A comparison of coplanar four-field techniques for conformal radiotherapy of the prostate

Background and purpose: Conformal radiotherapy of the prostate is an increasingly common technique, but the optimal choice of beam configuration remains unclear. This study systematically compares a number of coplanar treatment plans for four-field irradiation of two different clinical treatment volumes: prostate only (PO) and the prostate plus seminal vesicles (PSV). Materials and methods: A variety of four-field coplanar treatment plans were created for PO and PSV volumes in each of ten patients. Plans included a four-field ‘box’ plan, a symmetric plan having bilateral anterior and posterior oblique fields, a plan with anterior oblique and lateral fields, a series of asymmetric plans, and a three-field plan having anterior and bilateral fields for comparison. Doses of 64 and 74 Gy were prescribed to the isocentre. Plans were compared using the volume of rectum irradiated to greater than 50% ( V 50), 80% ( V 80) and 90% ( V 90) of the prescribed dose. Tumour control probabilities (TCP) and normal tissue complication probabilities (NTCP) for the rectum, bladder and femoral heads were also evaluated. Femoral head dose was limited such that less than 10% of each femoral head received 70% of the prescribed dose. Results: For the PO group, the optimal plan consisted of anterior oblique and lateral fields (Rectal V 80=23.8±5.0% (1 SD)), while the box technique ( V 80=26.0±5.8%) was less advantageous in terms of rectal sparing ( P=0.001). Similar results were obtained for the PSV group (Rectal V 80=43.9±5.0% and 47.3±5.5% for the two plan types, respectively, P=0.001). The three-field plan was comparable to the optimal four-field plan but gave higher superficial body dose. With dose escalation from 64 to 74 Gy, the mean TCP for the optimal plan rose from 52.0±2.8% to 74.1±2.0%. Meanwhile, rectal NTCP for the optimal plan rose by 3.5% (PO) or 8.4% (PSV), compared to 4.7% (PO) or 10.1% (PSV) for the box plan. Conclusions: For PO volumes, a plan with gantry angles of 35°, 90°, 270° and 325° offers a high level of rectal sparing and acceptable dose to the femoral heads for all patients, while for PSV volumes, the corresponding plan has gantry angles of 20°, 90°, 270° and 340°. Using these plans, the gain in TCP resulting from dose escalation can be achieved with a smaller increase in anticipated rectal NTCP.