Determination and clinical significance of bone pseudoprogression in hormone receptor-positive metastatic breast cancer

BACKGROUND: (99m)Technetium labeled methylene diphosphonate bone scans (BSs) are commonly used to monitor disease progression in bone for patients with metastatic breast cancer (MBC). However, new BS lesions may represent osteoblastic bone healing, which we now define as bone pseudoprogression. In this study, we aimed to assess the clinical significance and determination methods of bone pseudoprogression. METHODS: This retrospective analysis was conducted among 48 patients with hormone receptor-positive MBC treated with first-line endocrine therapy. Four months after initiating therapy, all the participants did not show extraosseous disease progression. Participants were divided into two groups according to the presence of new BS lesions. All the patients continued on treatment until explicit disease progression (extraosseous disease progression or progressive lysis on bone lesions). Explicit progression-free survival (PFS) and extraosseous objective response rate were analyzed between the two groups. RESULTS: New BS lesions were observed in 11 of 48 (22.9%) patients. All the new BS lesions appeared as osteoblastic bone lesions on computed tomography. For patients with new BS lesions, the median PFS was 26.57 months [95% confidence interval (CI) 15.46–37.68], which was similar to that (29.57 months; 95% CI 19.24–39.90) in patients without new BS lesions [hazard ratio: 1.098 (95% CI 0.482–2.503), p = 0.818]. Notably, 82.9% of patients without new BS lesions showed an extraosseous objective response, whereas 85.7% of patients with new BS lesions demonstrated an extraosseous objective response [odds ratio: 0.806 (95% CI 0.061–5.682), p = 0.999]. The median interval between bone pseudoprogression and true disease progression was 21.26 months (95% CI 10.11–32.42). CONCLUSIONS: Osteoblastic new BS lesions detected on follow-up BSs may represent bone pseudoprogression. Clinicians should raise awareness of bone pseudoprogression, thereby avoiding premature discontinuation of therapy and maximizing the opportunity to benefit from endocrine therapy. Due to the small sample size and retrospective nature of the study, large prospective clinical trials are needed to confirm our findings.