Cytology Misses Neoplasia Calls in Up to 45% of Canine Splenic Masses

Cytology and histopathology agreed on whether a splenic mass was neoplastic in only 55% to 67% of 33 dogs, according to a retrospective study published in JAVMA (doi:10.2460/javma.26.01.0006). In 24% of dogs, the two diagnostics outright disagreed.

The authors, led by Janet Grimes, DVM, MS, DACVS, a professor of small animal surgery at the University of Georgia College of Veterinary Medicine, reviewed medical records from 33 dogs that had a splenic mass or nodule plus both cytologic and histopathologic evaluation within 90 days of each other. Cytology came from ultrasound-guided fine-needle aspirates; histopathology came from splenectomy or necropsy specimens.

The agreement range is wide because 7 dogs received no definitive cytologic diagnosis, so agreement could not be fully evaluated for them. Even at the top of the range, the numbers sit below what many clinicians might assume from a screening test — and they reiterate that histopathology remains the gold standard for a definitive diagnosis.

What changes at the exam table?

The practical takeaway splits by lesion size, Grimes said. For dogs with larger non-ruptured masses, splenectomy is indicated regardless of cytology, because the rupture risk alone justifies surgery even for benign lesions.

For smaller nodules, ultrasound-guided fine-needle aspirates can still guide decisions. If cytology reveals neoplasia, splenectomy is indicated. A non-neoplastic result — for example, extramedullary hematopoiesis (EMH) — does not rule out underlying malignancy, so Grimes recommends active surveillance rather than reassurance:

• Static nodule: continue monitoring with ultrasound.
• Regressing nodule: consistent with EMH or nodular hyperplasia.
• Growing nodule: recommend splenectomy.

A growing nodule may still be benign, but growth raises concern for hemangiosarcoma or eventual rupture, Grimes said. Dogs diagnosed with hemangiosarcoma at lower stages survive longer than dogs diagnosed at higher stages, and splenectomy carries minimal long-term complications. "To me, the risk-reward balance is in favor of splenectomy in this case," she said.

Why did this study find lower agreement?

Previous studies reported cytology accuracy ranging from 59% to 92% against histopathology for splenic disease — but those studies included dogs with diffuse splenomegaly. This study deliberately excluded splenomegaly and focused solely on focal masses and nodules.

That distinction matters. "Including cases of diffuse splenomegaly tends to increase the accuracy of cytology by increasing the probability of aspirating the active disease process," Grimes said. The disease processes behind diffuse splenomegaly may also be more amenable to cytologic diagnosis. Even so, the study's agreement rates fell within the previously published range despite the narrower case definition.

Can cytology identify the tumor type?

Beyond the neoplastic-versus-non-neoplastic question, the study compared specific neoplasia diagnoses. The tumor type — sarcoma versus lymphoma — differed between cytology and histopathology in 1 dog.

The mechanism is straightforward: cytology samples a small portion of the lesion and loses tissue architecture, which gives histopathology a substantial diagnostic advantage. Grimes advised counseling owners that clinical decisions rest on the best available presurgical information, but the final diagnosis and long-term prognosis may change once the entire tissue structure is examined.

What are the study's limits?

Grimes identified three constraints. First, her institution does not routinely aspirate ruptured splenic masses, so the findings apply only to non-ruptured lesions. Second, 18 of the 33 dogs had multiple nodules, and retrospectively matching the aspirated nodule to the corresponding histopathology slide introduces potential discrepancy. Third, although slide review by a pathologist strengthened the study, the archived slides typically represented the most diagnostic slide, which may not reflect the entirety of the lesion.

The bottom line for practice

"A neoplastic cytology result is somewhat reliable for guiding surgical intervention, a non-neoplastic result should not provide false reassurance," Grimes said. Cytology remains a valuable initial screening tool, but histopathology following splenectomy remains the definitive route to diagnosis in dogs with splenic masses — a conclusion clinicians can expect future focal-lesion studies to test against larger sample sizes.