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BACKGROUND Balloon catheter–based accelerated partial breast irradiation (APBI) is an alternative to whole-breast external-beam irradiation during breast-conserving therapy (BCT) for breast carcinoma, but it is limited by the size of the segmental mastectomy cavity. There are scant data on the average or optimal volume of resection (VR) in BCT. The objective of the current study was to evaluate the percentage of patients who would be eligible for balloon catheter–based APBI based on the selection criteria of the American Society of Breast Surgeons and the surgical VR. METHODS The authors reviewed the medical records of 443 patients with ductal carcinoma in situ (DCIS) or invasive carcinoma treated with BCT. Patient treatment and pathologic data were analyzed to assess VR and eligibility for APBI. RESULTS BCT was performed for 178 patients with DCIS and 267 patients with invasive breast carcinoma. The majority of invasive carcinomas (63.3%) were infiltrating ductal carcinomas. The median overall lumpectomy volume was 67.61 cm3, with no significant difference between DCIS and invasive carcinoma (P > 0.05). Although the majority (62.9–82.0%) of patients met the individual selection criteria for APBI, only 27.4% of the cohort was found to be eligible for any type of APBI when the selection criteria were considered together. Based on VR, only approximately one-half of the patients initially eligible for APBI would be candidates for immediate balloon catheter–based APBI using the 70 cm3 balloon device (13.3%). However, with the new, larger 125 cm3 balloon device, approximately three-fourths of patients initially eligible for APBI would be eligible for balloon catheter–based APBI at the time of the initial surgical procedure (20.7%). Although not evaluated in the current study, shrinkage of the lumpectomy cavity with time may increase the number of patients eligible based strictly on VR criteria. Patients with a very large VR (≥ 125 cm3) were more likely to have invasive carcinoma (P = 0.02; hazard ratio [HR], 7.4) and tumors ≥ 5 cm on final pathology (P < 0.01; HR, 22.0). CONCLUSIONS Approximately one-fifth to one-fourth of patients presenting for BCT may be eligible for balloon catheter–based APBI according to accepted national guidelines and VR. VR must be considered when selecting patients for balloon catheter–based APBI, because a minority of patients will have a lumpectomy cavity that exceeds the size limit of the current balloon device. Cancer 2004. © 2003 American Cancer Society.