Introduction

• Most neoplasias are specific organ related.
• Most abdominal organs have a specific tumor disorder either as a primary or secondary condition, but all are rare.
• The most common form of abdominal neoplasia is intestinal lymphoma, but this is still a rare condition.
• Some disseminated tumor types including lymphoma can involve many different organs or individual organs.
• Dissemination of tumors in the abdomen can be hematogenous or by direct extension Gastrointestinal: neoplasia.
• Secondary organ failure is common.

Pathogenesis

Etiology

• Tumor transformation occurs when progressive genetic mutations in cells allow for dysregulation of replication and cellular survival.
• Some are rapid and progressive, eg multicentric lymphoma, mesothelioma, dysgerminoma and malignant melanoma, while others are slower, eg granulosa cell tumor Ovary: neoplasia - granulosa / theca cell, lipoma Abdomen: lipoma - pedunculated) and some are asymptomatic.
• Functional tumors (pheochromocytoma Pheochromocytoma of the adrenal medulla and granulosa cell tumors have clinical effects relating to their secretory hormones.
• The majority of pheochromocytomas are believed to be asymptomatic, diagnosed only at post-mortem examination performed for other reasons. These are likely to be non-functioning tumors that are not secreting hormones.
• Primary liver tumors are rare, but metastatic disease to the liver is not uncommon, eg up to 40% of lymphoma cases will have infiltration of the liver.
• Splenic tumors may be primary or metastatic. Lymphoma and melanoma are the most common metastatic splenic tumors in the horse.
• Pancreatic neoplasia is very rare in horses, with variable and inconsistent clinical signs reported, and diagnosis only confirmed at post-mortem examination to date (adenocarcinoma and adenoma).

Predisposing factors

General

• Retained testicles are inclined to produce testicular tumors, as the increased temperature within the abdomen predisposes to malignant transformation of cells.
• Female horses are liable to granulosa cell tumors.
• Age is a common factor in melanoma and lipoma.
• Diffuse forms of lymphoma are more common in younger horses (<12 years) while focal forms are more common in older horses (>12 years) but there are exceptions to these rules.

Pathophysiology

• Functional tumors have direct effects relating to their secretions.
• Others have obstructive effects on the tubular structures or exert effects through space occupying expansion.

Timecourse

• Usually slow/insidious onset with progressive escalation and terminal rapid progression.
• Diagnosis often only made late in the course because of late presentation and difficulty in identification.

Diagnosis

Treatment

Prevention

Outcomes