Bone Density As A Marker Of Response To Radiotherapy For Bone Metastatic Lesions In Patients With Breast Cancer And Clinical Prognostic Analysis For Patients With Bone-only Metastasis In Breast Cancer

?Objective?To assess local response to radiotherapy(RT)in a quantitative manner by evaluating bone density of metastases.The previous studies showed a low incidence of bone-only metastasis in breast cancer patients.The purpose of this study was to define the prognostic factors affecting survival following bone-only metastasis in breast cancer patients.?Methods?The first part: Spinal and pelvic bone metastases in 44 patients with breast cancer who were treated between May 2010 and December 2016 were retrospectively assessed.The primary endpoint was to compare bone density in the irradiated and unirradiated bone metastases before,1–3 months after,4–6 months after,and 7–9 months after RT.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.The second part: All women(n=96)diagnosed with bone-only metastasis from breast cancer between 2012-2015 at 307 Hospital of PLA were studied retrospectively.?Results? The first part: 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),while 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].The second part: The median time from the diagnosis of bone-only metastasis to the last follow-up or death was 55.2 [95% confidence interval(CI),29.4-60.6] months.The median time from the diagnosis of bone-only metastasis to the extra-bone metastasis or death was 22 [95% CI,15.6-28.4] months.Patients with Ki-67?14%(P=0.037)or radiotherapy(P=0.029)for bone metastasis had a longer extra-bone progression-free survival(EB-PFS),and patients with a solitary bone metastasis(P=0.003)or endocrine therapy(P=0.017)had a longer overall survival after diagnosis of bone metastasis(BM-OS).Among patients with hormone receptor(HR)+ tumors,longer recurrence-free interval(RFI)(P=0.032),solitary bone metastasis(P=0.004)and endocrine therapy(P=0.018)were significantly associated with longer BM-OS.?Conclusions?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.Bone density could be used to quantitatively assess the response to radiotherapy for metastatic bone lesions.Increased bone density was also observed in the irradiated bone metastases.Endocrine therapy could prolong overall survival after diagnosis of bone metastasis in patients with HR(+)tumors.Radiotherapy prolongs extra-bone progression-free survival in patients with bone-only metastases warrant further investigation.