MRI-based contouring of functional sub-structures of the lower urinary tract in gynaecological radiotherapy

Research in radiation-induced urinary morbidity is limited by lack of guidelines for contouring and dose assessment of the lower urinary tract. Based on literature regarding anatomy, physiology and imaging of the lower urinary tract, this study aimed to provide advice on contouring of relevant sub-structures, reference points and reference dimensions for gynaecological radiotherapy.

210 MRIs for Image-Guided Adaptive Brachytherapy (IGABT) were analysed in 105 locally advanced cervical cancer patients treated with radio(chemo)therapy. Sub-structures (trigone, bladder neck and urethra) were contoured and trigone height (TH) and width (TW) were measured. Internal urethral ostium (IUO) and Posterior inferior border of pubic symphysis-urethra (PIBS-U) points were used to identify proximal and middle/low urethra, respectively. Urethra reference length (URL) was defined as IUO and PIBS-U distance. TH, TW and URL were also quantified on 54 MRIs acquired for External Beam Radiotherapy (EBRT).

Median absolute differences in volumes and dimensions between first and second IGABT fraction were 0.7 cm³, 4.3 cm³, 0.2 cm, 0.3 cm and 0.2 cm for trigone, bladder neck, TH, TW and URL, respectively. Mean(±SD) TH and TW were 2.7(±0.4)cm and 4.4(±0.4)cm, respectively, with no significant difference (p = 0.15 and p = 0.06, respectively) between IGABT and EBRT. URL was significantly shorter in EBRT than in IGABT MRIs (p < 0.001).

This study proposed relevant urinary sub-structures and dose points and showed that standardized contouring is reproducible. Trigone reference dimensions are robust despite different bladder filling and treatment conditions. Standardized contouring and reference points may improve understanding of urinary morbidity.