**3. Results**

All 40 controls had an intracytoplasmic immunoreactivity for SDHA and SDHB similar to that found in human tissues (Figure 1A,B).

In the 35 sections examined, there was a lack of both SDHA and SDHB immunoreactivity in four samples (11.4%) (Figures 2 and 3); 25 samples (71.4%) lacked SDHB immunoreactivity but had SDHA immunoreactivity. The lack of SDHB immunoreactivity correlated with both age (with younger animals predisposed (≤10 years old: 62.07%,>10 years old: 37.93%; *p* < 0.0421) and sex (male: 75.86%, female: 24.14%; *p* < 0.05).

**Figure 1.** (**A**) Succinate dehydrogenase A (SDHA) and (**B**) Succinate dehydrogenase B (SDHB) immunoreactivity in normal canine adrenal glands as controls.

**Figure 2.** Succinate dehydrogenase A (SDHA)/ Succinate dehydrogenase B (SDHB) immunoreactivity for all samples.

**Figure 3.** SDHA and SDHB immunoreactivity. (**A**,**C**,**E**)—SDHA immunohistochemistry. (**B**,**D**,**F**)—SDHB immunoreactivity. (**A**) and (**B**) represent a case with both SDHA and SDHB immunoreactivity; (**C**) and (**D**) have SDHA but lack SDHB immunoreactivity; (**E**) and (**F**) represent a case lacking both SDHA and SDHB immunoreactivity. SDHA—Succinate dehydrogenase A, SDHB—Succinate dehydrogenase B, Original objective 40×.

Out of the 34 cases with associated signalment and clinical information, 12 were from females and 24 were from males (Table 1). The mean patient age was 10.4 years (standard deviation: 2.5 years). Out of 23 samples that had a clinical evidence of invasion, 19 (82.6%) lacked SDHB immunoreactivity. Of these, 14 samples (73.6%) had a vascular invasion (caudal vena cava, phrenicoabdominal vein, and/or renal vein), while two out of these 19 (10.53%) metastasized to the renal vein or regional lymph node. Out of the 12 samples with no reported invasion, 10 (34.84%) lacked immunoreactivity to SDHB. However, we lacked an intraoperative surgical report for four of these samples, which means that these may have an invasion that was not stated in the sample submission form. Two samples out of these 12 described adhesion to the omentum, splenic vessels, apex of the left pancreatic limb, and/or the left kidney. Out of the five samples that had both SDHA and SDHB immunoreactivity, three (60%) also had an invasion. There was no significant correlation between invasions and the immunohistochemical status, age, sex, breed, or animal weight.


**Table 1.** Immunohistochemical findings and clinical information for all cases.

SDHA—Succinate dehydrogenase A, SDHB—Succinate dehydrogenase B, M—Male, F—Female.
