Dosimetric studies of mixed field photon beam intensity modulated radiation therapy (IMRT) for prostate cancer using pencil beam (PB) and collapsed cone convolution (CCC) algorithms using Oncentra MasterPlan treatment planning system (v. 4.3) are investigated in this study. Three different plans were generated using 6？MV, 15？MV, and mixed beam (both 6 and 15？MV). Fifteen patients with two sets of plans were generated: one by using PB and the other by using CCC for the same planning parameters and constraints except the beam energy. For each patient’s plan of high energy photons, one set of photoneutron measurements using solid state neutron track detector (SSNTD) was taken for this study. Mean percentage of in the rectum is , , and for 6？MV, 15？MV, and mixed-energy plans, respectively. Mean percentage of in bladder is , and for 6？MV, 15？MV, and mixed-energy plans, respectively. Mixed fields neutron contribution at the beam entrance surface is 45.62% less than at 15？MV photon beam. Our result shows that, with negligible neutron contributions, mixed field IMRT has considerable dosimetric advantage. 1. Introduction Radiation therapy (RT), either alone or in combination with chemotherapy and/or surgery, plays a major role in treating prostate cancer. Deep-seated prostate cancer is the second common cancer in men [1, 2], which is widely treated with intensity modulated radiation therapy (IMRT) technique with minimal dose to surrounding organs at risk (OAR). High energy beams are used for 3-dimensional conformal radiation therapies to treat prostate, while 6？MV, low photon energy is used in IMRT. With a higher surface dose from low energy, low dose gradient and higher conformity are achieved at planning target volumes to minimize bladder and rectal dose [3–5]. Femoral heads receive higher dose in 3D-CRT and low energy IMRT beams, if the beam angle is not well optimized [6]. High energy is always limited in IMRT because of the probability of photoneutron production, where multileaf collimators (MLC) are continuously moved during the treatment, and higher beam on time (MU) is required [7, 8]. Treatment planning systems (TPS) optimization gives conformity in the target region [9–11]. To achieve tolerable radiation in the rectum and bladder within the dose constraints given, TPS will optimize the dose fluence at the target. MLC positions and monitor units are generated by these conditions. Modern linacs are given the choice to select two or three optimization algorithms in a single TPS to get an appropriate plan in individual treatments. Many studies [12–15]
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