ICBT combined with IMRT boost (IC/IMRT)
After whole pelvic EBRT, five fractions of brachytherapy with an IMRT boost were performed based on the MRI and pelvic examination findings. The prescription dose for HR-CTV was 6 Gy per fraction. CT and MRI scans were acquired while the Fletcher CT/MRI compatible applicator (Elekta AB, Stockholm, Sweden) was in the patient. The applicator was subsequently fixed to a board, which can slide between the mobile bed, treatment couch, and computed tomography (CT) couch. According to GEC-ESTRO recommendations, gross tumor volume (GTV), HR-CTV, and intermediate risk CTV (IR-CTV) were identified from the fusion of CT and MRI images. OARs include bladder, rectum, sigmoid colon and small intestine 26, 27. To achieve an ideal combination,the interval between brachytherapy and IMRT boost was to be completed within 6~10 minutes. The total cumulative dose of EBRT and brachytherapy boost were evaluated in terms of equivalent dose in 2 Gy per fraction (EQD2), using α/β = 3 Gy for OAR and α/β = 10 Gy for targets. The treatment planning aimed to achieve D90 > 86 Gy for HR-CTV and D90 > 75 Gy for IR- CTV from combined EBRT and IC/IMRT boost. Dose volume constraints for cumulative dose to the OAR were mean D2cc < 90 Gy for the bladder, and mean D2cc < 75 Gy EQD2 for rectum and sigmoid.
The IMRT plan was optimized using the ICBT plan base dose plan by an inverse dose optimization tool which allows the use of DVH constraints on the total dose of ICBT. A seven-field gantry angle IMRT plan was devised in order to avoid hotspots when optimizing the boost plan. The IC/IMRT plan was evaluated three dimensionally to determine the coverage of the target and the sparing of OARs. HDR brachytherapy treatment was delivered immediately after the approval of the plan. Its execution varied between a few minutes to ten minutes according to the source activity (Figure 1). When HDR brachytherapy treatment finished, the patient was transferred to the linac (Elekta Synergy) with the applicator still in patient. The IMRT boost was guided by the applicator position on Cone Beam CT (CBCT). Therefore, this procedure ensured that the IMRT plan dose gradients were aligned with the ICBT dose gradients. The applicator was not removed until the IMRT boost ended in order to minimize any tissue movement and deformation. This procedure was repeated two times a week.