Optimizing Respiratory Gated Intensity Modulated Radiation Therapy Planning and Delivery of Early-Stage Non-Small Cell Lung Cancer

Stereotactic ablative body radiotherapy (SABR) is the standard of care for inoperable early-stage non-small cell lung cancer (NSCLC) patients. However, thoracic tumours are susceptible to respiratory motion and, if unaccounted for, can potentially lead to dosimetric uncertainties. Respiratory gating is one method that limits treatment delivery to portions of the respiratory cycle, but when combined with intensity-modulated radiotherapy (IMRT), requires rigorous verification. The goal of this thesis is to optimize respiratory gated IMRT treatment planning and develop image-guided strategies to verify the dose delivery for future early-stage NSCLC patients. Retrospective treatment plans were generated for various IMRT delivery techniques, including fixed-beam, volumetric modulated arc therapy (VMAT), and helical tomotherapy. VMAT was determined the best technique for optimizing dose conformity and efficiency. A second treatment planning study that considered patients exhibiting significant tumour motion was conducted. Respiratory ungated and gated VMAT plans were compared. Significant decreases in V20Gy and V50%, predictors for radiation pneumonitis and irreversible fibrosis, respectively, were observed. The predominant uncertainty of respiratory gating lies in the ability of an external surrogate marker to accurately predict internal target motion. Intrafraction triggered kV imaging was validated in a programmable motion phantom study as a method to determine how correlated the internal and external motion are during ungated and gated VMAT deliveries and to identify potential phase shifts between the motions. KV projections acquired during gated VMAT delivery were used to reconstruct gated cone-beam CT (CBCT), providing 3D tumour position verification. Image quality and target detectability, in the presence of MV scatter from the treatment beam to the kV detector, was evaluated with various imaging parameters and under real-patient breathing motion conditions. No significant difference in image quality was observed for the CBCT acquisitions with or without the presence of MV scatter. This thesis explores the benefits of combining respiratory gating with IMRT/VMAT for the treatment of early stage NSCLC with SABR, and evaluates advanced on-board imaging capabilities to develop dose delivery verification protocols. The results of this thesis will provide the tools necessary to confidently implement a respiratory gated radiotherapy program aimed at improving the therapeutic ratio for early-stage NSCLC

Evaluation and verification of the QFix Encompass couch insert for intracranial stereotactic radiosurgery

The QFix EncompassTM stereotactic radiosurgery (SRS) immobilization system consists of a thermoplastic mask that attaches to the couch insert to immobilize patients treated with intracranial SRS. This study evaluates the dosimetric impact and verifies a vendor provided treatment planning system (TPS) model in the Eclipse TPS. A thermoplastic mask was constructed for a Lucy 3D phantom, and was scanned with and without the EncompassTM system. Attenuation measurements were performed in the Lucy phantom with and without the insert using a pinpoint ion chamber for energies of 6xFFF, 10xFFF and 6X, with three field sizes (2 × 2, 4 × 4, and 6 × 6 cm2 ). The measurements were compared to two sets of calculations. The first set utilized the vendor provided Encompass TPS model (EncompassTPS ), which consists of two structures: the Encompass and Encompass base structure. Three HU values for the Encompass (200, 300, 400) and Encompass Base (-600, -500, -400) structures were evaluated. The second set of calculations consists of the Encompass insert included in the external body contour (EncompassEXT ) for dose calculation. The average measured percent attenuation in the posterior region of the insert ranged from 3.4%-3.8% for the 6xFFF beam, 2.9%-3.4% for the 10xFFF, and 3.3%-3.6% for the 6X beam. The maximum attenuation occurred at the region where the mask attaches to the insert, where attenuation up to 17% was measured for a 6xFFF beam. The difference between measured and calculated attenuation with either the EncompassEXT or EncompassTPS approach was within 0.5%. HU values in the EncompassTPS model that provided the best agreement with measurement was 400 for the Encompass structure and -400 for the Encompass base structure. Significant attenuation was observed at the area where the mask attaches to the insert. Larger differences can be observed when using few static beams compared to rotational treatment techniques

