Background: A remarkable number of patients presents with multiple primary malignancies (MPM) over their lifetimes. In most cases inherited syndromes, iatrogenic, or viral factors are implicated, while in some cases it is not possible to ascertain a clear aetiopathogenesis.

Methods: Starting from a series of 315 patients with MPM, we focused our attention on those with extremely infrequent combinations of tumours. We retrospectively analysed patients’ characteristics, type of first and second tumour and the interval between the two tumours. We made a comparison between our own data and data from surveillance, epidemiology, and end results cancer registries, the largest global series on this topic.

Results: Six patients presented with unusual associations, namely, central nervous system (CNS)/colon, testis/stomach, colon/CNS, CNS/kidney, uterus/soft tissue, and bone/breast. The median age was 50.5 years at the diagnosis of second neoplasm and the male:female ratio was 1:1. All six patients underwent surgery for both tumours. The median interval between the first and the second tumour was 11.3 years (range 1–36 years). Five patients were given chemotherapy as adjuvant systemic treatment, and two of them with CNS tumours also received radiotherapy.

Discussion: We analysed the behaviour of these rare tumours as first and second neoplasms. More frequent combinations and possible aetiological factors were evaluated.

Conclusions: Follow-up for patients recovering from a first tumour must be strict, as there is the risk of developing MPM, even after a long time period. Advancement in biomolecular knowledge and cooperation among different specialists are strongly needed to reduce mortality related to MPM and to foresee their occurrence.