Multiple Testicular Tumors in Dogs: Understanding Collision Lesions and Their Clinical Relevance

Testicular tumors are among the most frequently encountered neoplasms in male dogs, ranking second only to skin tumors in prevalence^1,2. With reported occurrence rates ranging from 5–15% and reaching as high as 27% in some studies^1,3, these tumors represent a significant area of interest, not only in veterinary oncology but also in comparative pathology. While most clinicians are familiar with solitary lesions, the occurrence of multiple unilateral testicular tumors remains an under-recognized yet clinically important phenomenon. 

These tumors are typically seen in older dogs, particularly those above 10 years of age, with peak incidence between 7 and 12 years^1,4. Although most cases present unilaterally, bilateral involvement is not uncommon. Importantly, different histological tumor types may coexist within the same testis, a condition sometimes referred to as a collision tumor¹. However, the term “multiple unilateral testicular tumors” more accurately reflects the independent origin of each neoplastic population. 

From a histopathological standpoint, the three most common primary tumors include¹: 

• Seminomas (SEM), arising from germinal epithelium  

• Sertoli cell tumors (SCT), originating from supporting cells  

• Interstitial (Leydig) cell tumors (ICT), derived from endocrine tissue  

Each of these tumors differs in biological behavior, growth rate, hormonal activity, and metastatic potential^1,5. Clinically, this diversity translates into variable presentations, from asymptomatic masses to hormonally driven syndromes. 

A critical risk factor across species is cryptorchidism, with undescended testes showing a 14-fold increased risk of neoplasia¹. This is particularly relevant in both veterinary and human medicine, emphasizing the importance of early intervention. 

Frequency and Patterns of Multiple Tumors 

The current study reports that 8.28% of testicular tumors in dogs are multiple in nature, aligning with previously reported ranges of 4.62% to 18%^1,5. The most common tumor combination observed was seminoma with interstitial cell tumor (SEM–ICT), consistent across multiple studies^1,4,6. 

Other combinations include: 

• SEM–SCT  

• SCT–ICT  

• Rarely, all three tumor types (SEM–ICT–SCT)  

Interestingly, no mixed tumors with overlapping histological features were included in this analysis due to strict selection criteria, ensuring that each tumor type represented a distinct neoplastic process. 

From a clinical perspective, recognizing these combinations is critical. Imaging modalities such as ultrasound may reveal lesions with differing echogenicity, which should prompt targeted biopsy sampling to avoid diagnostic errors⁵. Missing a secondary tumor type could lead to incomplete diagnosis and suboptimal treatment planning. 

Malignancy and Biological Behavior 

One of the most clinically reassuring findings is that multiple tumors do not appear to exhibit increased malignancy compared to solitary lesions of the same type. Histological evaluation revealed no significant deviations in architecture or aggressiveness. Even in cases of seminomas with higher mitotic activity, vascular or lymphatic invasion was not observed, suggesting relatively low metastatic potential. 

However, all tumor types can induce pressure atrophy of the surrounding germinal epithelium¹, which may impact fertility and endocrine function. 

Comparative Insights: Relevance to Human Medicine 

Although testicular tumors are less common in humans, accounting for 1–1.5% of all male neoplasms, they remain the most common malignancy in young men aged 25–35 years. The coexistence of multiple tumor types within a single testis is rare but documented, with reported incidence ranging from 1.83% to over 20%¹. 

Dogs serve as a valuable comparative model due to similarities in tumor histology and shared environmental exposures⁷. This makes veterinary findings particularly relevant for translational research. 

Practical Takeaways for Clinicians 

For practicing clinicians, especially those involved in diagnostic pathology or surgical management, this study highlights several key considerations. The presence of multiple tumors should always be suspected in older dogs presenting with testicular enlargement. Imaging findings showing heterogeneous lesions must be interpreted carefully, and multiple sampling sites should be considered during biopsy or excision. 

Additionally, while malignancy may not be significantly higher, the complexity of diagnosis increases, reinforcing the importance of thorough histopathological evaluation. 

In conclusion, multiple unilateral testicular tumors are not rare and should be actively considered in clinical practice. Their recognition not only improves diagnostic accuracy but also enhances our understanding of tumor biology across species.  

References: 

1. Kuberka M, Prządka P, Dzimira S. Histological Analysis of Multiple Unilateral Testicular Tumors in Dogs. Life. 2025 Nov 19;15(11):1772. https://doi.org/10.3390/life15111772 

2. Baioni E, Scanziani E, Vincenti MC, Leschiera M, Bozzetta E, Pezzolato M, Desiato R, Bertolini S, Maurella C, Ru G. Estimating canine cancer incidence: findings from a population-based tumour registry in northwestern Italy. BMC veterinary research. 2017 Jun 28;13(1):203. https://link.springer.com/content/pdf/10.1186/s12917-017-1126-0.pdf 

3. Ciaputa R, Gach J, Baranowski K, Dzimira S, Janus I, Kandefer-Gola M, Żebrowski K, Nowak M. Prevalence of tumours and tumour-like lesions in domestic and exotic animals from Lower Silesia and its surrounds in Poland in 2014–2017. Journal of veterinary research. 2022 Aug 30;66(3):427. https://sciendo.com/2/v2/download/article/10.2478/jvetres-2022-0036.pdf 

4. Nascimento HH, Santos AD, Prante AL, Lamego EC, Tondo LA, Flores MM, Fighera RA, Kommers GD. Testicular tumors in 190 dogs: clinical, macroscopic and histopathological aspects. Pesquisa Veterinária Brasileira. 2020 Jul;40(7):525-35. https://www.scielo.br/j/pvb/a/RXCDQJzbjSbrZGYX3xY99Yf/?format=pdf&lang=en 

5. Cicero C, Bertolotto M, Hawthorn BR, Antonelli CT, Sidhu PS, Ascenti G, Nikolaidis P, Dudea S, Toncini C, Derchi LE. Multiple, synchronous lesions of differing histology within the same testis: ultrasonographic and pathologic correlations. Urology. 2018 Nov 1;121:125-31. https://arts.units.it/bitstream/11368/2940779/1/cicero%20urology.pdf 

6. Manuali E, Forte C, Porcellato I, Brachelente C, Sforna M, Pavone S, Ranciati S, Morgante R, Crescio IM, Ru G, Mechelli L. A five-year cohort study on testicular tumors from a population-based canine cancer registry in central Italy (Umbria). Preventive veterinary medicine. 2020 Dec 1;185:105201. https://cris.unibo.it/bitstream/11585/819159/5/Manuscript_IP_EM160420.pdf 
7. Sultan F, Ganaie BA. Comparative oncology: Integrating human and veterinary medicine. Open veterinary journal. 2018 Jan 30;8(1):25. https://pmc.ncbi.nlm.nih.gov/articles/PMC5806664/pdf/OpenVetJ-8-25.pdf