Clinical utility of Gafchromic film in an MRI-guided linear accelerator

BACKGROUND: The purpose of this study is to comprehensively evaluate the suitability of Gafchromic EBT3 and EBT-XD film for dosimetric quality assurance in 0.35 T MR-guided radiotherapy. METHODS: A 0.35 T magnetic field strength was utilized to evaluate magnetic field effects on EBT3 and EBT-XD Gafchromic films by studying the effect of film exposure time within the magnetic field using two timing sequences and film not exposed to MR, the effect of magnetic field exposure on the crystalline structure of the film, and the effect of orientation of the film with respect to the bore within the magnetic field. The orientation of the monomer crystal was qualitatively evaluated using scanning electron microscopy (SEM) compared to unirradiated film. Additionally, dosimetric impact was evaluated through measurements of a series of open field irradiations (0.83 × 0.83-cm(2) to 19.92 × 19.92-cm(2)) and patient specific quality assurance measurements. Open fields were compared to planned dose and an independent dosimeter. Film dosimetry was applied to twenty conventional and twenty stereotactic body radiotherapy (SBRT) patient specific quality assurance cases. RESULTS: No visual changes in crystal orientation were observed in any evaluated SEM images nor were any optical density differences observed between films irradiated inside or outside the magnetic field for both EBT3 and EBT-XD film. At small field sizes, the average difference along dose profiles measured in film compared to the same points measured using an independent dosimeter and to predicted treatment planning system values was 1.23% and 1.56%, respectively. For large field sizes, the average differences were 1.91% and 1.21%, respectively. In open field tests, the average gamma pass rates were 99.8% and 97.2%, for 3%/3 mm and 3%/1 mm, respectively. The median (interquartile range) 3%/3 mm gamma pass rates in conventional QA cases were 98.4% (96.3 to 99.2%), and 3%/1 mm in SBRT QA cases were 95.8% (95.0 to 97.3%). CONCLUSIONS: MR exposure at 0.35 T had negligible effects on EBT3 and EBT-XD Gafchromic film. Dosimetric film results were comparable to planned dose, ion chamber and diode measurements

Assessment of Intrafraction Breathing Motion on Left Anterior Descending Artery Dose During Left-Sided Breast Radiation Therapy.

PURPOSE: To use 4-dimensional computed tomography (4D-CT) imaging to predict the level of uncertainty in cardiac dose estimates of the left anterior descending artery that arises due to breathing motion during radiation therapy for left-sided breast cancer.METHODS AND MATERIALS: The fast helical CT (FH-CT) and 4D-CT of 30 left-sided breast cancer patients were retrospectively analyzed. Treatment plans were created on the FH-CT. The original treatment plan was then superimposed onto all 10 phases of the 4D-CT to quantify the dosimetric impact of respiratory motion through 4D dose accumulation (4D-dose). Dose-volume histograms for the heart, left ventricle (LV), and left anterior descending (LAD) artery obtained from the FH-CT were compared with those obtained from the 4D-dose.RESULTS: The 95% confidence interval of 4D-dose and FH-CT differences in mean dose estimates for the heart, LV, and LAD were ±0.5 Gy, ±1.0 Gy, and ±8.7 Gy, respectively.CONCLUSION: Fast helical CT is a good approximation for doses to the heart and LV; however, dose estimates for the LAD are susceptible to uncertainties that arise due to intrafraction breathing motion that cannot be ascertained without the additional information obtained from 4D-CT and dose accumulation. For future clinical studies, we suggest the use of 4D-CT-derived dose-volume histograms for estimating the dose to the LAD

Dosimetric planning study of respiratory-gated volumetric modulated arc therapy for early-stage lung cancer with stereotactic body radiation therapy

