Dose to level I and II axillary lymph nodes and lung by tangential field radiation in patients undergoing postmastectomy radiation with tissue expander reconstruction

BACKGROUND To define the dosimetric coverage of level I/II axillary volumes and the lung volume irradiated in postmastectomy radiotherapy (PMRT) following tissue expander placement. METHODS AND MATERIALS Twenty-three patients were identified who had undergone postmastectomy radiotherapy with tangent only fields. All patients had pre-radiation tissue expander placement and expansion. Thirteen patients had bilateral expander reconstruction. The level I/II axillary volumes were contoured using the RTOG contouring atlas. The patient-specific variables of expander volume, superior-to-inferior location of expander, distance between expanders, expander angle and axillary volume were analyzed to determine their relationship to the axillary volume and lung volume dose. RESULTS The mean coverage of the level I/II axillary volume by the 95% isodose line (V(D95%)) was 23.9% (range 0.3 - 65.4%). The mean Ipsilateral Lung V(D50%) was 8.8% (2.2-20.9). Ipsilateral and contralateral expander volume correlated to Axillary V(D95%) in patients with bilateral reconstruction (p = 0.01 and 0.006, respectively) but not those with ipsilateral only reconstruction (p = 0.60). Ipsilateral Lung V(D50%) correlated with angle of the expander from midline (p = 0.05). CONCLUSIONS In patients undergoing PMRT with tissue expanders, incidental doses delivered by tangents to the axilla, as defined by the RTOG contouring atlas, do not provide adequate coverage. The posterior-superior region of level I and II is the region most commonly underdosed. Axillary volume coverage increased with increasing expander volumes in patients with bilateral reconstruction. Lung dose increased with increasing expander angle from midline. This information should be considered both when placing expanders and when designing PMRT tangent only treatment plans by contouring and targeting the axilla volume when axillary treatment is indicated.