Background Reduction mammoplasty or mastopexy is performed as an additional balancing procedure in patients with large or ptotic breasts who undergo breast-conserving surgery (BCS). Radiation therapy on breasts that have undergone surgery may result in changes in the volume. This study presents a comparative analysis of patients who received post-BCS balancing procedures to determine whether volume changes were larger in breasts that received radiation therapy than on the contralateral side. Methods Thirty-six participants were selected among patients who received BCS using the inverted-T scar technique between September 2012 and July 2017, were followed up for 2 or more years, and had pre-radiation therapy computed tomography images and post-radiation therapy images taken between 12 and 18 months after completion. The average age of the participants was 53.5 years, their average body mass index was 26.62 kg/m2. Results The pre- and post-radiation therapy volumes of the breasts receiving BCS were 666.08±147.48 mL and 649.33±130.35 mL, respectively. In the contralateral breasts, the volume before radiation therapy was 637.69±145.72 mL, which decreased to 628.14±166.41 mL after therapy. The volume ratio of the affected to the contralateral breasts was 1.05±0.10 before radiation therapy and 1.06±0.12 after radiation therapy. Conclusions The ratio of the volume between the two breasts immediately after surgery and at roughly 18 months postoperatively was not significantly different (P=0.98). For these reasons, we recommend a simultaneous single-stage balancing procedure as a reasonable option for patients who require radiation therapy after BCS without concerns regarding volume change.