A high-energy photon beam that is more than 10 MV can produce neutron contamination. Neutrons are generated by the [γ,n] reactions with a high-Z target material. The equivalent neutron dose and gamma dose from activation products have been estimated in a LINAC equipped with a 15-MV photon beam. A Monte Carlo simulation code was employed for neutron and photon dosimetry due to mixed beam. The neutron dose was also experimentally measured using the Optically Stimulated Luminescence (OSL) under various conditions to compare with the simulation. The activation products were measured by gamma spectrometer system. The average neutron energy was calculated to be 0.25 MeV. The equivalent neutron dose at the isocenter obtained from OSL measurement and MC calculation was 5.39 and 3.44 mSv/Gy, respectively. A gamma dose rate of 4.14 µSv/h was observed as a result of activations by neutron inside the treatment machine. The gamma spectrum analysis showed 28Al, 24Na, 54Mn and 60Co. The results confirm that neutrons and gamma rays are generated, and gamma rays remain inside the treatment room after the termination of X-ray irradiation. The source of neutrons is the product of the [γ,n] reactions in the machine head, whereas gamma rays are produced from the [n,γ] reactions (i.e. neutron activation) with materials inside the treatment room. The most activated nuclide is 28Al, which has a half life of 2.245 min. In practice, it is recommended that staff should wait for a few minutes (several 28Al half-lives) before entering the treatment room after the treatment finishes to minimize the dose received.

Background: This study was to investigate the dose variation between anisotropic analytical algorithm (AAA) and Acuros XB (AXB) calculation algorithms in heterogeneous inside planning target volume (PTV) of pituitary carcinoma.Methods: AAA and AXB algorithms were used for dose calculation by Eclipse treatment planning system in Varian TrueBeam linear accelerator machine with 120 MLC. Twelve cases of pituitary carcinoma were planned by intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)techniques. The target overlapped with the air cavity. IMRT plans were done by 9 static fields and VMAT plans were done by 2-3 arc rotation fields. The PTV was prescribed the dose of 50.4 Gy. The dosimetry was analyzed by dose to 95% and 98% of volume of PTV, maximum dose, dose coverage, and conformity index (CI) of PTV. The normal organ was analyzed by mean dose (Dmean) and maximum dose (Dmax). Results:The dose coverage of PTV using AXB calculation was less than using AAA calculation in the air cavity region for both IMRT and VMAT. The dose coverage of PTV was any statistical significantly difference between AAA and AXB algorithms. For the normal organs, the mean and maximum dose agreed well for AAA and AXB algorithms.Conclusions: AXB can calculate the dose more accurate than AAA for dose calculation in inhomogeneous region. AXB algorithms should be selected for dose calculation in clinical.