DISCUSSION
KA is generally characterized by the rapid growth of dome-shaped nodules with a central plug of keratin3. KAs have been considered a variant of cutaneous squamous cell carcinoma (cSCC) and are often reported as KA-type cSCC1. The literature on purported metastases from KAs is rare. Jacqueline et al. analysed 445 cases of KA from 113 published articles and found that none of the cases resulted in death or distant metastases4. KA usually invades the surrounding tissues and does not metastasize. In the present case, two nodules were observed arising from the bridge of the nose at 1.0 cm and 3.4 cm away from the primary focus region. This is the first report of skip metastasis of KA on the nose.
The treatment of KA includes chemotherapy, surgery, photodynamic therapy, and others. Surgical resection is the first-line treatment for KA5 as most KA patients prefer it. In consideration of the cosmetic request of the present patient, however, radiotherapy was applied. The literature has a few reports on radiotherapy for KA, with studies suggesting radiotherapy as a highly effective treatment for KA with excellent cosmetic outcomes5, 6. There is no uniform standard for the total, fractional dose, radiation dose, and time of radiotherapy for KA. However, considering the similarity between KA and SCC, we applied a high dose of 45.6 Gy of radiotherapy for this tumour, which resulted in complete remission of the primary lesion.
The danger triangle of the face consists of the area from the bridge of the nose to the corners of the mouth7. In this region, the veins are valveless and communicate freely through the angular vein with superior ophtalmic8. Thus, infection in this area can spread easily without blocking the valves, especially with an external force. In our case, the patient occasionally squeezed the lesion both before and during the treatment. One month after the radiotherapy was completed, two rapidly growing nodules were observed near the original site of KA, and a biopsy confirmed the same pathology as earlier. It is highly suspected that the external pressure applied by the patient forced the tumour cells to gradually grow through the valveless veins of the facial veins.
KA tends to invade the surrounding skin area, but it may present as adjacent tissue skip metastasis under a trigger by an external force in the valveless area of the danger triangle of the face. Nodules present in this area should be treated with vigilance and any external force should be forbidden.