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Patient Dose From Kilovoltage Radiographic Images During Synchrony Treatments On the Radixact System

W Ferris*, W Culberson, Univ of Wisc Madison, Madison, WI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The Synchrony™ motion management system on the Radixact™ uses real-time tracking through kilovoltage (kV) radiographs to compensate for the presence of intrafraction target motion during helical tomotherapy (HT) deliveries. The purpose of this work is to use measurements and simulations to estimate patient dose accumulated from kV images when treated using Synchrony.

Methods: A model of the kV imaging system on the Radixact was created using MCNP6. Doses in a water tank and half-value layers (HVLs) were measured using an Exradin A12 ion chamber following the AAPM TG-61 protocol. These measurements were acquired for several tube potentials and were used to benchmark the MCNP6 model. The benchmarked model was used to calculate volumetric dose to patient computed tomography (CT) datasets. The CT datasets were converted into MCNP geometries by discretizing HU values into 8 tissue types and 30 total densities. Dose accumulated throughout a patient’s treatment was calculated by scaling the dose per mAs per image angle by the number of images acquired per angle throughout all fractions.

Results: The point doses calculated using MCNP agreed with measured point doses within the uncertainty of the measurements. Volumetric kV dose was calculated for four patient CT datasets with various disease sites and imaging protocols. For 100 images per fraction and conventional fractionation, median dose to soft tissue from kV images ranged from 15 to 114 mGy, median dose to bone ranged from 44 to 252 mGy, and maximum body dose ranged from 468 mGy to 2068 mGy among the four patient datasets.

Conclusion: Patient dose from kV images during Synchrony treatments may reach more than a Gray when patients are imaged over the course of a treatment using conventional fractionation. Calculations of patient dose from this work can be used to aid in treatment planning for Synchrony deliveries.


Monte Carlo, Image-guided Therapy, Tomotherapy


TH- External beam- photons: Motion management (intrafraction)

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