Dose to the non-involved uterine corpus with MRI guided brachytherapy in locally advanced cervical cancer.

Background And Purpose: This study evaluates the impact of MRI guided adaptive brachytherapy (BT) on uterine corpus dose.

Material And Methods: 84 patients with median follow-up of 18 months were analysed. MRI based BT was done according to GEC-ESTRO guidelines. Non-involved uterine corpus at the time of BT was contoured and the uterine corpus dose (D90 and D98) was evaluated for (1) standard loading pattern with source loading to the tip of the tandem and (2) optimised dose plan. Tandem lengths and heights of the 85 Gy isodose were recorded.

Results: Dose optimisation resulted in a reduction of active tandem length of 0.4±0.4 cm leading to lowering the D90 to the non-involved uterine corpus from 63.8±9.5 Gy to 56.7±7.5 Gy EQD2 (p<0.0001). Mean active tandem length was 5.0±1.0 cm, and the height of the 85 Gy isodose was 5.7±1.0 cm in optimised plans.

Conclusions: MRI guided dose optimisation lowered the dose to the uterine corpus. However, a total EBRT+BT dose larger than 50 Gy was obtained in 99% of patients. Assuming that 45-50 Gy is sufficient to eradicate microscopic disease, the lowering of uterus corpus dose is not expected to induce additional uterine corpus recurrences in the setting of MRI guided adaptive BT. This hypothesis should be tested in a larger number of patients as e.g. the EMBRACE study.