Adaptation of a Matrixx Evolution Two-Dimensional Array Detector for Dose Assessment In High-Dose-Rate Brachytherapy with a Cobalt-60 Source
Abstract
Purpose
High-dose-rate (HDR) brachytherapy (BT) is characterised by steep dose gradients and high doses delivered per treatment fraction in a short time. These features, together with the numerous steps involved in treatment delivery, make rigorous dose assessment and quality assurance (QA) indispensable rather than optional. Dose assessment in HDR BT relies heavily on point doses, though the field is transitioning towards 3D volume-based assessments. The current study aims to evaluate the feasibility of using the MatriXX Evolution 2D array detector system, typically designed for patient-specific QA and treatment machine QA in external beam radiotherapy (EBRT), for dose assessment in HDR BT using a cobalt-60 source.
Methods
To adapt the 2D array detector system for BT dose measurement, an in-house Perspex slab phantom with two holes, centred 4.1 cm apart to accommodate the Bebig universal applicators (LAA1400-RU), was constructed to sandwich the 2D array detector. BT treatment plans for the assessment were created with an HDRplus TPS that uses the AAPM-TG 43 dose calculation formalism. Since the formalism was based on water, depth-dependent conversion factors were experimentally obtained to scale the doses measured in the in-house phantom to those in water for comparison with the TPS-calculated doses. The measured dose distributions were compared to their TPS-calculated counterparts using a gamma passing criterion of 3%/3 mm (5% threshold).
Results
Treatment plans with distances of 1.31 cm and 3.31 cm from the source plane to the measurement plane recorded gamma passing rates of 93.7–96.9% (94.8±0.3142%, with standard error of ±0.1815%) and 95.0–98.3% (96.6±0.3411%, with standard error of ±0.1969%), respectively.
Conclusion
The MatriXX Evolution 2D array detector system is adaptable for HDR BT dose assessment using a cobalt-60 source, providing reliable and consistent measurements. Measurements at 3.31 cm from the source plane achieved the lowest dose discrepancies, which may be recommended for QA.