ESTRO 2024 - Abstract Book

S3496

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

677

Digital Poster

Replacing Electron therapy with VMAT

David Carnegie, Julie Fox, Andrew Bromiley, Polly Darby, Adam El-Qmache, John McLellan

NHS Grampian, Radiotherapy, Aberdeen, United Kingdom

Purpose/Objective:

Electron beam therapy has been a staple in radiotherapy departments since the advent of the modern LINAC and has found niche treatment applications that photons have traditionally struggled with. In this work we present a series of patient case studies based on real treatments quantifying the differences in coverage and OAR dose between electron beam and VMAT treatment plans using the Eclipse planning system. In every case, VMAT can provide an overall superior plan to electrons and requires less QA and staffing resources.

Material/Methods:

Electron treatments have historically been calculated by hand and can only give assurance that a point dose at a certain depth in water is within a certain percentage of the prescription. This methodology requires lots of assumptions (e.g. homogenous media) and dose distributions are not readily available to view in the same that we are used to with modern photon plans.

An electron dose calculation algorithm (Electron Monte Carlo 16.1.0, Varian) allows direct comparison between electron and photon treatments in a common TPS workspace.

Using the Eclipse planning system a series of photon treatment plans were created for patients who would have traditionally been automatically sent for electron beam therapy (e.g. superficial SCC) and then equivalent electron plans (using custom cut-outs) were created and the dose distributions compared. Five real patients were selected and two phantom simulated cases were created for this work. Cases were selected based on a subjective assessment of how difficult it would be to generate an acceptable VMAT plan, the idea being to challenge VMAT as an alternative for rare and difficult cases. Since electron treatments are traditionally not planned on a CT data set, clinicians were asked to contour a CTV to allow physicists to create optimised VMAT plans. Where this was not possible, physicists created CTV’s based on CT markers. Bolus has been used where appropriate and all plans have been normalised to the median dose in the target structure.

Common DVH parameters such as D95%, D105% coverage and Dmax to OAR’s were used to quantify plan quality and compare results.

Results:

For all seven test cases examined here the VMAT plan consistently meets the D95%>95% and the D1%<105% constraints and most OAR constraints as well. The VMAT plans out-perform electron treatments, in all the cases investigated, in terms of the specified dose constraints. The electron beam treatments however consistently