P&A Colloquium – Dr. Patricia Oliver, Dalhousie University
Sep 17, 2025
10:30AM to 11:30AM
Date/Time
Date(s) - 17/09/2025
10:30 am - 11:30 am
BSB 108
Dr. Patricia Oliver
Assistant Professor
Dalhousie University
TITLE: Using Monte Carlo-based transit dosimetry to verify radiotherapy treatment delivery accuracy
ABSTRACT:
Transit dosimetry involves measuring the 2D radiation intensity pattern exiting through the patient during radiotherapy treatment delivery. This technique can provide a convenient means of verifying that the treatment plan was delivered as intended. Differences between predicted and measured transit dosimetry images can be indicative of treatment delivery errors.
Online adaptive radiotherapy (OART) would especially benefit from transit dosimetry. In OART, a new treatment plan is created every day, while the patient is on the table, based on the cone beam computed tomography (CBCT) dataset acquired that day. As such, there is no convenient option for pre-treatment, measurement-based quality assurance (QA) testing. However, transit dosimetry would provide a measurement during treatment delivery.
This presentation will cover the development and validation of a Monte Carlo (MC) model of the Ethos radiotherapy system (Varian Medical Systems, Palo Alto, USA). MC simulations are used to generate transit dosimetry prediction images based on CBCT datasets and treatment plan parameters. The MC model is validated by comparing against measurements for a series of scenarios with increasing complexity. First, tests with nothing in the beam are carried out including (1) square, open fields, (2) more complex, static MLC patterns, and (3) sliding window IMRT fields. Next, an IMRT plan is delivered to an anthropomorphic phantom to test the accuracy of the CBCT-based computational phantom model. Furthermore, CBCT acquisition modes with and without advanced reconstruction features (using the HyperSight imaging system) are compared. A prostate case is studied to investigate the impact of rectal gas adjacent to the prostate. Correlations between 2D transit dosimetry images and 3D patient dose distributions are explored, with the goal of better quantifying action levels for transit dosimetry.