Uncertainties when using only one MRI-based treatment plan for subsequent high-dose-rate tandem and ring applications in brachytherapy of cervix cancer

Background and purposeThis retrospective study compares individual MRI based 3D treatment planning for each intracavitary applicator insertion and the use of only one MRI treatment plan for cervical cancer brachytherapy.Materials and methodsGTV, high risk (HR) CTV and OAR were delineated and analysed for 14 patients. Data using the individual approach were taken from the actual irradiated plans. The “single plan procedure” was simulated by matching the dose distribution of the first plan to the MRI datasets of each subsequent implantation. Total doses from brachytherapy were added up and normalized to 2 Gy fractionation (EQD2).ResultsThe mean D90 for HR CTV was 6 Gy higher when using one plan than when using individual treatment plans. The D2cc increased 3.5 Gy for the bladder, 4.2 Gy for the rectum and 5.8 Gy for the sigmoid. The use of only one treatment plan would have resulted in 2, 1 and 5 extra cases exceeding total D2cc constraints for bladder (90 Gy), rectum (75 Gy) and sigmoid (75 Gy), respectively.ConclusionThe use of only one treatment plan for several applications results in higher dose to target and OAR structures. CT, clinical examination and X-ray findings can help to reduce certain situations of overdosage, when individual MRI based treatment planning is not available for each fraction.