Case Report of Denovo Musculoskeletal Metastasis from Hormonal Positive and Her2 Negative Receiving CDK4 Inhibitor

Skeletal muscle metastasis from breast cancer is a very rare presentation. We reported a case with breast cancer metastatic to skeletal muscle, bone, peritoneum and lymph node at presentation detected in FDG PET-CT. A Core needle biopsy was performed from the left breast tail lesion. Immunohistochemistry showed hormonal positivity, Her2 negativity with low KI67. Patient was treated with combined CDK4 inhibitor and non-steroidal aromatase inhibitors with regressive course in the PET-CT after the 1st 3 months. More studies should investigate the prognosis and proper management of skeletal muscle metastasis in breast cancer.

The prevalence of metastases to skeletal muscle from post-mortem studies of patients with cancer ranges from 0.03% to 17.5%. For the patients who were treated from skeletal muscle metastasis, genital tumors (24.6%) were the most frequent malignancies metastasizing into the skeletal musculature, followed by gastrointestinal tumors (21.3%), urological tumors (16.4%), and malignant melanoma (13.1%). There is other rare tumor that may have muscle metastasis as bronchial carcinoma (8.2%), thyroid gland carcinoma (4.9%), and breast carcinoma (3.3%) [2]. There is no consensus on treating it as different entity but treating as other metastatic sites and according to the biological types. It is Journal of Biosciences and Medicines usually associated with poor prognosis [3].

Case Report
Our patient is 70 years old female patient with no previous medical history or comorbidities and she had not any history of regular medication intake. She month of treatment, she had neutropenia and anemia she received blood transfusion (250 cc/day for 2 days) and we had to delay Palbociclib for 1 week. In July 2019, she was improving clinically with no complaint and patient was assessed using PET-CT. It was done in comparison with the previous one that showed good therapeutic effect. She was advised to continue to be assessed after another 3 months. In October 2019, another assessment was done using PET-CT that showed sustained good therapeutic response. Her CA15.3 was 168 U/mL compared to 141 U/mL in September 2019. We advised to continue for another 3 months then to be assessed. In January 2020, CA15.3 was 406 U/mL and PET-Ct showed mild disease progression in the form of metabolic reactivation of the osseous lesions. She was shifted to Fulvestrant-Everolimus till June 2020 and CA15.3 was 1980 U/mL and was planned to start Chemotherapy using Paclitaxel, but her performance status was 2 so she was advised for supportive treatment and palliative care then unfortunately she died in July 2020 at her home.

Discussions
Skeletal muscle metastases are rare likely due to inability to remove lactic acid associated with angiogenesis from the microenvironment, the activation of lymphocytes and NK cells in skeletal muscles, and mechanical tumor destruction from motion [3]. The incidence of subclinical skeletal muscle metastasis has been reported to range from 0.2% to 17.5% in the autopsies which is considered higher than thought [4]. It is usually uncommon for breast cancer and if present it is always presented at disseminated stage [5]. It is usually presented as painful swelling at the involved muscle [6]. These metastases are very challenging to be year and our case also survived for 1 year and four months [9].
To our knowledge, all the patients have relapsed in the form of skeletal muscle metastases either alone or associated with other metastatic sites, and our patient was first diagnosed with metastatic breast cancer to skeletal muscle and bone.
The prognosis and appropriate treatment of skeletal muscle metastasis are currently uncertain. Further studies are needed to determine the prognosis and therapeutic strategies for skeletal muscle metastasis in breast cancer.

Consent
The patient's son (The judicial witness) gave his free well approval for informed consent for the case report to be published without any influences.