Density of bone metastatic lesions increases after radiotherapy in patients with breast cancer

The aim of this study was to assess local response to radiotherapy (RT) in a quantitative manner by evaluating the bone density of metastases. Spinal and pelvic bone metastases in 44 patients with breast cancer who were treated between May 2010 and December 2016 were retrospectively assessed. Bone density values of irradiated and unirradiated bone metastases before, 1–3 months after, 4–6 months after, and 7–9 months after RT were compared. At each time point, mean bone density ± standard deviation values were measured in Hounsfield units (HU) from computed tomography (CT) scans. Student’s t-test was used for statistical analyses of the differences in bone density and for univariate analysis of the prognostic factors for differences in bone density at various time points after RT. Mean bone densities in irradiated and unirradiated bone metastases before RT were 297.31 ± 211.93 HU and 326.29 ± 228.61 HU, respectively. At the subsequent three time points examined, the mean bone density values in the irradiated and unirradiated bone metastases were: 61.97 ± 78.58 HU (P = 0.000) and 36.93 ± 52.49 HU (P = 0.001); 149.07 ± 133.27 HU (P = 0.000) and 68.40 ± 101.10 HU (P = 0.000); and 183.94 ± 168.30 HU (P = 0.000) and 88.21 ± 159.49 HU (P = 0.004), respectively, in each case. Patients receiving bisphosphonates exhibited greater increases in bone density in their metastases 1–3 months after RT (83.04 ± 82.18 HU vs 26.86 ± 60.55 HU, respectively; P = 0.044), whereas chemotherapy before RT was associated with significantly lower increases in bone density at the subsequent three time points [(37.53 ± 67.66 HU vs 93.63 ± 80.36 HU, P = 0.027), (99.30 ± 107.92 HU vs 180.24 ± 127.85 HU, P = 0.030), and (126.07 ± 141.77 HU vs 236.28 ± 158.22 HU, P = 0.024), respectively, in each case]. Comparing bone density values determined from CT scans appears to be a practicable and reproducible method for assessing local response to RT for bone metastasis of breast cancer. Increased bone density was also observed in the irradiated bone metastases.