclipse v15.6 Scripting API. The script generates two treatment plans: a head-first VMAT-based plan for upper body coverage using four isocenters and a total of eight full arcs; and a feet-first AP/PA plan with three isocenters that covers the lower extremities of the patient. PTV was the entire body cropped 5 mm from the patient surface and extended 3 mm into the lungs and kidneys. Two plans were generated for each case: one to a total dose of 1200 cGy in 8 fractions and a second one to a total dose of 1320 cGy in 8 fractions. Plans were calculated using the AAA algorithm and 6 MV photon energy. One plan was created and delivered to an anthropomorphic phantom containing 12 OSLDs for in-vivo dose verification. For the plans prescribed to 1200 cGy total dose the following dosimetric results were achieved: median PTV V100% = 94.5%; median PTV D98% = 89.9%; median lungs Dmean = 763 cGy; median left kidney Dmean = 1058 cGy; and median right kidney Dmean = 1051 cGy. For the plans prescribed to 1320 cGy total dose the following dosimetric results were achieved: median PTV V100% = 95.0%; median PTV D98% = 88.7%; median lungs Dmean = 798 cGy; median left kidney Dmean = 1059 cGy; and median right kidney Dmean = 1064 cGy. Maximum dose objective was met for all cases. The dose deviation between the treatment planning dose and the dose measured by the OSLDs was within ±4%. In summary, we have demonstrated that scripting can produce high-quality plans based on predefined dose objectives and can decrease planning time by automatic target and optimization contours generation, plan creation, field and isocenter placement, and optimization objectives setup.

PURPOSE/OBJECTIVE:MRI is the gold-standard imaging modality for brain tumor diagnosis and delineation. The purpose of this work was to investigate the feasibility of performing brain stereotactic radiosurgery (SRS) with a 0.35 T MRI-guided linear accelerator (MRL) equipped with a double-focused multileaf collimator (MLC). Dosimetric comparisons were made versus a conventional C-arm mounted linac with a high-definition MLC. METHODS:). Quality assurance measurements were performed with Gafchromic EBT-XD film following an absolute dose calibration protocol. RESULTS:). CI and CGI ranged from 1.12-1.65 and 81.2-88.3, respectively. Gamma analysis pass rates (3%/1mm criteria) exceeded 97.6% for six clinical targets planned and delivered on the MRL. The mean measured versus computed absolute dose difference was -0.1%. CONCLUSIONS:The MRL system can produce clinically acceptable brain SRS plans for spherical lesions with diameter ≤ 2.25 cm. Large lesions (> 2.25 cm) should be treated with a linac capable of delivering non-coplanar beams.

PURPOSE/OBJECTIVE:Routine quality assurance (QA) of cone-beam computed tomography (CBCT) scans used for image-guided radiotherapy is prescribed by the American Association of Physicists in Medicine Task Group (TG)-142 report. For CBCT image quality, TG-142 recommends using clinically established baseline values as QA tolerances. This work examined how image quality parameters vary both across machines of the same model and across different CBCT techniques. Additionally, this work investigated how image quality values are affected by imager recalibration and repeated exposures during routine QA. METHODS:Cone-beam computed tomography scans of the Catphan 604 phantom were taken on four TrueBeam® and one Edge™ linear accelerator using four manufacturer-provided techniques. TG-142 image quality parameters were calculated for each CBCT scan using SunCHECK Machine™. The variability of each parameter with machine and technique was evaluated using a two-way ANOVA test on a dataset consisting of 200 CBCT scans. The impact of imager calibration on image quality parameters was examined for a subset of three machines using an unpaired Student's t-test. The effect of artifacts appearing on CBCTs taken in rapid succession was characterized and an approach to reduce their appearance was evaluated. Additionally, a set of baselines and tolerances for all image quality metrics was presented. RESULTS:All imaging parameters except geometric distortion varied with technique (P < 0.05) and all imaging parameters except slice thickness varied with machine (P < 0.05). Imager calibration can change the expected value of all imaging parameters, though it does not consistently do so. While changes are statistically significant, they may not be clinically significant. Finally, rapid acquisition of CBCT scans can introduce image artifacts that degrade CBCT uniformity. CONCLUSIONS:This work characterized the variability of acquired CBCT data across machines and CBCT techniques along with the impact of imager calibration and rapid CBCT acquisition on image quality.

