Abstract
There is a growing interest in the study of radiation-induced secondary cancer. The aim of this work is (i) to estimate the peripheral doses attributable to intensity modulated radiotherapy (IMRT) and mega-voltage cone beam computed tomography (MV-CBCT) for some organs at risk (OARs) which surround the target being treated (Nasopharynx) in paediatric patients. (ii) To estimate the risk of radiation-induced secondary cancers attributable to patient setup verification imaging dose using MV-CBCT for Nasopharyngeal Carcinoma (NPC) in paediatric patients and comparing it with that attributable to the therapeutic dose using IMRT. Intensity modulated radiotherapy treatment planning of 10 NPC paediatric patients was carried out on KonRad release 2.2.23. The additional radiation doses to the patients attributable to MV-CBCT were calculated also using Xio Version 4.4. A paediatric phantom and thermoluminescent dosimeters (TLDs) were used to measure the patient doses attributable to IMRT. These doses were then compared with the calculated doses. The risk of induced secondary cancers attributable to IMRT and MV-CBCT was calculated and compared to each other. The absorbed doses (mean dose) for the OARs (Brain, Brain stem, spinal cord, thyroid, oesophagus, mandible, heart, optic nerve, lung and eye) were higher for the therapeutic dose than for the imaging dose used in the verification of patient position before and during the treatment. The risk of induced secondary cancers in thyroid, oesophagus and lung (the only organs from the OARs which have tabulated values for risk calculations) was higher for therapeutic dose (7.29±0.73%, 2.62±0.17% and 6.76±0.87%, respectively) than for verification imaging dose (0.14±0.00%, 0.06±0.00%, 0.10±0.03% respectively). The risk of secondary cancers attributable to verification imaging dose using MV-CBCT is very small compared to therapeutic dose using IMRT. Therefore, it is important to focus on the risk of secondary cancers attributable to therapeutic dose especially when using IMRT, where the produced leakage radiation is considerably high compared to some other techniques (such as conformal radiotherapy).
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