BI33 Expect the unexpected

Abstract Multiple primary malignant neoplasms (MPMNs) refer to the rare occurrence of two or more cancers in an individual at the same time. While uncommon, MPMN is observed in patients with cutaneous melanoma, with reported incidences ranging from 1.5% to 20%. Factors such as prolonged survival due to early detection through screening programmes, increased awareness of melanoma risk factors, and other malignancy risk factors may also contribute to the detection of MPMN. We present the case of a 66-year-old White man with a history of superficial spreading malignant melanoma diagnosed on his right upper back. He later developed generalized plaque psoriasis, complicated by psoriatic arthritis. After not responding to topical treatments and apremilast, and experiencing short-term remission with phototherapy, he began treatment with methotrexate in consultation with rheumatologists. He was aware of the immunosuppressive nature of this medication, which could potentially cause a relapse of his cutaneous melanoma, but it was necessary to manage both his skin and joint symptoms. Ten years after his melanoma diagnosis, he was incidentally diagnosed with invasive ductal breast carcinoma following the discovery of a lump on his left chest. He underwent a left mastectomy, and his sentinel lymph node biopsy was negative. He was prescribed tamoxifen for the following 5 years. The published literature has shown a statistically significant increased risk of breast cancer among female survivors of cutaneous melanoma, but data for male survivors are limited. A potential link between melanoma and breast cancer has been noted in patients with a strong family history of cancer, suggesting that genetic mutations, such as those in the BRCA2 and CDKN2A genes, may play a role in both malignancies. In this case, the patient’s daughter was diagnosed with breast cancer, requiring a lumpectomy shortly after his own diagnosis, which highlights the potential influence of familial factors. Methotrexate may be a contributing factor. Although no established link exists between low-dose methotrexate and an increased risk of breast cancer, it has been associated with a higher risk of melanoma. Additionally, immune dysregulation caused by autoinflammatory conditions such as psoriasis is known to increase cancer risk. Several registry-based and hospital-based studies have demonstrated a significant association between these malignancies, although the underlying causes remain unclear. Further research, particularly through comprehensive population-based epidemiological studies, is needed to explore the effects of genetic and biological factors in MPMN. This research is crucial for the development of early detection and prevention strategies. Finally, patients diagnosed with melanoma should continue regular surveillance, not only for potential melanoma recurrence but also to detect new primary cancers.