Variability of the depth of supraclavicular and axillary lymph nodes in patients with breast cancer: is a posterior axillary boost field necessary?

PURPOSE: To determine the variability of the depth of supraclavicular (SC) and axillary (AX) lymph nodes in patients undergoing radiation therapy for breast cancer and to relate this variability with the patient's anterior/posterior (A/P) diameter. The dosimetric consequences of the variability in depth are explored and related to the need for a posterior axillary boost field. METHOD AND MATERIALS: In 49 patients undergoing treatment-planning computed tomography (CT) scanning in the treatment position, the maximum depth of the SC and AX lymph nodes was measured on CT images. The A/P diameter was measured at the location of the SC and AX, respectively. The relationship between the SC/AX lymph node depth and patient diameter was determined using linear regression. For an anterior SC and AX field, the relative dose to the SC and AX lymph nodes were calculated for a 6 MV photon beam. RESULTS: The maximum depth of the SC lymph nodes ranged from 2.4 to 9.5 cm (median, 4.3 cm). The depth was less than 3 cm in 4 patients, 3-6 cm in 39 (80%), and greater than 6 cm in 6 patients. There was a linear relationship between the SC lymph node depth and the A/P diameter. The depth of the SC lymph nodes in cm equals approximately one-half of the A/P diameter minus 3.5 (r(2) = 0.69). In 94% (46 of 49) of patients, the SC lymph node depth was between one-fifth and one-half of the A/P diameter. The depth of the axillary lymph nodes ranged from 1.4 to 8 cm (median, 4.3 cm). The depth was less than 3 cm in 8 patients, 3-6 cm in 32 (65%), and greater than 6 cm in 9 patients. The AX lymph node depth in cm equals approximately one-half of the A/P diameter minus 3 (r(2) = 0.81). In all patients, the AX lymph nodes were shallower than mid-depth. The depth of the SC and AX lymph nodes was within +/- 1 cm in 53% (26 of 49) of patients. The AX lymph nodes were located at >/= 1 cm shallower or greater depth than the SC in 24.5% (12 of 49) and 22.5% (11 of 49) of patients, respectively. If an anterior 6-MV beam only is used to treat the SC and AX lymph nodes in these 49 patients, the dose to the AX is within +/- 5% of the SC dose in 53% (26 of 49) patients and is 90% or more of the dose delivered in the SC in 90% (44 of 49) of patients. CONCLUSION: The maximum depth of the SC and AX lymph nodes varies widely and is related to the patient's size represented by the A/P diameter. In most patients, the AX lymph nodes lie at approximately the same depth or shallower than the SC. Therefore, the rationale for a posterior axillary boost field needs to be further assessed. When the AX and SC lymph nodes are deep, opposed supraclavicular and axillary fields and/or the use of a higher energy beam might be reasonable.