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Evaluation of the optimal co-planar field arrangement for use in the boost phase of dose escalated conformal radiotherapy for localized prostate cancer

¹ The Academic Unit of Radiotherapy & Oncology
² The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Trust, Downs Road, Sutton, Surrey, SM2 5PT, UK

Correspondence: Dr V S Khoo, The Academic Department of Radiation Oncology, Christie Hospital, Wilmslow Road, Withington, Manchester M20 4BX, UK.

The aim of this study was to determine the optimal co-planar beam arrangement from a variety of three-field (3F), four-field (4F) and six-field (6F) plans for the boost phase of a dose escalated conformal radiotherapy schedule. Three selected plans (3F 0°, 90°, 270° plan, 4F 45°, 90°, 270°, 315° plan and 6F 40°, 90°, 115°, 245°, 270°, 320° plan) were compared with reference plans (3F 0°, 120°, 240° plan, 4F 0°, 90°, 180°, 270° plan, 6F 55°, 90°, 125°, 235°, 270°, 305° plan and 6F 50°, 90°, 130°, 230°, 270°, 310° plan) in 10 patients. Doses of 64 Gy and 74 Gy were prescribed to the isocentre using 6 MV photons. The boost planning target volume comprised the prostate gland alone without a margin. Plans were compared by means of rectal volumes irradiated to >50% (V₅₀), >80% (V₈₀) and >90% (V₉₀) of the prescribed dose. Irradiated volumes were also measured for the bladder (V₉₀) and the femoral heads (V₇₀). All optimal 3F, 4F and 6F plans gave lower irradiated rectal V₈₀ and V₉₀ levels than their corresponding reference plan. The 3F (0°, 90°, 270°) plan consistently provided lower irradiated rectal levels at V₅₀ to V₉₀, with acceptable bladder and femoral head doses compared with the other plans in the study. When the 6F (50°, 90°, 130°, 230°, 270°, 310°) plan used at our institution for the boost phase was compared with the 3F (0°, 90°, 270°) plan, the rectal V₅₀ was reduced from 20.8±5.2% to 12.6±5.1%, the rectal V₈₀ was reduced from 8.7±2.9% to 6.5±3.1% and the rectal V₉₀ was reduced from 5.5±2.1% to 3.9±2.0% (all p<0.001). The bladder V₉₀ and the femoral heads V₇₀ levels were equivalent. For the boost phase when escalating the dose from 64 Gy to 74 Gy, the co-planar plan that allowed optimal rectal sparing was a 3F beam arrangement using gantry angles of 0°, 90° and 270°. This 3F plan provided improved rectal sparing compared with the 6F (50°, 90°, 130°, 230°, 270°, 310°) beam arrangement currently used at our institution, with equivalent and acceptable bladder and femoral head doses.