140: Prostat IMRT Using Unflattened Beams

Abstract

IMRT is a common treatment modality for prostate radiotherapy. With IMRT, the optimization of the fluence map renders the flattening filter unnecessary. Flattening-filter free beam characteristics have been investigated recently, but clinical evaluations have not been conducted. In this study we compare IMRT treatment plans for prostate therapy generated with and without a flattening filter. Both low-energy and high-energy treatment plans were compared. IMRT plans were generated for 10 early stage prostate patients using clinical dose prescriptions and beam orientations. Plans were generated with Eclipse 8.0 (Varian Medical Systems), which we commissioned with beam data measured on a Varian Clinac 21EX operated with and without the flattening filter. For each patient four plans were generated, one with and one without the flattening filter at 6 MV, and one with and one without the flattening filter at 18 MV. The plans were normalized so that 98% of the PTV received 75.6 Gy. For all 4 treatment modalities it was possible to generate clinically acceptable plans in terms of target coverage and critical structure sparing. Comparing the 6 MV plans, the unflattened treatment plans required 2 times fewer monitor units (on average) than plans using the flattened beam. Dosimetrically, treatment plans using the unflattened beam had more homogeneous PTV coverage. Although the minimum PTV dose was statistically the same between the flattened and unflattened therapies, the maximum dose was, on average, 3.5% lower with the unflattened beam. DVH data of critical structures showed a small but consistent reduction in the rectal and femoral head dose. The dose to the bladder was nearly identical between flattened and unflattened treatment plans. Comparing 18 MV plans, the unflattened treatment plans required 2.4 times fewer monitor units (on average) than plans using the flattened beam. Treatment plans using the unflattened beam were very similar dosimetrically to plans using the flattened beam. The minimum PTV dose was statistically the same between the flattened and unflattened therapies, while the maximum dose was 4% higher for the unflattened beam. DVH data of critical structures showed nearly identical rectal doses between the flattened and unflattened plans. Bladder and femoral head doses were also the same within statistical uncertainty. Clinically acceptable IMRT plans for prostate cancer can be developed with unflattened beams. At both 6 MV and 18 MV they require substantially fewer monitor units than comparable treatments with conventional flattened beams. Dosimetrically, flattened and unflattened beams generated similar treatment plans. At 6 MV, the plans were slightly better when generated with the unflattened beam as compared to the flattened beam. At 18 MV, the plans were slightly poorer when generated with the unflattened beam as compared to the flattened beam, however, optimization of parameters such as gantry angle was not done for the unflattened beams.