PROSTAT VE AKCİĞER KANSERLERİNİN VMAT (VOLÜMETRİK AYARLI RADYOTERAPİ) VE YART (YOĞUNLUK AYARLI RADYOTERAPİ) TABANLI TEDAVİLERİNDE TERMAL FOTONÖTRON KONTAMİNASYONUNUN İNCELENMESİ
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Yılmaz, E. Investigation of Thermal Photoneutron Contamination in VMAT (Volumetric Modulated Arc Therapy) and IMRT (Intensity-Modulated Radiation Therapy)-Based Treatments of Prostate and Lung Cancers, Hacettepe University Graduate School of Health Sciences, MSc. Thesis in Radiotherapy Physics Program, Ankara, 2022. It is known that linear accelerators used in radiotherapy cause neutron contamination, particularly produced in the head that is not calculated by the treatment planning systems (TPS) for tissues and organs outside the target area. In order to eliminate this uncertainty in the TPS, target volumes for prostate cancer with lymph node involvement and lung cancer with mediastinal and hilar masses and related critical organs were contoured on the Rando phantom in order to obtain measurements with more accurate approaches and similarities with clinical applications. Following, using the RayStation TPS, intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) treatment plans were prepared for these two separate regions at 10 MV and 15 MV energies. Irradiations were made using an Elekta Versa HD model linear accelerator. Thermal neutron measurements were made with bubble detectors using equivalent cork molds for the lung region and water equivalent wax molds for the prostate region in the phantom. It was observed that in-field thermal photoneutron doses were 50-60% higher than out-of-field doses 10 cm from the field center in VMAT and IMRT plans in both treatment areas. In the plans made with 15 MV, while 70-80% higher neutron dose was read in the prostate region, this value was measured as 50-60% in the lung region. While the in-field photoneutron doses were found to be negligible compared to the photon dose, the thermal neutron doses in the out-of-field doses were calculated to be in the 1-3% band compared to the photon doses. In wide-field irradiation in the prostate region, 1.2 μSv/MU dose per thermal neutron-induced fraction was measured, especially in the gonads and kidneys. In conclusion, the contribution of photoneutrons to equivalent doses should be considered in order to reduce the uncertainty in secondary cancer risk, particularly in low-dose areas out of the field, in high nominal energy IMRT applications.