Programa de Engenharia Nuclear/Instituto Alberto Luiz Coimbra de Pós-graduação e Pesquisa de Engenharia (COPPE) - Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.
PLoS One. 2013;8(2):e55430. doi: 10.1371/journal.pone.0055430. Epub 2013 Feb 6.
An increasing number of studies have shown that post-mastectomy radiotherapy presents benefits associated with the patients survival and a significant fraction of the treated patients makes use of tissue expanders for breast reconstruction. Some models of tissue expanders have a magnetic disk on their surface that constitutes heterogeneity in the radiation field, which can affect the dose distribution during the radiotherapy treatment. In this study, the influence of a metallic heterogeneity positioned in a breast tissue expander was evaluated by means of Monte Carlo simulations using the MCNPX code and using Eclipse treatment planning system. Deposited energy values were calculated in structures which have clinical importance for the treatment. Additionally, the effect in the absorbed energy due to backscattering and attenuation of the incident beam caused by the heterogeneity, as well as due to the expansion of the prosthesis, was evaluated in target structures for a 6 MV photon beam by simulations. The dose distributions for a breast treatment were calculated using a convolution/superposition algorithm from the Eclipse treatment planning system. When compared with the smallest breast expander volume, underdosage of 7% was found for the largest volume of breast implant, in the case of frontal irradiation of the chest wall, by Monte Carlo simulations. No significant changes were found in dose distributions for the presence of the heterogeneity during the treatment planning of irradiation with an opposed pair of beams. Even considering the limitation of the treatment planning system, the results obtained with its use confirm those ones found by Monte Carlo simulations for a tangent beam irradiation. The presence of a heterogeneity didn't alters the dose distributions on treatment structures. The underdosage of 7% observed with Monte Carlo simulations were found for irradiation at 0°, not used frequently in a clinical routine.
越来越多的研究表明，乳房切除术后放疗与患者的生存相关，且相当一部分接受治疗的患者会使用组织扩张器进行乳房重建。一些组织扩张器模型的表面有一个磁盘，这会使放射野产生不均匀性，从而影响放射治疗过程中的剂量分布。在这项研究中，使用 MCNPX 代码和 Eclipse 治疗计划系统，通过蒙特卡罗模拟评估了位于乳房组织扩张器中的金属异质性的影响。计算了对治疗具有临床重要性的结构中的沉积能量值。此外，通过模拟评估了由于异质性以及由于假体扩张引起的反向散射和入射束衰减对靶结构中 6 MV 光子束吸收能量的影响。使用 Eclipse 治疗计划系统的卷积/叠加算法计算了乳房治疗的剂量分布。与最小的乳房扩张器体积相比，当对胸壁进行正面照射时，最大乳房植入物体积的剂量不足 7%，这是通过蒙特卡罗模拟发现的。在使用对向射野进行照射的治疗计划中，不存在异质性时，剂量分布没有明显变化。即使考虑到治疗计划系统的局限性，使用其获得的结果也证实了在切线束照射时通过蒙特卡罗模拟得到的结果。异质性的存在不会改变治疗结构上的剂量分布。在临床常规中不常使用的 0°照射时，通过蒙特卡罗模拟发现的 7%的剂量不足。
