Abstract
This work is concerned with physics-aspects of safety, quality control (QC) and dosimetry audit in high dose rate (HDR) gynaecological brachytherapy. A survey of brachytherapy QC practice across the UK was conducted. Areas of least consistency were addressed, including test method development and establishment of clinical performance requirements. ‘End to end’ dosimetry auditing was not being utilised and its implementation was the main focus of this work. Three candidate dosimeters were evaluated for use in audit: Fibre optic thermoluminescence detector, Gafchromic EBT3® radiochromic film, and Presage® radiochromic plastic. Film dosimetry was selected, fully characterised, triple-channel dosimetry evaluated, and uncertainty reduction methods implemented. A novel ‘end to end’ audit methodology was developed, the BRachytherapy Applicator Dosimetry (BRAD) system, to measure dose distributions around clinical brachytherapy applicators and compare to treatment planning system calculations. MCNP5 Monte Carlo code was used to support the design of the BRAD system and validate the use of film dosimetry. 46 radiotherapy centres in the UK were audited. Delivery of the intended prescription dose was confirmed to be within clinically acceptable levels at all centres, mean difference 0.6% for plastic and 3.0% for metal applicators (±3.0% k=1). The intended dose distribution was faithfully delivered to the film-measured dose planes with a mean gamma passing rate of 97.8% at 3% (local) 2 mm criteria. Two audits had results that required follow-up and both were resolved. Each audit included a review of local brachytherapy physics practice and opportunities for improvement were reported, including imaging, applicator reconstruction, planning procedures, QC tests, and staff training. The brachytherapy audit provided the first comprehensive validation of ‘end to end’ clinical brachytherapy dosimetry, from applicator imaging to treatment delivery, combined with a review of clinical physics practice. The BRAD system is retained in the Institute of Physics and Engineering in Medicine (IPEM) phantom library.