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.