2995 - The Tumor Bed Size Change after Surgery of Breast Cancer Patients Undergoing Breast Conserving Therapy

Purpose/Objective(s): To evaluate the volumetric change in the postoperative tumor bed right after breast conserving therapy (BCT), before, and during radiotherapy (RT), in patients with breast cancer in the Japanese surgical way; suturing of the breast cavity, where approximately the outer one-third of the mammary gland on the skin side would be opposed.

Materials/Methods: Patients with breast cancer who were planned to undergo BCT at our institution were eligible. Computed tomography scans (CTs) of the breast were obtained on the next day of BCT, before the start of RT, and before the tumor bed boost. Four surgical clips were placed around the tumor bed after tumor excision (on the cranial, caudal, medial, and lateral sides of the excised mammary gland, on the anterior surface of the pectoralis major muscle). We created a sphere employing these four clips and calculated the volume of these spheres (tumor beds) on each CTs and compared the size. Multiple variables, such as age, breast size, excised volume, the main quadrant where the tumor existed, the days from BCT to start of RT, and the days from start of RT to tumor bed boost, were examined for correlation with the volumetric change.

Results: Out of 46 eligible patients, 42 agreed to participate. All three scans could be obtained from 38 patients, and in three only the latter two scans and in one only the former two scans could be obtained. One patient underwent bilateral BCT, and therefore we could obtain two series from this patient. The median age was 51 (range 37-79) years. The median breast size was 370 (range 103-858) cm, and³ the median tumor excision volume was 126 (range 3-374) cm³ . The main quadrants where the tumor exited were A in 13, B in 7, C in 16, and D in 7. The median time from BCT to start of RT was 41 (range 22-295) days, and the median time from start of RT to start of tumor bed boost was 24 (range 18-43) days. The tumor bed volume from surgery to start of RT decreased at least 20% in 18 patients (46%) and at least 50% in 3 patients (8%). In 8 patients, the tumor bed volume increased. It increased at least 20% in 3 patients (8%) and at least 50% in 2 patients (5%). In no patient, the tumor bed volume from start of RT to start of tumor bed boost decreased 20% or more, but it increased in 30 patients. It increased at least 20% in 12 patients (29%) and at least 50% in 1 patient (2%). The tumor bed volume change from BCT to start of RT showed correlation with days from BCT to start of RT. Median days for the group with 20% or more volume reduction were 51.5, and the median days for the group with less than 20% volume reduction were 35 (p = 0.02). The tumor volume increase between the start of RT and the tumor bed boost showed correlation with the main quadrant where it existed. Percent of tumor beds with 20% or more increase was 17% in A, 29% in B, 6% in C, and 100% in D (p<0.01).

Conclusion: The tumor bed decreases after BCT to RT has an inverse correlation with days from BCT to the start of RT. However, during RT in most of the cases, the tumor bed volume increased and showed a correlation with the main quadrant where the tumor existed.