Assessment of dose to functional sub-structures in the lower urinary tract in locally advanced cervical cancer radiotherapy.

PURPOSE: To provide an analysis of dose distribution in sub-structures that could be responsible for urinary toxicity after Image-Guided Adaptive BrachyTherapy (IGABT) in Locally Advanced Cervical Cancer (LACC).

METHODS: 105 LACC patients treated with radiochemotherapy and IGABT were selected. Sub-structures (bladder wall, trigone, bladder neck, urethra) were contoured on IGABT-planning MRIs. D2cm3 and D0.1cm3 , ICRU Bladder-Point (ICRU BP) and Posterior-Inferior Border of Symphysis points (PIBS, PIBS + 2 cm, PIBS - 2 cm) doses were extracted. Internal-Urethral-Ostium (IUO) and PIBS-Urethra (PIBS-U) points were defined as urethral dose surrogates. Finally, the Vaginal Reference Length (VRL) was extracted. Values were converted into total EBRT + BT equivalent dose in 2 Gy fractions using α/β = 3 and T1/2  = 1.5 h.

RESULTS: Median D2cm3 for bladder and trigone were 71.7[interquartile-range:66.5;74.1]Gy and 57.8[53.3;63.6]Gy, respectively, while median D0.1cm3 were 82.2[77.6;89.1]Gy and 70.7[62.0;76.7]Gy, respectively. Median ICRU BP dose was 63.7[56.5;70.5]Gy and correlated with trigone D2cm3 and D0.1cm3 , while bladder and trigone D2cm3 had poor correlation (R2  = 0.492), as well as D0.1cm3 (R2  = 0.356). Bladder neck D0.1cm3 was always lower than trigone D0.1cm3 and higher than IUO. Correlation between PIBS + 2 cm and IUO was poor (R2  = 0.273), while PIBS and PIBS-U were almost equal (R2  = 0.990). VRL correlated with dose to bladder base.

CONCLUSIONS: The study confirmed that ICRU BP and trigone doses correlate. Bladder D2cm3 is not representative of trigone dose because hotspots are often placed in the bladder dome. VRL is a good indicator for bladder base sparing. In addition to D2cm3 and D0.1cm3 for whole bladder, ICRU BP, trigone D2cm3 and D0.1cm3 , IUO and PIBS are useful for lower urinary tract reporting.