Regional Nodal Irradiation Targeting the Undissected Axilla Following Axillary Lymph Node Dissection (ALND) Is Associated with Substantial Radiation Dose to the Dissected Axilla

Abstract

Adjuvant breast cancer radiotherapy including regional nodal irradiation (RNI) following axillary lymph node dissection (ALND) for node-positive disease typically targets the undissected axilla, excluding the dissected axilla in an effort to reduce lymphedema risk. Although it is well documented that a portion of the low axilla receives dose from tangent fields, incidental dose to the dissected axilla from RNI targeting the undissected axilla is not well characterized. We hypothesized that the dissected axilla receives a substantial dose or radiation despite being excluded from the treatment plan. We evaluated the radiation dose delivered to the dissected axilla during adjuvant breast or chest wall radiotherapy with regional nodal irradiation intending to target only the undissected axilla. The records of 18 women with Stage IIA to IIIB breast cancer treated with lumpectomy or mastectomy and ALND were reviewed under an IRB approved protocol. All patients completed adjuvant radiotherapy to the breast or chest wall, and undissected regional lymphatics. Regional lymphatics were contoured based on RTOG breast consensus guidelines, and target volumes included the breast or chest wall, the undissected axilla, the supraclavicular region, and internal mammary lymph nodes at the discretion of the treating radiation oncologist. The dissected axilla was not included in the target volume. Treatments were delivered using 3D conformal monoisocentric (n=12) or dual isocentric multifield (n=6) techniques. The dissected axilla was contoured after treatment completion to evaluate dose delivered incidentally. For uniformity of analysis, boost plans were not included in the evaluation. The volume of the dissected axilla covered by 100% (V100%), 95% (V95%), and 50% (V50%) of prescription dose was evaluated, in addition to the mean dose to the dissected and undissected axilla. Variance is expressed in standard deviations. The mean V100%, V95%, and V50% to the dissected axilla was 47.1±7.1%, 85.2±6.4%, and 97.2±4.9%, respectively. Mean dose to the dissected axilla and the undissected axilla were 97.0±6.4% and 101.4±4.2% of prescription dose, respectively. On univariate analysis, mean lung dose (p=0.11), mean normalized dose to the undissected axilla (p=0.37), dissected volume (p=0.54), type of surgery (p=0.41), and presence of expander (p=0.74), did not correlate with normalized dose to dissected volume. The dissected axilla receives substantial radiation dose incidentally despite being excluded from breast or chest wall RNI target volumes. Given significant dose delivered to the dissected axilla, these results have potential implications for analysis of lymphedema endpoints from clinical trials, including Alliance A11202, which incorporate adjuvant radiotherapy directed to the undissected axilla following ALND. Future studies are needed to analyze quantitatively the impact of dose to the dissected axilla on clinical lymphedema risk.