Local infiltration into neighboring tissue
- Anal cancer develops from basal epithelial cells in the anal canal and often grows as an ulcerating tumor.
- It can spread circularly in the anal canal, grow up into the rectum, or out through the anus. When there is infiltration through the anal musculature, the tumor can grow in the vagina. In men, it can in rare cases grow into the urethra or prostate.
- In some cases, the tumor grows in the lower rectum, without any distinct connection to the anal canal.
- Perianal cancer grows as a perianal ulcer or as hyperkeratotic tissue.
Spreading to regional lymph nodes (N-stage)
Lymph node metastasis occurs in up to 30% of the patients with anal cancer.
Spreading to lymph nodes can develop in two different ways:
- Anterior spreading in the groin via subcutaneous lymph pathways in the pelvic floor. This is more frequent in anal cancer than rectal cancer. Lymph node metastases in the groin can grow and attach to the underlying pelvic floor and to the femoral veins. These lymph nodes sit characteristically medial to the symphysis.
- Spreading upward to the perirectal lymph nodes (outside the mesorectum) and mesorectal lymph nodes (situated in mesorectal fat tissue around the rectum) and to lymph nodes along the internal iliac veins on the pelvic wall and to the presacral lymph nodes. This is especially the case with large tumors.
Hematogenic spreading occurs in less than 10% of the patients at the time of diagnosis, and most frequently for large primary tumors.
Metastasis to the:
- liver – most commonly affected organ
- lungs – second most commonly affected organ