Duplicating a tandem and ovoid distribution with IMRT: A feasibility study

Abstract

Brachytherapy plays an important role in the definitive treatment of cervical cancers but is not available at all centers. We have investigated whether we can achieve identical isodose distribution with similar sharp dose falloff using IMRT. CT scans of a tandem and ovoid patient were electronically transferred to HDR and IMRT treatment planning systems after contouring the target, rectum and bladder, thereby, ensuring identical structures in both planning systems. A conventional plan (7Gy in 5 fractions) was generated for delivery with an HDR after loader. The 150, 125, 100, 75, 50 and 25% isodose curves were drawn on each slice and then transferred to IMRT TPS. A 7-field IMRT plan using 6 MV x-ray beams was generated and compared with the HDR plan using the following indices: isodose conformity to the target and 125% volume, DVHs and integral dose. The resultant isodose distribution demonstrated good agreement between the HDR and IMRT plans in the 100 & 125% isodose range. Though the dose fall off in the HDR plan was much steeper, the maximum dose was substantially higher than the IMRT plan. Integral dose for the HDR and IMRT plan for the target, rectum and bladder were found to be 6.69, 1.07 and 1.02 J for HDR while the respective values for IMRT were 3.47, 1.79 and 1.34. Our preliminary results indicate that it is possible to replicate the HDR distribution using a standard IMRT. Radiobiological and patient positioning differences between both the techniques merit further consideration.