EP-1566: Biologically Optimized IMRT Plans for Prostate Cancer Using Population-Based Tumour Biology

The following organs at risk were contoured: thyroid gland, heart, lungs, esophagus and contralateral breast.Treatment Planning: Prescription dose was 42,56 Gy in 16 fractions of 2,66 Gy.The 3D-CRT technique consisted of tangential beams for the breast/thoracic wall, one anterior beam (15° or 345°) for the medial periclavicular region and an anterior (15° or 345°) and posterior (165° or 195°) beam for the lateral periclavicular and axillary regions ( Fig. 1.).For the VMAT technique tangential arcs of 24 degrees were chosen as these provide the best sparing of lung and heart and further minimize the low dose delivery to the rest of the body (integral dose).We analyzed PTV coverage including the conformation number (CN) and dose to the OARs to compare the techniques. ResultsResults: Table 1 shows the results.Mean V95% for the PTV was 95,3% for 3D-CRT and 97,5% for VMAT.CN was higher for the VMAT technique, indicating that PTV-coverage has improved at the same time as limiting the volume receiving a lower dose.Coverage was especially better with VMAT for lymph node levels 3-4.This came at a cost of a slightly higher dose to the thyroid gland.Dose to the lungs as well as the heart were lower with VMAT. Conclusion Conclusion:We developed a VMAT-only planning method for locoregional breast irradiation, which is straightforward, robust, can be combined with respiratory control and creates very conformal and homogeneous treatment plans with improved PTV coverage and low doses to the organs at risk.