Internal mammary node coverage: an investigation of presently accepted techniques

D W Arthur, M R Arnfield, L A Warwicke, M M Morris, R D Zwicker
International Journal of Radiation Oncology, Biology, Physics 2000 August 1, 48 (1): 139-46

PURPOSE: Recent publications have generated a renewed interest in regional nodal treatment to include the ipsilateral supraclavicular and internal mammary nodes (IMN). The purpose of this study is to evaluate three presently accepted treatment techniques for coverage of the intact breast and ipsilateral lymph node regions and to construct recommendations regarding the utilization of these techniques.

METHODS AND MATERIALS: Anatomic data were obtained from five randomly selected patients with computerized tomography (CT) in treatment position. Three patients presented with cancer of the left breast and two with cancer of the right. Using the Pinnacle 3-D planning system, normal tissue volumes of breast, ipsilateral lung, heart, sternum, and the IMN target were delineated for each patient. Three accepted techniques used to treat ipsilateral breast, internal mammary and supraclavicular nodes (extended tangents, 5-field, partly wide tangents) were configured and compared to a supraclavicular field matched to standard tangential fields. A dosage of 50 Gy in 25 fractions was prescribed to the target volume. Dose-volume histograms (DVH) were generated and analyzed with regard to target volume coverage and lung/heart volumes treated.

RESULTS: All of the treatment techniques covering IMN include at least 10% more lung and heart volume than that covered by standard tangential fields. The relative lung and heart volumes treated with each technique were consistent from patient to patient. The 5-field technique clearly treats the largest volume of normal tissue; however, most of this volume receives less than 50% of the dose prescribed. The percent of heart and ipsilateral lung treated to 20 Gy, 30 Gy, and 40 Gy have been calculated and compared. Due to the increase in chest wall thickness and depth of IMN superiorly, complete coverage was not achieved with any technique if the IMN target extended superiorly into the medial supraclavicular field where dose fall-off resulted in a significant underdosing at depth. For the same anatomic reasons, the 5-field technique underdosed 10-15% of the IMN target volume in 4 of the 5 cases. This technique also yielded a greater dose heterogeneity, which was not seen with the other techniques evaluated and correlated with the change of anterior chest wall thickness.

CONCLUSIONS: Anatomic variation in chest wall thickness and IMN depth strongly suggests the routine use of multislice CT planning to ensure complete coverage of the target volume and optimal sparing of normal tissue. All of the techniques can be constructed to look acceptable at central axis. To cover the superior most aspect of the IMN chain either high tangential fields, a supraclavicular field photon beam of energy >6 MV, or an AP/PA supraclavicular setup should be considered. The 5-field technique has the most difficulty in compensating for the increased depth of the IMN in the superior aspect of the tangent fields with up to +/-40% variation of the dose noted in isolated areas within the target volume. Based on our evaluation, the partly wide tangent technique offers many advantages. It provides optimal coverage of the target volume, reduces coverage of normal tissue volumes to an acceptable level, and is easily reproducible with a high degree of dose homogeneity throughout the target.

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