Testicular cancer. Light micrograph of a section through a seminoma tumour in tissue from a testis (testicle). Seminomas are malignant (cancerous) tumours that tend to spread to lymph nodes via the lymphatic system. They are the most common form of testicular cancer and occur most often in men between the age of 30 to 45 years. Here the characteristic tightly packed polygonal cells can be seen with clear cytoplasm (white) and round central nuclei (dark spots).