Calculation of Equivalent Dose to Target Volume and Critical Organs for Combination of IMRT and MRI Guided High-dose-rate Brachytherapy Using Deformable Image Registration in Cervical Cancer

Abstract

To quantify the dose on MRI guided HDR brachytherapy from intensity modulated radiation therapy (IMRT) using deformable image registration method. Six cervical cancer patients who were treated with IMRT and 3D Intracavitary MRI guided HDR brachytherapy were retrospectively analyzed to evaluate the dose from IMRT on the MRI based brachytherapy plan. All patients had external beam radiation therapy (EBRT) of 45 Gy with 1.8 Gy per fraction with IMRT followed by 3D MRI guided HDR brachytherapy with Ring and Tandem applicator. Three of them had pelvic and paraortic nodes treated with 55 Gy in 25 fractions as SIB (simultaneous integrated boost). Prescription dose for brachytherapy was 5.5-6.0 Gy per fraction to HRCTV (high risk clinical target volume) with total 5 fractions. Total doses for HR CTV D90 and dose to 2cc of bladder, rectum and sigmoid (organs at risk-OARs) from EBRT and brachytherapy were summated and normalized to a biologically equivalent 2 Gy fraction size (EQD2). As per GEC ESTRO guidelines it is assumed that target volume (HRCTV) and critical organs get 45 Gy in 25 fractions with EBRT and thus, EQD2 contribution from EBRT to HRCTV and OARs are 44.3 Gy and 43.2 Gy, respectively for all 6 patients. Each brachytherapy fraction dose is then converted to EQD2 and added to that EBRT EQD2 for total dose. For this study we fused the MRI for each HDR fraction with external beam planning CT using deformable image registration method (Velocity v2.7). MRI image for each fraction from brachytherapy was used as a reference image and planning CT image was deformed to match to each MRI fraction based on intact uterus as well as bony anatomy. The dose contribution was calculated for HRCTV and critical organs including rectum, bladder and sigmoid. The mean EQD2 ±SD contribution from EBRT for HR CTV D90 was 46.2±0.7 Gy. Mean EQD2 to 2cc of bladder, rectum and sigmoid was 46.2±1.3 Gy, 46.3 ±1.0 Gy, 48.4±2.6 Gy, respectively. The difference in EQD2 based on uniform dose to target volume and critical organs and from deformable registration was statistically significant (p < 0.0001). The true dose to HRCTV and OARs were higher in comparison to calculation based on assumption of uniform dose distribution from EBRT. For composite dose calculation of EBRT and brachytherapy, EBRT contribution has been assumed to be uniform for all patients. However, true dose to HRCTV and OARs appears to be different and higher than calculated dose based on uniform dose distribution. Deformable registration method helps to quantify and individualize dose contribution from EBRT and future outcome analysis of image based brachytherapy should take this factor into account.