ESTRO 2024 - Abstract Book

S3270

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

1 National University of Singapore, Department of Physics, Singapore, Singapore. 2 Nanyang Technological University, School of Physical and Mathematical Sciences, Singapore, Singapore. 3 National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore

Purpose/Objective:

It is commonly understood that metal content in post-surgical implants can render a non-negligible impact during conventional X-ray radiotherapy treatment. However, little research has been done for its effects in proton therapy. This study investigates the potential dose perturbations due to titanium mesh/screws and aneurysm clips following brain surgery such as craniotomy or aneurysm repair, that may be present during proton therapy treatment.

Material/Methods:

Three spread out Bragg peak (SOBP) plans were generated with RayStation 10A (RaySearch Laboratories, Stockholm, Sweden), each utilizing a field size of 10x10 cm. The National Cancer Centre Singapore Hitachi ProBeat proton therapy machine was used for the experimental measurements. Gafchromic EBT3 films were placed between plastic water phantoms, with the top-most film placed between a 1.5cm thick Superflab bolus and a plastic water phantom ( Figure 1 ). Three experiments were conducted which involved placing the mesh and clips in different parts of a SOBP based on clinical relevance. In experiment 1, the mesh and screws were placed at a water equivalent depth of 1.5cm in a SOBP of 6cm range, 5cm modulation width and 10cm field size. The films were placed at depths: 0mm, 1mm, 3mm, 5mm, 10mm, 20mm, 30mm from the mesh and screws. In experiment 2, the mesh and screws were placed at depth 1.5cm at the plateau region of the SOBP of 8cm range, 5cm modulation width and 10cm field size. The films were placed at depths: 0mm, 5mm, 10mm, 20mm, 22mm, 25mm, 30mm, 40mm, 50mm from the components. In the last experiment, the surgical clips were placed at depth 3.0 cm using the SOBP as the second experiment. The films were placed at depths: 0mm, 1mm, 3mm, 5mm, 10mm, 20mm, 30 mm. The schematics of the experiments are shown in Figure 1 . The exposed films were scanned using Epson Expression 12000XL (Epson America Inc., CA, USA) one day after exposure to ensure optical density growth had stabilized, as per AAPM TG-235. Film calibration was performed with doses of 0Gy, 0.5Gy, 1Gy, 1.5Gy, 2Gy, 2.5Gy, 3Gy, 4Gy, 6Gy and 8Gy. Owing to a higher sensitivity in the lower dose region (0-3Gy), the red channel was selected to convert scanned films into dose maps using MATLAB R2023b. The mean dose of the vertical screw was determined via a 2D area profile within, along with four mean dose area profiles in its vicinity ( Figure 2c ). Similarly, the mean dose of the horizontal screw and four clips were determined via line profiles within, along with two line profiles parallel and at its vicinity ( Figure 2d ). The metal mesh could not be observed in all scanned films. The mean dose in the vicinity of each item served as reference where no perturbation was expected. The dose perturbation ratio is obtained by taking the ratio of the mean dose within to the mean dose in its vicinity. Figure 1. (a) Experimental setup, and (b-d) schematic setups for all three experiments performed. Yellow and grey rectangles represent the Superflab bolus and plastic water phantoms respectively. The screws/clips are represented as a red line.