放射線治療計画における計画標的体積（PTV）は，照射位置のばらつきによる等線量曲線の偏位を補償するための余白（PTVマージン）を，腫瘍細胞の存在範囲に加えることで作成する．今回，必要なPTVマージンの値に照射野の形状が及ぼす影響を検討することを目的に，van Herkの理論値（正確法，近似法）の妥当性を検討した．部分的に凸形～凹形を有する7種類の照射野を作成し，照射方向は対向2門，4 MV X線と10 MV X線でそれぞれ線量分布を計算した．線量分布にばらつきを加える数値シミュレーションを行い，等線量曲線（処方線量の95%）の偏位を理論値と比較した．正確法は照射野辺縁が平坦な場合はシミュレーションと一致したが，照射野の凹凸が大きくなると不一致が見られた．近似法は多くの場合不一致が見られたが，不一致の範囲は正確法より小さかった．PTVマージンの算出に際しては，照射野形状の考慮が必要である． The planning target volume (PTV) on the radiotherapy plan is created by adding a margin (PTV margin) to the volume in which tumor cells spread, for compensating the deviation of the isodose curve due to geometrical uncertainties. In this study, the validity of van Herk's theoretical value (precise method, approximation method) was evaluated for the purpose of examining the impact of the irradiation field shape on necessary PTV margin value. Seven types of irradiation fields with partially convex to concave shapes were prepared, and the dose distributions were calculated with the parallel opposed fields, using 4 MV X-ray and 10 MV X-ray data. Numerical simulation was carried out to add geometrical uncertainties to the dose distribution, and the deviation of the isodose line (95% of the prescribed dose) was compared with the theoretical value. The precise method was consistent with the simulation when the edge of the irradiated field was flat, but inconsistency was observed when the irregularities of the irradiation field was increased. Although the approximation method was found to be inconsistent in many cases, the range of inconsistency was smaller than that of the precise method. When calculating the PTV margin, the influence of the irradiation field shape should be taken into account.