加速部分乳房照射における外科用クリップを用いた位置照合精度

摘要

早期乳癌に対する乳房温存療法は，全乳房照射を行うことで温存乳房内の再発リスクを約1/3に減少させることが報告されている.これに対して，同側乳房内の局所再発の70〜80%が腫瘍床近傍に生じるという事実を受け，腫瘍床近傍のみを標的として，大線量を小分割で照射し，短期に治療を完遂させる加速部分乳房照射(accelerated partial breast irradiation: APBI)が提案された.%The purpose of this study was to evaluate the accuracy of positional verification during overall radiation treatment periods in accelerated partial breast irradiation using one or more surgical clips. We first investigated the appropriate computed tomography (CT) slice thickness and detectability of clips for a matching criterion in a phantom study. Next, clinical investigations were carried on 12 patients with multiple clips positioned around the lumpectomy cavity. During radiation treatment planning, a 5-mm region of interest (5-mm ROI) was defined by adding a three dimentional (3D) margin of 5 mm to each clip. During treatment, the clips on two orthogonal kilovoltage X-ray images acquired were moved so as to be included in the corresponding 5-mm ROI on digitally reconstructed radiographs (DRRs). Positional accuracy was calculated using the displacement of each clip in the verification images. The displacements of each clip acquired in all setups were then calculated throughout the overall radiation treatment period and the factors affecting the displacement of clips were investigated. Positional accuracy was also investigated in setups using skin marks and in setups using the bone structure around the thorax. We demonstrated in a phantom study that a CT slice thickness of 2.5 mm was appropriate. In our clinical investigations, 91% of the clips were included in the 5-mm ROI. The interfractional displacement of clips was large, with a long distance between the isocenter and each clip at the time of radiation treatment planning.