Sherif Reham S, Attalla Ehab M, Elshemey Wael M, Madian Noha G
Department of Biophysics, Faculty of Science, Cairo University, Giza, Egypt.
Radiotherapy & Nuclear Medicine Department, National Cancer Institute, Cairo University, Cairo, Egypt.
J Med Imaging Radiat Oncol. 2017 Jun;61(3):402-409. doi: 10.1111/1754-9485.12562. Epub 2016 Dec 25.
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).
对辐射诱发的继发性癌症的研究兴趣与日俱增。本研究的目的是：（i）估算儿科患者中，针对围绕治疗靶区（鼻咽）的一些危及器官（OARs），调强放疗（IMRT）和兆伏级锥形束计算机断层扫描（MV-CBCT）所产生的外周剂量。（ii）估算儿科鼻咽癌（NPC）患者使用MV-CBCT进行患者摆位验证成像时，辐射诱发继发性癌症的风险，并将其与IMRT治疗剂量所导致的风险进行比较。
在KonRad 2.2.23版本上，对10例儿科NPC患者进行调强放疗治疗计划。还使用Xio 4.4版本计算了MV-CBCT给患者带来的额外辐射剂量。使用儿科体模和热释光剂量计（TLDs）测量IMRT给患者带来的剂量。然后将这些剂量与计算剂量进行比较。计算并比较IMRT和MV-CBCT诱发继发性癌症的风险。
对于危及器官（脑、脑干、脊髓、甲状腺、食管、下颌骨、心脏、视神经、肺和眼睛），治疗剂量的吸收剂量（平均剂量）高于治疗前和治疗期间用于验证患者位置的成像剂量。甲状腺、食管和肺（OARs中仅有的有风险计算表格值的器官），治疗剂量诱发继发性癌症的风险（分别为7.29±0.73%、2.62±0.17%和6.76±0.87%）高于验证成像剂量（分别为0.14±0.00%、0.06±0.00%、0.10±0.03%）。
与IMRT治疗剂量相比，使用MV-CBCT进行验证成像剂量导致继发性癌症的风险非常小。因此，关注治疗剂量尤其是使用IMRT时诱发继发性癌症的风险很重要，因为与其他一些技术（如适形放疗）相比，IMRT产生的泄漏辐射相当高。
