Dose escalation in brachytherapy for cervical cancer: impact on (or increased need for) MRI-guided plan optimisation

Objective: The aim of this study was to assess the impact of dose escalation on the proportion of patients requiring MR image-guided optimisation rather than standard Manchester-based CT-guided planning, and the level of escalation achievable. Methods: 30 patients with cervical cancer treated with external beam radiotherapy and image-guided brachytherapy (IGBT) had MR images acquired at the first fraction of IGBT. Gross tumour volume and high-risk clinical target volume (HR CTV) were contoured and treatment plans retrospectively produced for a range of total 2-Gy equivalent (EQD2) prescription doses from 66 Gy(alpha/beta=10) to 90 Gy(alpha/beta=10) (HR CTV D90). Standard Manchester system-style plans were produced, prescribed to point A and then optimised where necessary with the aim of delivering at least the prescription dose to the HR CTV D90 while respecting organ-at-risk (OAR) tolerances. Results: Increasing the total EQD2 from 66 Gy(alpha/beta=10) to 90 Gy(alpha/beta=10) increased the number of plans requiring optimisation from 13.3% to 90%. After optimisation, the number of plans achieving the prescription dose ranged from 93.3% (66 Gy(alpha/beta=10)) to 63.3% (90 Gy(alpha/beta=10)) with the mean +/- standard deviation for HR CTV D90 EQD2 from 78.4 +/- 12.4 Gy(alpha/beta=10) (66 Gy(alpha/beta=10)) to 94.1 +/- 19.9 Gy(alpha/beta=10) (90 Gy(alpha/beta=10)). Conclusion: As doses are escalated, the need for non-standard optimised planning increases, while benefits in terms of increased target doses actually achieved diminish. The maximum achievable target dose is ultimately limited by proximity of OARs. Advances in knowledge: This work represents a guide for other centres in determining the highest practicable prescription doses while considering patient throughput and maintaining acceptable OAR doses.