Topographic distribution of inguinal lymph nodes metastasis: Significance in determination of treatment margin for elective inguinal lymph nodes irradiation of low pelvic tumors

Purpose: To study the distribution of gross inguinal lymph node metastasis and, in particular, its correlation with major pelvic bony structures on a simulation film. Methods and Materials: Thirty-seven cases of low pelvic tumors having gross inguinal lymph node metastasis that were treated with radiation therapy between November 1987 and December 1992 were segregated for study. The patient's nodes were palpated and marked with lead wire before the simulation film was taken. The geometric center of the usually round or elliptical node on the film was assumed to be the origin of the previously uninfested node. A total of 84 such labeled nodes was obtained from these 37 cases. These centers were transferred to and mapped collectively on a new simulation film showing major pelvic bony structures of left hemipelvis and upper femur. Results: Distribution of gross inguinal lymph nodes was found confined to the following area, as related to major pelvic bony structure: laterally, just abutting the tangential line that passes through lateral border of the femoral head; medially: 3 cm away from the body's midline axis; superiorly: 1 cm below the line that joins both upper borders of the femoral head; inferiorly: 2.5 cm below the low borders of ischial tuberosity. According to this rectangular boundary, three nodes were out of field, nine nodes near the border less than 1 cm margin. This area adequately covered 86% (72 of 84) of the studied nodes. Conclusion: Distribution study is important in determining the treatment margin. In general, an additional 1-2 cm beyond the area described above is the recommended treatment margin for elective inguinal lymph nodes irradiation with high confidence level of coverage.