Bernese Mountain Dog Histiocytic Sarcoma

Colleague,

Your suspicion for disseminated histiocytic sarcoma (HS) in a Bernese Mountain Dog with this presentation is unfortunately very high on my differential list as well. This is a challenging disease, and your initial workup is pointing us in the right direction.

First, we need to confirm the diagnosis. The most direct path would be ultrasound-guided fine-needle aspirates of the splenic and/or hepatic lesions. Cytology can be highly suggestive, showing large, pleomorphic round cells, often with multinucleation and phagocytosis. However, definitive diagnosis and exclusion of other aggressive round cell tumors requires histopathology and often immunohistochemistry (IHC for CD18).

For complete staging, in addition to the abdominal ultrasound and bloodwork you've performed, I would strongly recommend:

Three-view thoracic radiographs: To screen for pulmonary nodules, which are common with disseminated HS.

Coagulation panel: Given the thrombocytopenia and splenic involvement, assessing for subclinical DIC is prudent.

Bone marrow aspirate: This could be considered to confirm systemic involvement, especially given the thrombocytopenia.

Regarding treatment, lomustine (CCNU) is indeed the standard of care. The typical protocol is 70-90 mg/m² PO every 3-4 weeks. It's critical to monitor for its two main toxicities:
* Myelosuppression: Profound and delayed neutropenia and thrombocytopenia are common. Check a CBC at nadir (day 7-10) and before each subsequent dose.
* Hepatotoxicity: This is cumulative and idiosyncratic. Monitor liver enzymes before every dose and consider concurrent use of a hepatoprotectant like SAMe or milk thistle.

Now, for the honest conversation about prognosis. Even with treatment, disseminated HS carries a grave prognosis. With lomustine, the reported median survival time is unfortunately only around 4-6 months. Without treatment, survival is typically measured in weeks. The goal of chemotherapy in this setting is not curative; it is to slow progression and provide a period of good quality of life. Palliative care with prednisone can offer short-term improvement but doesn't significantly alter the timeline.

This is a tough diagnosis for any owner. My approach is to present the lomustine protocol as the best option to gain some good quality time, while being very transparent about the expected outcome and the need for frequent monitoring.

Let me know what the cytology shows.

— Dr. Rachel Torres 🧫

Colleagues, the signalment and findings are indeed highly suggestive of disseminated histiocytic sarcoma. My focus here will be on ensuring we have a complete and accurate imaging-based stage, which is crucial for the prognostic and therapeutic discussion that Dr. Torres will lead.

For complete staging of suspected disseminated HS, a multi-modality imaging approach is the standard of care.

Abdominal Ultrasound: The reported hepatic and splenic masses are classic findings. A complete abdominal ultrasound for staging should meticulously evaluate all organs and, critically, all accessible lymph nodes. Pay close attention to the splenic, mesenteric, and medial iliac lymph nodes for enlargement or changes in architecture. Also, assess the renal cortices and adrenal glands, as these are other potential sites of involvement. Remember, while these findings strongly support HS, ultrasound cannot definitively differentiate it from other aggressive neoplasms like hemangiosarcoma or lymphoma without cytology or histology.

Thoracic Imaging: This is a non-negotiable step.

* Baseline: Three-view thoracic radiographs are the minimum standard for a metastatic screen. We will be looking for a diffuse, unstructured interstitial or nodular interstitial pattern, hilar lymphadenopathy, and potentially pleural effusion.
* Gold Standard: I strongly recommend a staging thoracic CT scan. The sensitivity for detecting pulmonary metastatic nodules is far superior to radiography. CT can identify nodules down to 1-2mm, whereas radiographs may not detect lesions smaller than 7-9mm. Given the aggressive nature of this disease, a negative set of thoracic radiographs does not rule out pulmonary metastasis, and CT provides the most accurate information for prognosis.

Other sites: While less common, HS can manifest in other locations. Carefully review the appendicular and axial skeleton included on the thoracic radiographs for any subtle lytic or proliferative bone lesions. If any lameness is reported, dedicated skeletal radiographs of the affected limb are indicated.

In summary, complete staging requires a thorough abdominal ultrasound and, ideally, a thoracic CT. This provides the most accurate assessment of disease burden.

I will defer to Dr. Torres for the discussion of chemotherapy protocols and prognosis based on staging, and to Dr. Vasquez for managing the patient's specific hematologic and biochemical derangements.

Dr. Vasquez, Internal Medicine here. This is a classic and heartbreaking presentation for a Berner, and you're right to have disseminated histiocytic sarcoma (DHS) at the very top of your list. My role here is to ensure we have a definitive diagnosis and the patient is stable before we hand off to oncology for treatment.

First, we need tissue. While the clinical picture is highly suggestive, initiating chemotherapy without a confirmed diagnosis is risky. Fine-needle aspirates of the liver and spleen are a good first step and may give us a tentative diagnosis if we see a clear population of malignant histiocytes. However, cytology can be ambiguous, and histopathology with immunohistochemistry (IHC) is the gold standard. Stains for CD18 are essential for confirming the histiocytic lineage. This can be achieved with ultrasound-guided trucut biopsies.

Before any invasive sampling, given the thrombocytopenia, I'd strongly recommend a coagulation panel (PT/PTT). We need to ensure the patient can safely tolerate a biopsy procedure. I'd also request a manual slide review of the CBC by a clinical pathologist to look for neoplastic cells in circulation and characterize the thrombocytopenia.

While DHS is overwhelmingly likely, my job is to ask, "What else could this be?" Anaplastic large cell lymphoma can present similarly, as can other aggressive metastatic sarcomas or carcinomas. Less likely, but still on the list, would be a severe systemic fungal infection (though the discrete masses make this less probable). The histopathology will differentiate these for us.

I'll defer to Dr. Lin for a detailed interpretation of the ultrasound findings and to Dr. Torres for the specifics of staging, lomustine protocols, and prognosis, as that is her area of expertise. My focus is on getting you that definitive diagnosis as safely and efficiently as possible.

Confidence Level: High confidence in this diagnostic approach.