Skin malignancy most commonly arises from the keratinocytes of the epidermis. Other cell types can also cause cancer, such as melanocytes or Merkel cells.
The most common malignancy of humans; the incidence is hard to estimate as it is usually not considered a reportable cancer. It typically occurs in sun exposed skin; other aetiological factors include immunosuppression and familial germline mutations (eg. Gorlin syndrome). A number of basal cell carcinomas show mutations in the Sonic Hedgehog Pathway, usually in the PTCH1 gene; this allows unrestricted activity of SMO1 which promotes the usual suspects. Common features among the basal cell carcinoma subtypes are small basaloid cells with scant cytoplasm with a loose fibromyxoid stroma. There are several subtypes:
- Nodular BCC (60-80%): Cells are arranged in lobules or nests with peripheral palisading with some mitotic activity. The lobules are separated by fibromyxoid stroma.
- Superficial BCC (10%): Cells extend into the dermis from the epidermis. There is often a cleft seen on histopathology between the malignancy and the dermis. There is peripheral pallisading along the tumour border.
- Micronodular BCC (5-10%): Cells are arranged in small nodules with a collagenous stroma. They often extend outside of the macroscopic lesion.
- Infiltrative BCC (5%): Cells are arranged in cords and strands instead of nodules, and often spread beyond the macroscopic lesion which is often pale and slightly raised.
- Morphaeic BCC (< 5%): Shares the appearance of the infiltrative type but with a sclerotic stroma.
Immunohistochemistry is negative for S100 and positive for keratin; cytokeratin 20 is usually absent in contrast to Merkel Cell Carcinoma where it is positive.
- A: Breast Malignancy
- B: Thoracic Tumours
- C: Head and Neck Malignancy
- D: Skin Malignancy
- E: Male Reproductive System
- F: Female Reproductive System Malignancies
- G: Urinary Tract
- H: Gastrointestinal Malignancy
- I: Central Nervous System
- J: Haematological Malignancy
- K: Sarcoma
- L: Paediatric Tumours
- M: Endocrine Malignancy
- N: Non Malignant Disease
- P: Effects of Radiation on Normal Tissue
- Q: Diagnostic Dilemmas