Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Purpose: Intensity-modulated bolus electron conformal therapy (IM-BECT) shapes homogeneous dose distributions to superficial, irregularly-shaped target volumes, avoiding nearby critical structures. This dissertation provides research useful for translating IM-BECT to the radiotherapy clinic by: (1) developing and testing a factory quality assurance (QA) method for fabricated Passive Radiotherapy Intensity Modulator for Electrons (PRIME) devices and (2) verifying an IM-BECT planning and delivery process using end-to-end tests.

Methods: The IM-BECT planning and delivery process determines beam parameters and treatment devices (cutout, bolus, PRIME device) using .decimal’s (Sanford, FL) ElectronRT (eRT) planning system; calculates dose using a pencil-beam redefinition algorithm modified for PRIME devices; and performs QA of fabricated treatment devices. (1) A kilovoltage x-ray imaging procedure, developed for factory QA of PRIME devices, was tested by comparing well-fabricated ones with those having significant errors. (2) The IM-BECT planning process and delivery was verified using end-to-end tests for three anthropomorphic phantoms. Planar dose distributions, measured using TLD-100 or EBT3 radiochromic film, were compared with calculated dose distributions for 16 MeV IM-BECT treatment plans.

Results: (1) Kilovoltage x-ray-based QA revealed intensity differences under the modulated region (measured -calculated) were <3% for well fabricated PRIME devices, as compared to <5% and <7% for devices having large random or systematic fabrication errors, respectively. (2) TLD-100 measured dose distributions in the retromolar trigone phantom had dose difference (measured-calculated) distributions characterized by 0.36±1.64% (average ± standard deviation), similar to 0.36±1.90% without the PRIME device. EBT3 radiochromic film measured dose distributions in temple and postmastectomy phantoms fabricated from machinable blue wax. Density difference between phantom wax (r=0.92g·cm-3) and film polyester substrate (r=1.35g·cm-3) required a 10º gantry rotation and a depth-dependent correction. Dose difference distributions for the two phantoms were characterized by 0.45±1.60% and 0.30±1.36%, respectively, similar to 0.18±1.69% and 0.13±1.72% without PRIME devices.

Conclusions: (1) The kilovoltage x-ray imaging and analysis method is recommended for factory QA of PRIME devices, as it evaluates dose errors due to fabrication inaccuracies. (2) Excellent agreement of delivered (measured) and planned (calculated) dose distributions (end-to-end tests) for three phantom sites verifies this study’s planning and delivery process being suitable for IM-BECT patients.

Date

7-7-2023

Committee Chair

Pitcher, Garrett M.

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