Conformal irradiation of concave-shaped PTVs in the treatment of prostate cancer by simple 1D intensity-modulated beams

Background: In the case of concave-shaped PTVs including prostate (P) and s eminal vesicles (SV), intensity-modulated radiation therapy (IMRT) should i mprove the therapeutic ratio of the treatment of prostate cancer. Purpose: Comparing IMRT by simple 1D modulations with conventional 3D confo rmal therapy (i.e. non-IMRT) in the treatment of concave-shaped PTVs includ ing P+SV. Materials and methods: For five patients having a concave-shaped PTV (P+SV) previously treated at our Institute with conformal radiotherapy, conventio nal 3- and 4-fields conformal plans were compared with IMRT plans in terms of biological indices. IMRT plans were generated by using five equi-spaced beams with a partial shielding of the rectum obtainable with our single-abs orber modulation technique (Fiorino C. Lev A. Fusca M. Cattaneo GM, Rudello F. Calandrino R. Dynamic beam modulation by using a single dynamic absorbe r. Phys. Med. Biol. 1995;40:221-240). The modulation was one-dimensional an d the shape of the beams was at single minimum in correspondence with the ' core' of the rectum; the beam intensity in the minimum was set equal to 20 or 40% of the open beam intensity. All plans were simulated on the CADPLAN TPS using a pencil-beam based algorithm (with 18 MV X-rays). Tumour control probability (TCP) and normal tissue complication probabilities (NTCPs) (fo r rectum, bladder and femoral head) were calculated for all situations when varying the isocentre dose from 60 to 90 Gy. Dose distributions were corre cted taking dose fractionation into account through the linear-quadratic mo del; for the TCP/NTCP estimations the Webb-Nahum and the Lyman-Kutcher mode ls were respectively applied. Three different scores were considered: (a) i ncrease of TCP while keeping rectum NTCP equal to 5% (TCP(5%)); (b) increas e of the uncomplicated tumour control probability (P+); (c) increase of the biological-based scoring function (S+), developed by Mohan et al. (Mohan R , Mageras GS, Baldwin B, Clinically relevant optimization of 3D conformal t reatments. Med. Phys, 1992;19:933-944). The impact of the uncertainty in th e knowledge of the parameters of the biological models was investigated for TCP(5%). Results: (a) The average gain in TCP(5%) when considering IMRT against non- IMRT conformal plans was 7.3% (range 5.0-13.5%); (b) the average increase o f P+ was 3.4% (range: 1.0-8.5%); and (c) the average increase of S+ was 5.4 % (range 2.9-12.4%). The largest gain was found for one patient (patient 5) showing a significantly larger overlapping between PTV and rectum. Conclusions: Simple 1D-IMRT may clearly improve the therapeutic ratio in th e treatment of concave-shaped PTVs including P and SV. In the range of clin ically suitable values, the impact of the uncertainty of the parameters n a nd sigma(alpha) does not significantly alter the main results concerning th e gain in TCP(5%). The reported gain in terms of P+ and S+ should be consid ered with great caution because of the intrinsic uncertainties of the model 's parameters and, for bladder, because the 'true' DVH (considering variati ons of the shape and dimension due to variable filling) may be very differe nt from the DVH calculated on a single CT scan. Further investigations shou ld consider inversely-optimised 1D and 2D-IMRT plan in order to compare the m in terms of cost-benefit. (C) 2000 Elsevier Science Ireland Ltd. All righ ts reserved.