The purpose of this feasibility study is to develop a fully automated procedure capable of generating treatment plans with multiple fractionation schemes to improve speed, robustness, and standardization of plan quality. A fully automated script was implemented for spinal stereotactic radiosurgery/stereotactic body radiation therapy (SRS/SBRT) plan generation using Eclipse v15.6 API. The script interface allows multiple dose/fractionation plan requests, planning target volume (PTV) expansions, as well as information regarding distance/overlap between spinal cord and targets to drive decision-making. For each requested plan, the script creates the course, plans, field arrangements, and automatically optimizes and calculates dose. The script was retrospectively applied to ten computed tomography (CT) scans of previous cervical, thoracic, and lumbar spine SBRT patients. Three plans were generated for each patient - simultaneous integrated boost (SIB) 1800/1600 cGy to gross tumor volume (GTV)/PTV in one fraction; SIB 2700/2100 cGy to GTV/PTV in three fractions; and 3000 cGy to PTV in five fractions. Plan complexity and deliverability patient-specific quality assurance (QA) was performed using ArcCHECK with an Exradin A16 chamber inserted. Dose objectives were met for all organs at risk (OARs) for each treatment plan. Median target coverage was GTV V100% = 87.3%, clinical target volume (CTV) V100% = 95.7% and PTV V100% = 88.0% for single fraction plans; GTV V100% = 95.6, CTV V100% = 99.6% and PTV V100% = 97.2% for three fraction plans; and GTV V100% = 99.6%, CTV V100% = 99.1% and PTV V100% = 97.2% for five fraction plans. All plans (n = 30) passed patient-specific QA (>90%) at 2%/2 mm global gamma. A16 chamber dose measured at isocenter agreed with planned dose within 3% for all cases. Automatic planning for spine SRS/SBRT through scripting increases efficiency, standardizes plan quality and approach, and provides a tool for target coverage comparison of different fractionation schemes without the need for additional resources.

This paper presents a practical method for converting dose measured with thermoluminescent dosimeters (TLD) to dose in lung and bone for 6 MV and 15 MV photon beams. Monte Carlo (MC) simulations and Burlin cavity theory calculations were performed to calculate [Formula: see text], the dose-to-TLD to dose-to-medium conversion factor. A practical method was proposed for converting TLD-measured-dose to dose-in-medium using the TLD dose calibration in water and [Formula: see text] dose-to-medium to dose-to-water conversion factor. Theoretical calculations for [Formula: see text] were performed using photon spectrum weighted parameters and were compared with MC simulations. Verification of the proposed method was done using phantoms having either bone or lung equivalent slabs stacked in between solid water slabs. Percent depth dose (PDD) curves were measured using 0.089 cm thick LiF:Mg,Ti (TLD-100) dosemeters placed at various depths within these phantoms. They were then corrected with [Formula: see text] factors using the proposed dose conversion method, and were compared with the MC simulations. For 6 MV beam, the MC calculated [Formula: see text] factors were 0.942 and 1.002 for bone and lung, and for 15 MV it was 0.927 and 1.005 for bone and lung, respectively. The difference between the MC simulated and spectrum weighted theoretical [Formula: see text] factors were within 3% for both lung and bone. The PDD curves measured with TLD-100 chips that were corrected using the proposed method agreed well within 1.5% of the MC simulated PDD curves for both the water/lung/water and water/bone/water (WBW) phantoms. The dose-to-medium correction using MC simulated [Formula: see text] is convenient, easy, and accurate. Therefore, it can be used instead of Burlin cavity theory, especially in media with high atomic numbers such as bone for accurate dose quantification.

INTRODUCTION/BACKGROUND:Spine stereotactic body radiation therapy (SBRT) achieves favorable outcomes compared to conventional radiotherapy doses/fractionation. The spinal cord is the principal dose-limiting organ-at-risk (OAR), and safe treatment requires precise immobilization/localization. Therefore, image guidance is paramount to successful spine SBRT. Conventional X-ray imaging and alignment to surrogate bony anatomy may be inadequate, whereas magnetic resonance imaging (MRI) directly visualizes the dose-limiting cord. This work assessed the dosimetric capability of the ViewRay (ViewRay Inc. Oakwood Village, OH) magnetic resonance (MR) guided linac (MR-Linac) for spine SBRT. METHODS:Eight spine SBRT patients without orthopedic hardware who were previously treated on a TrueBeam using volumetric modulated arc therapy (VMAT) were re-planned using MR-Linac fixed-field intensity-modulated radiation therapy (IMRT). Phantom measurements using film, ionization chamber, and a commercial diode-array assessed feasibility. Plans included a variety of prescriptions (30-50 Gy in 3-10 fractions). RESULTS:: 20.0±2.6Gy vs. 24.5±2.0Gy, p=0.0996). Delivery time increased but was acceptable (14.39±1.26min vs. 9.57±1.19min). Ionization chamber measurements agreed with planned dose to within 2.5%. Film and diode measurements demonstrated accurate/precise delivery of dose gradients between the target and the cord. CONCLUSION/CONCLUSIONS:Spine SBRT with the MR-Linac is feasible as verified via re-planning eight clinical cases followed by delivery verification in phantoms using film, diodes, and an ionization chamber. Real-time visualization of the dose-limiting cord during spine SBRT may enable cord-based gating, reduced margins, alternate dose schemas, and/or adaptive therapy.