Aggressive systemic mastocytosis (SM) should be considered when older patients develop unexplained osteoblastic bone lesions (areas of dense bone growth) along with fatigue, weight loss and low blood counts, especially when no primary cancer is found, according to a case report published recently in the European Journal of Case Reports in Internal Medicine.
Early recognition of this rare blood disorder can prevent unnecessary oncology testing and allow timely treatment that may improve symptoms and stabilize blood counts.
“ASM [aggressive SM] should be considered in the differential diagnosis of diffuse osteoblastic bone lesions, especially in patients presenting with constitutional symptoms and unexplained cytopenias in the absence of an identifiable primary malignancy,” stated this study’s authors.
The case report describes an 83-year-old woman who was admitted to the hospital after two months of worsening fatigue, unintentional weight loss, low-grade evening fever and night sweats. She had a history of degenerative joint disease and needed partial help with daily activities.
Read more about testing and diagnosis of SM
On exam, she appeared pale with a distended but non-tender abdomen. Laboratory testing revealed pancytopenia — low levels of red blood cells, white blood cells and platelets. Her kidney and liver function were normal, and C-reactive protein was 3.9 mg/dl. A urinary tract infection caused by extended-spectrum beta-lactamase-producing Klebsiella pneumoniae resolved with antibiotics.
Imaging initially raised concern for metastatic cancer. A computed tomography scan showed dense, hardened areas throughout the spine and other central bones, along with enlargement of the liver and spleen and swollen lymph nodes near the liver.
This pattern of bone changes is very uncommon in SM and more often points doctors toward cancers such as breast, prostate or neuroendocrine tumors. Although bone involvement occurs in up to 90% of people with SM, it more typically appears as bone thinning, areas of bone breakdown or a mix of changes, rather than only dense, sclerotic lesions.
A biopsy of one of the bone spots in her pelvis helped clarify the diagnosis. The sample showed large numbers of abnormal mast cells crowding the bone marrow. Special laboratory stains confirmed these cells by detecting markers commonly found on mast cells, and a blood test showed an elevated tryptase level of 56.3 ng/mL, which meets World Health Organization criteria for SM. Although many SM cases are linked to changes in the KIT gene, including a mutation called D816V, no KIT mutation was found in this patient, a reminder that genetic testing can be negative and the disease can still be present.
The patient began cladribine with prophylactic acyclovir, fluconazole and trimethoprim-sulfamethoxazole. Her constitutional symptoms improved and her blood counts stabilized under hematology follow-up.
For patients, this case highlights the importance of thorough evaluation when scans suggest metastatic cancer but no primary tumor is found. Accurate diagnosis can spare emotional distress, avoid invasive cancer investigations and open the door to targeted treatment for a rare but manageable disease.
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