ESTRO 2024 - Abstract Book

S3576

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

ESTRO 2024

1450

Digital Poster

Hemi-Skin Electron Therapy: The relationship between shield height and treatment margin

William P Donahue 1 , Jason Ocana 1 , Gregory Niyazov 1 , Christopher Barker 2 , Samuel Hellman 1 , Seng Boh Lim 1

1 Memorial Sloan Kettering Cancer Center, Department of Medical Physics, New York, NY, USA. 2 Memorial Sloan Kettering Cancer Center, Department of Radiation Oncology, New York, NY, USA

Purpose/Objective:

Our institution recently constructed an adjustable shield for performing hemi-skin electron therapy. This has been used at our institution to treat the lower extremities of a patient while sparing the torso and head. Previous cases required custom shielding, stand design and measurement. The goal of this work was to commission a nomogram to determine the height of the shield required to ensure the needed margin for the target to receive the prescription isodose level.

Material/Methods:

We use 6 dual-fields to treat TSET patients at our institution. Each dual-field is created using two beams with gantry angles of 74.4° and 105.6° and a nominal SSD of 430 cm. The adjustable shield is designed to be placed between our scatter plate and the patient (Figure 1) and employs a counterbalanced lead acrylic plate that can be easily moved up and down. Using a large solid water phantom placed on our TSET stand, EBT-3 Gafchromic film (Ashland Inc.) was irradiated to measure profiles at 5mm depth (Figure 1). Irradiations were performed for 6 different shield heights spanning a height of 77 to 118cm above our treatment stand. Two pieces of film were placed in tandem to measure the profiles. We marked the beam centerline and the shield position using the in-room vertical laser and the light field projection of the shield edge, respectively. We only measured the profiles for the en face dual field configuration and delivered 430cGy to ensure sufficient signal in the film. We scanned the films using an Epson 11000XL (Epson Inc., Los Alamitos, CA) flatbed scanner. The films were calibrated with triple channel technique and the dose profiles were using the FilmQA Pro software (v 2016, Ashland Inc., Wilmington, DE). Each pair of films were manually registered and stitched together using the laser marks to generate a profile. A smoothing spline function was applied to the film measurements to reduce the impact of film noise on the analysis. Each profile was normalized to the average of the open field dose, defined as 20 cm from the shield edge. The distance from the shield edge was extracted from the normalized profiles for the 90%, 80%, 70%, 60%, and 50% isodose levels. Linear regression was performed for each isodose level to determine the required shield height to ensure a specific isodose level covers the treatment position, defined as the height above the TSET stand. Finally, to determine the sensitivity of the setup to positioning errors, we looked at the change in relative dose 1cm away from the prescription isodose levels.