Research on the advantages of heavy ions against X-rays in radiation therapy through radiobiological and dosimetric parameters

Interest in proton beams has increased due to superior normal-tissue sparing they afford. The goal of this study was to radiobiologically and dosimetrically compare the treatment of low-risk prostate cancer using intensity modulated proton therapy (IMPT) plans with X-ray volumetric modulated arc therapy (VMAT) plans with respect to target coverage and avoiding organs at risk. Treatment plans of 10 low-risk prostate cancer patients were compared using volumetric modulated arc therapy (VMAT) and two different intensity modulated proton therapy (IMPT) plans; one with standard 2 beams and the other experimental 9 beams. The target dose was delivered with two different schemes. The standard scheme was 2 Gy per fraction over 38 fractions for a total of 76 Gy and the hypofractionated scheme was 7.5 Gy per fraction over 5 fractions for a total of 37.5 Gy. Dosimetric indices for target coverage, dose received by organs at risk (OAR) and normal tissue complication probability (NTCP) of late rectal, urinary bladder and femoral head toxicities were analysed as well as the low dose region of peripheral normal tissue. Protons and X-rays both demonstrated very good target coverage (prescribed volume) and no significant difference for homogeneity and conformity. Protons showed superior femoral head sparing with respect to the VMAT technique (p < 0,05). There was no significant difference for sparing the rectum and the urinary bladder however the volume-sparing of peripheral normal tissue was superior with the proton plans. A dosimetric advantage but no radiobiological advantage revealed to choose between IMPT and VMAT techniques for the treatment of low-grade prostate cancer. Using the same safety margins and dose objectives, both IMPT and VMAT techniques demonstrated superior sparing of the rectum and bladder. However, the dose to the femoral heads and to peripheral normal tissue was significantly low with the proton plans. On the other hand, the radiobiological data showed no relevant difference in the form of NTCP values as well as the hypofractionated scheme.