Purpose: To evaluate the dosimetric potential of respiratory-gated volumetric modulated arc therapy (VMAT) to reduce the dose to normal lung when treating early-stage non-small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT). Methods and materials: Twenty patients with inoperable stage I/II NSCLC with motion greater than 5 mm were retrospectively planned with 4-dimensional computed tomography–based gated and nongated VMAT. Each plan was optimized using two 225° arcs with 10-MV flattening filter– free beams with maximum dose rate of 2400 MU/min. A host script was generated and used to optimize all 40 plans to minimize dosimetric bias. The main dosimetric parameters compared were percent volume of the lung receiving 20 Gy or more (V20Gy) and the absolute volume of lung minus the internal tumor volume receiving at least 50% of the prescription dose for normal lung (V50%). Other parameters considered were the maximum dose 2 cm from the planning target volume (D2cm), percent volume of the contralateral lung receiving 5 Gy or more (V5Gy), mean lung dose, maximum dose to normal structures, and monitor units. Results: There was a significant decrease in both parameters for the normal lung with gated VMAT. V20Gy, predictive for pneumonitis, decreased from (6.05 ± 2.06%) to (5.25 ± 1.75%) (P = .00009) and the absolute volume of lung minus the internal tumor volume receiving at least 50%of the prescription dose decreased from (158.17 ± 61.12 cm3) to (125.71 ± 49.46 cm3) (P = .00002). Also, there was a significant decrease in D2cm, contralateral V5Gy, mean lung dose, and monitor units. Conclusions: Respiratory-gated VMAT has the potential to reduce the dose to normal lung when treating early-stage NSCLC with SBRT for tumor motion greater than 5 mm

Evaluation of a magnetic resonance guided linear accelerator for stereotactic radiosurgery treatment

INTRODUCTION: The purpose of this study was to investigate the systematic localization accuracy, treatment planning capability, and delivery accuracy of an integrated magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform for stereotactic radiosurgery. MATERIALS AND METHODS: The phantom for the end-to-end test comprises three different compartments: a rectangular MR/CT target phantom, a Winston-Lutz cube, and a rectangular MR/CT isocenter phantom. Hidden target tests were performed at gantry angles of 0, 90, 180, and 270 degrees to quantify the systematic accuracy. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy. Single-isocenter IMRT treatment plans using 10-15 coplanar beams were generated to treat the multiple metastases. RESULTS: The end-to-end localization accuracy of the system was 1.0 ± 0.1 mm. The conformity index, homogeneity index and gradient index of the plans were 1.26 ± 0.22, 1.22 ± 0.10, and 5.38 ± 1.44, respectively. The average absolute point dose difference between measured and calculated dose was 1.64 ± 1.90%, and the mean percentage of points passing the 3%/1 mm gamma criteria was 96.87%. CONCLUSIONS: Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter

Evaluation of a magnetic resonance guided linear accelerator for stereotactic radiosurgery treatment

INTRODUCTION: The purpose of this study was to investigate the systematic localization accuracy, treatment planning capability, and delivery accuracy of an integrated magnetic resonance imaging guided Linear Accelerator (MR-Linac) platform for stereotactic radiosurgery. MATERIALS AND METHODS: The phantom for the end-to-end test comprises three different compartments: a rectangular MR/CT target phantom, a Winston-Lutz cube, and a rectangular MR/CT isocenter phantom. Hidden target tests were performed at gantry angles of 0, 90, 180, and 270 degrees to quantify the systematic accuracy. Five patient plans with a total of eleven lesions were used to evaluate the dosimetric accuracy. Single-isocenter IMRT treatment plans using 10-15 coplanar beams were generated to treat the multiple metastases. RESULTS: The end-to-end localization accuracy of the system was 1.0 ± 0.1 mm. The conformity index, homogeneity index and gradient index of the plans were 1.26 ± 0.22, 1.22 ± 0.10, and 5.38 ± 1.44, respectively. The average absolute point dose difference between measured and calculated dose was 1.64 ± 1.90%, and the mean percentage of points passing the 3%/1 mm gamma criteria was 96.87%. CONCLUSIONS: Our experience demonstrates that excellent plan quality and delivery accuracy was achievable on the MR-Linac for treating multiple brain metastases with a single isocenter