Variation in Post-Surgical Lumpectomy Cavity Volume With Delay in Initiation of Breast Irradiation Because of Chemotherapy

Purpose: The addition of a radiotherapy boost has been shown to improve local control in breast conservation therapy. Three dimensional planning provides more accurate targeting of the operative bed than clinical setup using the lumpectomy scar. However, contraction of the lumpectomy cavity over time may have implications for the volume of tissue included in the boost field. Methods and Materials: The clinical variables and treatment planning volumes for patients receiving whole-breast radiotherapy at a single institution between July 1, 2006, and December 31, 2007 were analyzed retrospectively. Results: Of the 93 patients identified, 29 received chemotherapy (CTX) and 64 did not; CTX was sequenced before radiotherapy in all patients. Patients receiving CTX were more likely to have higher T and N stage and a longer interval between definitive breast surgery and radiation. The lumpectomy specimens of women receiving CTX trended toward being larger than those of women not receiving CTX (113.4cm³ vs. 74.6cm³, p = 0.08). Despite this, the volume of the lumpectomy cavity measured on computed tomography was smaller in patients receiving CTX (9.1cm³ vs. 16.8cm³, p = 0.02), as was the volume of the planning target volume (56.6cm³ vs. 79.9cm³, p = 0.02). Conclusions: Patients receiving CTX were at higher risk for local recurrence. However, as a result of lumpectomy bed contraction, these patients received a boost to a smaller volume than patients not receiving CTX. This finding is counterintuitive and supports re-evaluation of the optimal size of the boost field. In addition, these results may have implications for patients treated with partial breast irradiation.