Background: Internal mammary irradiation is still an issue of great debate. Although treatment of internal mammary lymph nodes was routinely given in the majority of randomized trials, data in its value are still limited. The aim of this study is to determine the variability of position of the internal mammary lymph nodes using lymphoscintigraphy and to compare the dose of radiation that reaches these lymph nodes, the heart and lungs if only tangential fields are used.
Material and methods: This is a prospective study that included 30 breast cancer patients treated in the department of radiation oncology of the Egyptian National Cancer Institute, Cairo University, planned for postoperative radiotherapy. Lymphoscintigraphy was done for all patients for detection of the exact site of the internal mammary lymph nodes. Dose volume histogram (DVH) was done to measure the dose to the heart using CT planning. Two plans were done for each patient, the first with internal mammary field (plan I), and the second without but with contralateral crossing of the midline by 1cm (plan II).
Results: The mean percentage of the internal mammary lymph nodes included in the internal mammary field (plan I) was 70.03% while in plan II it was 3.05%. The mean dose percentage reaching the heart in plan I was 54.5% of total dose, while in plan II it was 9.16% of total dose with significant p value<0.001. The mean dose to the heart decreased as the heart volume increased, this significant difference between the 2 plans was maintained for the different heart volumes. Also, the radiation dose to the heart in plan I varied significantly (p: 0.001) between the right side vs the left side with a mean dose of 48.02% and 63.5%, respectively. The mean dose percentage reaching the lungs in plan I and plan II was 46.53% and 24.5% respectively, with significant p value<0.001.
Conclusion: If irradiation of internal mammary chain is intended, then a direct internal mammary field should be used. The planning of internal mammary field should be adjusted according to lymphoscintigraphy so as to include most of the draining internal mammary lymph nodes. The risk of late cardiac and pulmonary complications will increase when using direct internal mammary field, but the risk of cardiac complications will be less in irradiation of right side internal mammary lymph nodes compared to that of irradiation of the left side.