In vivo dosimetry in the treatment of localised pelvic malignancies with brachytherapy and stereotactic ablative radiotherapy

Abstract

Introduction
Movement of organs at risk (OARs) in brachytherapy and stereotactic ablative radiotherapy (SABR) can result in dosimetric uncertainties. This PhD investigated the need for in vivo dosimetry by defining these uncertainties and determining the minimum accuracy and optimal positions of dosimeters which monitor radiation dose in real-time.

Methods
Initially, brachytherapy practice at the Northern Ireland Cancer Centre was reviewed and compared with international practice. Treatment planning studies were then performed which quantified the dosimetric effect of OAR movement between planning and treatment. Patient subgroups within low dose rate (LDR) prostate brachytherapy likely to benefit most from in vivo dosimetry were investigated by analysing biochemical outcomes with data stratified by NCCN risk group, radiological T stage, Gleason score, initial PSA and prostate gland volume. Prostate sector analysis was performed to determine the optimal positions of dosimeters within the prostate. Clinical outcomes and the effect of inter-fraction OAR movement were analysed as part of the SPORT prostate SABR trial and an exploratory analysis of citrulline was performed.

Results
Outcomes following brachytherapy at the NICC were in keeping with international practice. The dosimetric effect of potential OAR movement between planning and treatment was greatest for the urethra in LDR prostate brachytherapy and for the rectum in HDR cervical brachytherapy. Patients with higher risk tumour characteristics treated with LDR prostate brachytherapy demonstrated statistically significantly greater biochemical recurrence rates. Rates of freedom from biochemical/clinical failure (BCF) were lowest when the dominant intraprostatic lesion (DIL) was located in the anterior base and highest when the DIL was located in the posterior midgland, with a statistically significant increase in freedom from BCF when the DIL sector D90 was > 110%. Analysis of the SPORT SABR trial demonstrated feasibility with acceptable toxicity. The positions of OARs at pre-fraction CBCT correlated better with gastrointestinal toxicities than at planning CT. Correlations of citrulline levels with late bowel toxicity suggested its potential as a predictive biomarker.

Conclusions
In vivo dosimetry has the potential to detect dosimetric discrepancies from the radiotherapy plan as a result of OAR movement during radiotherapy delivery. It would be particularly beneficial in the detection of deviations in rectal dose during HDR cervical brachytherapy and prostate SABR, and deviations in urethral dose during LDR prostate brachytherapy.

Thesis is embargoed until 31 December 2026.

Date of Award	Dec 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	ORIGIN project, Photonics Public Private Partnership
Supervisor	Suneil Jain (Supervisor), Kevin Prise (Supervisor) & Alan Hounsell (Supervisor)