Optimization of coplanar six-field techniques for conformal radiotherapy of the prostate

Abstract

Purpose: To determine the optimal coplanar treatment technique for six-field conformal radiotherapy of prostate only (PO) or prostate plus seminal vesicles (PSV). Methods and Materials: A series of 6-MV six-field coplanar treatment plans were created for PO and PSV volumes in 10 patients prescribed to both 64 and 74 Gy. All plans consisted of laterally-symmetric anterior oblique, lateral, and posterior oblique fields. The posterior oblique fields were varied through 20–45° relative to the lateral fields, and for each of these angles, the anterior oblique fields were varied through 25–65° relative to lateral. The plans were compared by means of rectal volumes irradiated to 80% or more of the prescribed dose (V 80); normal tissue complication probability (NTCP) for rectum, bladder, and femoral heads; and tumor control probability (TCP). Femoral head tolerance was designated as 52 Gy to no more than 10% volume. Results: For the PO group, anterior oblique fields at 50° from lateral and posterior oblique fields at 25° from lateral produced the lowest V 80,together with femoral head doses which were appropriate for most patients (V 80 = 24.4 ± 5.3% [1 SD]). Compared to a commonly-used six-field (reference) plan with both anterior and posterior oblique fields at 35° from lateral (V 80 = 26.3 ± 5.9%), this represented an improvement ( p = 0.001). For the PSV group, the optimal anterior and posterior oblique fields were at 65° and 30° from lateral, respectively (V 80 = 47.5 ± 6.3%). Relative to the reference plan (V 80= 49.4 ± 5.6%), this was a marginal improvement ( p = 0.07). Conclusion: The optimized six-field plans provide increased rectal sparing at both standard and escalated doses. Moreover, the gain in TCP resulting from dose escalation can be achieved with a smaller increase in rectal NTCP using the optimized six-field plans.