A Large Area Pixelated Silicon Array Detector for Independent Transit In Vivo Dosimetry

Abstract

A large area pixelated silicon array detector named “MP987” has been developed for in vivo dosimetry. The detector was developed to overcome the non-water equivalent response of EPID (Electronic Portal Imaging Device) dosimetry systems, due to the shortfalls of the extensive corrections required. The detector, readout system and software have all been custom designed to be operated independently from the linac with the array secured directly above the EPID, to be used in combination with the 6 MV imaging system. Dosimetry characterisation measurements of percentage depth dose (PDD), dose rate dependence, radiation damage, output factors (OF), profile measurements, linearity and uniformity were performed. Additionally, the first pre-clinical tests with this novel detector of a transit dosimetry characterization and a collapsed IMRT (intensity-modulated radiation therapy) study are presented. Both PDD and OF measurements had a percentage difference of less than 2.5% to the reference detector. A maximum change in sensitivity of 4.3 ± 0.3% was observed after 30 kGy of gamma accumulated dose. Transit dosimetry measurements through a homogeneous Solid Water phantom had a measured dose within error of the TPS calculations, for field sizes between 3 × 3 cm2 and 10 × 10 cm2. A four-fraction collapsed IMRT plan on a lung phantom had absolute dose pass fractions between the MP987 and TPS (treatment planning system) from 94.2% to 97.4%, with a 5%/5 mm criteria. The ability to accurately measure dose at a transit level, without the need for correction factors derived from extensive commissioning data collection procedures, makes the MP987 a viable alternative to the EPID for in vivo dosimetry. This MP987 is this first of its kind to be successfully developed specifically for a dual detector application.

Characterization of a flexible a‐Si:H detector for in vivo dosimetry in therapeutic x‐ray beams

“…5 However, film-based dosimetry is a passive technology that cannot provide the desired real-time feedback. For real-time large-area dosimetry, several solutions are available based solely on EPID or a combination of EPID and dedicated detectors, such as the solution proposed by Brace et al 6 These large-area pixelated detectors typically employ a 2D array of silicon detectors placed on the flat panel imager available on most clinical LINACs to monitor exit dose during patient treatments. Olaciregui-Ruiz et al 7 provide an extensive review of the requirements of EPID dosimetry.…”

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy

“…EPIDs are not specifically designed for patient dosimetry, and with better methods for managing patient position, they may be at risk of becoming obsolete. Several authors have demonstrated redesigned downstream dosimetry systems based on silicon with attempts to address such issues [ 10 , 11 ]. A multicentre study of commercial EPID dosimetry systems [ 12 ] found out-of-tolerance errors of up to 25%, which was due predominantly to patient anatomical changes and positioning errors; however, it concluded that EPID dosimetry was effective in intercepting important errors.…”