前立腺がんIMRTの治療計画における金属マーカの線量分布および線量評価指標への影響

摘要

The aim of this study was to evaluate the influence of metal markers on dose distributions and dose evaluation indices in intensity modulated radiation therapy (IMRT) plans for prostate cancer. The dose distribution calculation in the prostate IMRT was performed in a virtual phantom with and without insertion of the metal markers. The deviations of D_(max), D_(min), homogeneity index (HI), D_(mean), D_2, D_(98), and D_(95) of clinical target volume (CTV) and planning target volume (PTV) were obtained for estimation of the influence on the dose evaluation indices. Analytical anisotropic algorithm (AAA) and Acuros external beam (AXB) algorithms were employed for calculating the dose distributions. There were no deviations in any dose evaluation indices in dose distributions calculated by using AAA, whereas the maximum deviations for CTV and PTV by using AXB were +7.93% and +6.43% for D_(max), -16.61% and-1.77% for D_(min), +29.46% and +8.34% for HI, +0.15% and +0.02% for D_(mean), +1.50 % and +0.24% for D_2, respectively. Additional data were -0.20% in D_(98) (CTV) and -0.27% in D_(95) (PTV). This study suggests that local dose changes, which were produced around metal markers, affected dose distributions and the dose evaluation indices.%強度変調放射線治療（intensity modulated radiation therapy: IMRT)では，標的周辺に急峻な線量勾配を形成でき，リスク臓器への線量を減らすことが可能である.特に，前立腺がん治療では，膀胱と直腸への線量を減らすことができる.一方，IMRTを治療計画に従って実行するためには，治療時における標的位置の不確定度を可能な限り減らす必要がある.そこで，近年照射直前または，治療中の2方向のX線画像やcone beam computea tomography (CBCT)なとを用いた画像誘導放身す線治療(image guided radiation therapy: IGRT)が積極的に行われるようになってきた.IGRTでは，腫瘍の位置を認識するために金属マーカが使われることがある.つまり，腫瘍や腫瘍近傍に高原子番号の金属マーカ（例：金）を留置し，より高精度な位置合わせが可能となっている.しかし，高原子番号の金属マーカを含めた状態での治療計画において線量分布の不確定度が増加する可能性がある.