Dosimetric and biological comparison of conventional and modern image guided high-dose rate brachytherapy for gynaecological cancers : impact on intra- and inter-fractional applicator displacement

Abstract

Background and purpose: Brachytherapy is one of the essential treatments in gynaecological cancers, including intra-cavitary irradiation (ICIr) for cervical cancers and vaginal cylinder brachytherapy (VBT)for corpus uteri cancer, and the techniques are evolving. The advancement draws concerns at the impact on intra-and inter-fractional applicator displacement as the immobilization adopted was unchanged. Therefore, this planning study aims at comparing conventional and modern brachytherapy and investigating on the dosimeric and biological impact on applicator shift to define an acceptable displacement tolerance through virtual simulation. Materials and methods: Fifteenpatients for ICIr and seven for VBT were recruited. Modern plans, 3D for ICIr and multichannel (MC) for VBT, were computed as delivered to patients, while conventional plans, 2D for ICIr and single channel (SC) for VBT, were generated retrospectively. The applicators were then virtually shifted in all plans from original to inferiorly with addition of 2 mm up to 10mm and to clockwise and anti-clockwise roll direction with increment of 3° till15°.The normal conventional plans were evaluated against modern plans and all mimicked shifted plans were compared against originals. Results: For ICIr, 3D achieved statistically significantly lower bladder and sigmoid dose (p < 0.001) than 2D plans while maintaining adequate target dose. Impact of applicator displacement was found exist with 2 mm inferior and 9°roll shift in 2D ICIr; and with 2 mm inferior and 3°rotation displacement in 3D ICI. 3D ICIr was found to require a more rigid rotational tolerance than for 2D. For VBT, SC and MC VBT was statistically comparable in dosimetry (p > 0.05) for all structures but higher target dose in terms of absolute dose differences. Tolerance of applicator displacement was suggested to set at less than 6 mm inferior movement for SC; and less than 6mm inferiorly and 15°rotationallyfor MC VBT. Conclusions: Modern techniques in both ICIr and VBT brachytherapy were found better in dosimetry than conventional techniques. Conversely, the improvements have shown increase in the impact on applicator shifts and thus tightening of the displacement tolerance especially on rotational aspects was suggested.