Schneider Uwe, Lomax Anthony, Hauser Beat, Kaser-Hotz Barbara
Division of Medical Physics, Department of Radiation Oncology and Nuclear Medicine, City Hospital Zürich, 8063 Zürich, Switzerland.
Radiat Environ Biophys. 2006 May;45(1):39-43. doi: 10.1007/s00411-006-0034-7. Epub 2006 Feb 7.
It is often assumed that radiation-induced secondary cancer after proton therapy forms preferentially close to the distal fall-off of the spread-out Bragg peak because of an increased relative biological effectiveness (RBE) with regard to cancer induction of low-energy protons. In this study we analyze to what extent dose gradients distal to the Planning Target Volume (PTV) may, independently from the RBE, contribute to enhanced radiation carcinogenesis. The study is based on two dogs which, out of 30 dogs treated with proton therapy at the Paul Scherrer Institute (PSI), developed a secondary cancer. Both dogs were originally diagnosed and treated for a fibrosarcoma and developed an osteosarcoma 48 and almost 60 months, respectively, after radiotherapy. From the dose distributions of the initial radiotherapy for both dogs three-dimensional maps of secondary cancer complication probability (SCCP) were computed. The SCCP maps were analyzed in the regions where the dogs developed a secondary cancer. The SCCP maps showed an enhanced risk in the regions of the femur where the secondary cancers were detected, as compared to the SCCP of the total femur. Excess risk of radiation-induced cancer at the distal part of proton radiation fields can thus be explained using SCCP calculations on the basis of the physical dose distributions. Therefore, the occurrence of secondary cancer close to the distal dose gradients of proton therapy is not necessarily due to an increased RBE of low-energy protons. More extensive studies based on more patients will be necessary to further elucidate the factors influencing the development of secondary tumors.
人们通常认为，质子治疗后辐射诱发的继发性癌症优先在扩展布拉格峰的远端剂量跌落附近形成，这是因为低能质子在诱发癌症方面具有更高的相对生物效应（RBE）。在本研究中，我们分析了计划靶区（PTV）远端的剂量梯度在多大程度上可能独立于RBE而导致辐射致癌作用增强。该研究基于两只狗，在保罗·谢尔研究所（PSI）接受质子治疗的30只狗中有两只患了继发性癌症。这两只狗最初均被诊断患有纤维肉瘤并接受了治疗，分别在放疗后48个月和近60个月时患上骨肉瘤。根据两只狗初始放疗的剂量分布，计算了继发性癌症并发症概率（SCCP）的三维图。在两只狗发生继发性癌症的区域对SCCP图进行了分析。与整个股骨的SCCP相比，SCCP图显示在检测到继发性癌症的股骨区域风险增加。因此，基于物理剂量分布的SCCP计算可以解释质子辐射野远端辐射诱发癌症的额外风险。所以，质子治疗远端剂量梯度附近继发性癌症的发生不一定是由于低能质子的RBE增加所致。需要基于更多患者进行更广泛的研究，以进一步阐明影响继发性肿瘤发生的因素。
