Session Item

Evaluate intra-applicator target volume and organ at risk (OAR) dose variation in MRI guided adaptive brachytherapy (MRIGABT) for cervix cancer. Calculate cost implications of replan at each treatment fraction.

Radiotherapy treatment planning MRI (RTPMRI) were acquired before each brachytherapy fraction (1-4), in 14 patients treated with intrauterine brachytherapy (IUB) using the VeneziaTM applicator. Each patient had 2 IUB applicator insertions and received 28 Gy/4# following EBRT 25 Gy/25# (EMBRACE II protocol)1 A replan based on anatomy on MRI acquired on day 2 (D2) of each IUB applicator insertion was planned and delivered (D2-replan). Dose to the high risk clinical target volume (HRCTV)  and OARs were compared to conventional treatment where the treatment plan on day 1 of IUB applicator insertion is also delivered on day 2 (no replan). A paired students t-test was used to assess for statistical differences. Total biologically equivalent doses for EBRT and brachytherapy were calculated in 2 Gy equivalents using the EQD2 equation and reported as total dose (Gy). Cost of a replan at each treatment fraction in terms of additional radiology and physics resources required was calculated.

1)    EMBRACE - https://www.embracestudy.dk/UserUpload/PublicDocuments/EMBRACE%20II%20Protocol.pdf

Forty-six RTPMRI and IUB HDR treatment plans were analysed in 14 patients.  The mean (±SD) total dose (EBRT + HRCTV 90) received for D2-replan and D2-no replan was 90.5 (±4.3) and 88.7 (±4.8). There was a statistically significant difference in target dose between D2-replan and no replan P<0.05). OAR dose constraints exceeded EMBRACE dose constraints in 50% of patients if no replan was performed (rectum 18%, sigmoid 11%, bowel 17% and bladder 7%). No OAR total dose constraints were exceeded with D2-replan.

Daily MRIGABT replan at each treatment fraction results in improved HRCTV dosimetry and reduced risk of exceeding OAR dose constraints.  A replan ensures HRCTV D90 of >90Gy and better OAR sparing. In our institution, the cost of a replan is €603 (additional physics and radiology resources).  Based on current activity, this amounts to €18,090 per year (n=30 patients treated/year). It could be argued that 50% of patients did not need a replan because OARs were within tolerance and HRCTV dose achieved. Additional resources required for a replan mean that selection criteria for patients who will benefit most needs to be defined.