Splenic nodules and tumors in dogs are two different pathological changes with some differences. Simply put, splenic nodules are usually caused by interruption of the nutrient supply in the spleen, while tumors are abnormal proliferation of spleen cells.

Splenic nodules mainly appear as masses of different sizes in the spleen, sometimes caused by inflammatory reactions or fibrosis. Common causes include splenic congestion, inflammation, and connective tissue lesions. Usually, splenic nodules are small and uniform, and do not show abnormal growth trends.

In contrast, a tumor is an abnormal proliferation of spleen cells that forms a mass. The most common tumor in the dog's spleen is angiosarcoma or sarcomatoid metastatic tumor. Tumors are often diverse in size, shape, and rate of growth and may be single or multiple nodules.

The most reliable way to differentiate between splenic nodules and tumors is to perform histological examination, including tissue sections and cytology. This allows the nature of the lesion to be determined for further treatment and management.