Date Approved

8-9-2025

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

Allied Health Sciences

First Advisor

Kristen Vu

Second Advisor

Emilie Soisson

Academic Year

2024/2025

Abstract

Cervical spinal chordomas and chondrosarcomas present significant therapeutic challenges due to their proximity to the spinal cord and resistance to conventional radiotherapy. This retrospective study investigated the capability of proton arc therapy (PAT) to maintain or improve adequate target coverage while reducing dose to the spinal cord for cervical spinal chordomas and chondrosarcomas over standard fixed beam intensity modulated proton therapy (IMPT). While both IMPT and PAT use pencil beam scanning (PBS) to with energy modulation to deliver dose in discrete layers, IMPT relies on a limited number of fixed beam angles, whereas PAT delivers dose over an arc trajectory. Twelve adult cases of cervical spinal chordomas/chondrosarcomas that were previously treated with standard 3-4 field IMPT proton beams were selected and replanned with proton arc therapy. Proton arc therapy demonstrated superior spinal cord sparing and favorable linear energy transfer (LET) distributions compared to conventional IMPT. The McNamara model, based on the most comprehensive in vitro dataset to date, was used to estimate variable RBE values in this study, allowing for a more biologically accurate representation of spinal cord dose by incorporating LET and tissue-specific α/β ratios. Based on DVH analysis of the PAT replanned cases, the average spinal cord D0.1cc dose was reduced by 11.4% using a fixed RBE of 1.1 and by 11.6% using the McNamara variable RBE model. PAT also maintained roughly equivalent values for the prescription dose conformity indices. Robustness was evaluated with 3 mm isotropic setup uncertainty and 3.5% density uncertainty to ensure coverage and spinal cord constraints were met. These dosimetric advantages suggest that PAT could allow for dose escalation and reduced risk of late toxicities to these anatomically challenging cases.

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