Factors associating with bone-only metastasis in Chinese breast cancer patients in the absence of anti-HER2 targeted therapy.

Introduction: Bone-only metastasis (BOM) is a distinct clinical phenomenon in which cancer cells disseminate exclusively to the bone, without involvement of other distant organs. We investigated the factors associated with the BOM state vs. other states of metastasis in breast cancer patients with bone metastasis (BM) at their first relapse. The results could help tailor the screening and preventive therapy strategies for BM in breast cancer.

Methods: The study included 231 women who underwent mastectomy for primary unilateral non-metastatic breast cancer in 1997 or later and were subsequently diagnosed with BM at first relapse in 2008 - 2018 at the Fourth Hospital of Hebei Medical University in China. Factors such as patient age at primary breast cancer diagnosis, tumor clinicopathological characteristics, chemotherapy, radiotherapy, endocrine therapy (ET), time to progression (TTP), and others were analyzed. ET compliance was categorized from medication adherence. Multivariate logistic regression was used to estimate the odd ratio (OR) and p value.

Results: Only three (3.8%, 3/79) HER2-positive (HER2+) breast cancer patients (n = 79) used anti-HER2 targeted agents in the adjuvant setting. After excluding them, the remaining 228 patients were analyzed. They had an average age 47.3 years and median TTP 29.4 months at their first relapse. Overall, patients with BOM accounted for 26.8%. The BOM state was similarly in the hormone receptor-positive (HR+) patients (n = 182) and in the HR-negative (HR-) patients (n = 45) (28.6% vs 17.8%, p = 0.142). However, it was significantly lower in the HER2+ patients (n=76) than in the HER2-negative (HER2-) patients (n = 129) (13.2% vs. 31.8%, p = 0.003). Multivariate analyses showed that BOM was not associated with the HR+ (vs. HR-, OR 1.253, p = 0.723) and full ET compliance (vs. no/partial, OR 1.346, p = 0.545) status. Nonetheless, the BOM state was significantly associated with a lower chancer in the HER2+ patients d with a lower chance of BOM overall (OR 0.240, p = 0.008) and in the HR+ patients (OR 0.145, p = 0.005) but not in the HR- patients (OR 1.012, p = 0.991) than one in the HER2- patients. A lower chance of BOM state was also associated with TTP ≥ 24 months (p < 0.05). There were no other associated factors identified.

Conclusion: Differently from HR status and other clinicopathological factors, the HER2+ status is associated with a lower chance of the BOM state in breast cancer patients with first BM. Such association appears to be reflected in HR+ patients only.