Nodal dose estimation for individualised brachytherapy planning for patients with cervical cancer

Introduction Radiotherapy for locally advanced cervical cancers involves External Beam Radiotherapy (EBRT) and a High Dose Rate Brachytherapy (HDR-BT) boost. 50% of these patients present with Pelvic Lymph Nodes (PLN) metastases therefore total dose delivered to PLN is an important consideration. This study estimates differences between dose at point B (Manchester system): for plans prescribed to point A and plans optimised to cover a High Risk Clinical Tumour Volume (HR-CTV). Methods Retrospective analysis was completed for 14 consecutive patients. EBRT standard 4-field box technique 50.4 Gy in 28 fractions was delivered followed by HDR-BT 6–7 Gy in 3 fractions using Varian ring and tandem applicators. Standard plans were normalised to point A and, if necessary, dwell times manually adjusted to optimise HR-CTV coverage D90 ⩾ 100%. Doses at points B were calculated in 2 Gy Equivalent Dose (EQD2) using radiobiology Linear Quadratic model (LQm) using α / β = 10 for all HDR-BT fractions. Results and conclusions The average EQD2 for EBRT and HDR-BT combined treatment was 55.1 Gy for standard plans and ranged between 52.8 Gy and 56.5 Gy for optimised plans. Dose at points B for HDR cervix treatment depends on the intended dose to the HR-CTV, HR-CTV volume and loading pattern. For standard plans the dose is predictable, approximately 1.8 Gy per fraction, however for individualised plans there is much greater variation. As we move towards more individualised planning it is important to be aware of these differences in dose delivered to PLN and the relationship between dose to PLN and risk of metastases.