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

Purpose: To determine the optimal coplanar treatment technique for six-fiel d conformal radiotherapy of prostate only (PO) or prostate plus seminal ves icles (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 an d 74 Gy, All plans consisted of laterally-symmetric anterior oblique, later al, and posterior oblique fields, The posterior oblique fields were varied through 20-45 degrees relative to the lateral fields, and for each of these angles, the anterior oblique fields were varied through 25-65 degrees rela tive to lateral. The plans were compared by means of rectal volumes irradia ted to 80% or more of the prescribed dose (V-80); normal tissue complicatio n probability (NTCP) for rectum, bladder, and femoral heads; and tumor cont rol 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 degrees from later al and posterior oblique fields at 25 degrees from lateral produced the low est 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-fi eld (reference) plan with both anterior and posterior oblique fields at 35 degrees from lateral (V-80 = 26.3 +/- 5.9%), this represented an improvemen t (p = 0.001), For the PSV group, the optimal anterior and posterior obliqu e fields were at 65 degrees and 30 degrees from lateral, respectively (V-80 47.5 +/- 6.3%), Relative to the reference plan (V-80 = 49.4 +/- 5.6%), thi s 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 f rom dose escalation can be achieved with a smaller increase in rectal NTCP using the optimized six-field plans. (C) 2000 Elsevier Science Inc.