44 New research has found that several rare forms of cancer carry a surprisingly high familial risk, even though scientists have yet to pinpoint consistent genetic explanations. The findings, drawn from Sweden’s national cancer registry and supplemented by a wide-ranging review of existing genetic studies, suggest that subtle genetic influences may be at play in cancers of the oesophagus, stomach, small intestine, testis, thyroid, and bone. The findings were published in the journal Hereditary Cancer in Clinical Practice. The analysis shows that people with a close family member who has had one of these rare cancers face a two to sevenfold increased risk of developing the same cancer themselves. This contrasts with more common cancers like breast, prostate, and colorectal, where familial risks are comparatively lower despite more attention and better-established genetic markers. Testicular cancer, for example, shows a fivefold familial risk, yet large studies have failed to uncover high-risk mutations commonly seen in other hereditary cancers. Instead, the pattern appears to reflect a polygenic model, where hundreds of small-effect genetic variants work together to increase risk. In one UK-based study, more than 80 percent of familial testicular cancer cases were found to be linked to this type of polygenic influence. Similarly, non-medullary thyroid cancer presents a strong familial signal but shows few cases within extended families. It has been associated with a range of rare syndromes and genes, although no dominant genetic variant has been identified that could explain the high familial clustering seen in population data. Perhaps the most striking finding relates to small intestinal carcinoid tumours, where the familial risk between siblings was found to be among the highest recorded in the Swedish registry. Yet even in this context, genetic sequencing has failed to uncover a consistent pattern of inherited mutations. Some individual gene variants, such as mutations in the IPMK or MUTYH genes, have been proposed, but these appear to account for only a fraction of familial cases. The study also reviewed stomach and oesophageal cancers, where familial risk is evident but more modest. Here, dominant environmental factors such as infection and lifestyle complicate efforts to uncover germline genetic influences. The shifting epidemiology of these cancers, particularly in the case of oesophageal adenocarcinoma, further complicates genetic research. Bone cancers show similarly elevated familial risks, particularly in younger individuals, but research in this area is hampered by the low number of cases and the diversity of tumour types. Some rare inherited syndromes are implicated, including those involving TP53 and BRCA1, but once again these explain only a small portion of cases. The overall picture points to a complex interplay of factors in rare cancers. While familial patterns are strong, they often do not follow the typical paths seen in hereditary conditions caused by high-risk mutations. Instead, a combination of rare and common variants, perhaps compounded by shared environmental exposures, may underlie the inherited risk. As such, the authors suggest that future studies should focus on large-scale international collaborations to improve genetic discovery in these understudied cancers.