Assessing dosimetric uncertainties in Papillon+ contact x-ray brachytherapy for rectal cancer: impact of beam quality and tumour geometry.

Objective.To investigate the evolution of beam quality in the Papillon+ contact x-ray therapy device and its dosimetric impact, a fast and straightforward beam quality index (QI) measurement method was proposed and validated. Additionally, the dosimetric consequences of tumour intrusion into the applicator tube were examined through Monte Carlo (MC) simulations and, for the first time, direct in-applicator dose measurements.Approach.QI measurements were performed every four weeks from mid-2022. Half-value layer (HVL) and depth-dose profile measurements in a plastic water low energy range phantom were conducted twice, with a two-year interval. Dose distributions for tumour intrusion scenarios (2.5-10 mm) were assessed using MC simulations and Gafchromic film measurements inside the applicator tube. Gross tumour volume (GTV) dose calculations (physical and EQD2) were performed for varying tumour thicknesses, intrusion depths, and diameters.Main results.All methods consistently demonstrated progressive beam hardening over time, with the HVL increasing from 0.77 mm Al to 0.93 mm Al. This spectral shift, attributed to x-ray tube ageing and radiation-induced material changes, was reversibly corrected after service replacement of the flattening filter, restoring the soft spectrum (HVL ≈ 0.69 mm Al). Tumour intrusion produced the strongest dosimetric effect, with surface dose increases up to 2.4-fold at 10 mm intrusion, exceeding MC predictions. GTV D90 decreased with tumour thickness but increased sharply with intrusion depth, whereas tumour diameter and surrounding medium had a negligible impact.Significance.This work highlights the importance of systematic beam quality follow-up in contact x-ray brachytherapy systems. Routine QI monitoring, together with well-defined beam-quality specifications and action limits, enables early detection of deviations, timely maintenance, and sustained compliance of the delivered beam quality in clinical practice. Tumour positioning relative to and protrusion into the applicator remains the dominant factor affecting dose distribution, underscoring the need for precise clinical setup and ongoing device characterisation to ensure clinical accuracy and patient safety.