Two primary tumours metastasizing to the liver in a collision phenomenon

Collision tumour is the definition given to two originally separate primary tumours that developed by chance in close proximity or possibly merged as they arose at the same site. Although the liver is a frequent site of malignant diseases, only few cases of collision tumours have been described. The most common histological combinations are hepatocellular carcinomas (HCCs) with cholangiocarcinomas (CCs) or HCC with sarcomas. The combination of HCC/CCs has been further classified in three different groups, where the cells are either separated, collided or intermingled or according to different genetic patterns into collision tumours in which two independent neoplastic clones develop at close proximity, single clonal tumours with a homogeneous genetic background to both components (histological diversity is thus a manifestation of divergent differentiation potential of a single clone) and single clonal process in which genetic heterogeneity in the process of clonal evolution within the tumour parallels histological diversity, and tumours in this category are mainly composed of mosaics of closely related sub-clones. Interesting findings are also present with the other histological types. In a case of HCC with sarcoma, definite transitional features between the two separate pathological types exists, while in a case of HCC and neuroendocrine pattern, the two tumours are separated by fibrous bands, and even in areas where they collide, there is no transition or intermingling of cells. Another interesting characteristic is that the proportions of the different cell types can be quite variable within the tumour mass. In the case of HCC with sarcoma, the histology usually shows an extensive proliferation of sarcoma-like neoplastic cells besides a minute area of definite liver cell carcinoma in the centre of the tumour mass. However, for HCC combined with a rhabdomyosarcoma, postmortem findings generally demonstrate that recurrent liver metastases following surgery only consist of the hepatocellular component. Whereas collision tumour is a definition usually given to primary tumours, the ‘collision phenomenon’ involves the presence of a secondary deposit in the tumoural mass. It refers to the combination of a primary with a metastasis from another distant tumour or from two metastases originating from primaries metastasizing in the same site. Cases of collision phenomenon are rarer than collision tumours and have been reported in organs different from the liver such as the thyroid (thyroid primary + metastatic liposarcoma), lymph nodes (lymphoma primary + metastatic breast) and maxillary sinus (maxillary sinus primary + metastatic HCC) or two metastasis harbouring in a mesorectal lymph node draining cells from a rectal and a prostatic adenocarcinoma. To date, the existence collision phenomena composed of liver metastases with cells originating from two different primary cancers has still not been presented. A 66-year-old gentleman was diagnosed with a high-grade penile leiomyosarcoma 2.5 years ago, for which he underwent a partial amputation of the penis. At that time, his imaging had shown no abnormality and specifically, no liver or lung metastases. Eight months later, he developed a primary colorectal neoplasm (pT3 N1) for which he underwent a low anterior resection without adjuvant chemotherapy. One year following resection of his colorectal tumour (January 2007), he developed an isolated 4-cm liver metastasis in segment VIII of the liver. Although a second lesion had been noted in his lung, this had been stable for 20 months and was thought to represent an old scar. A biopsy of the liver lesion performed on April 2007 revealed the evidence of an unusual metastasis composed of a mixture of cells from a colorectal and sarcoma primary (Fig. 1). The histology from the lower anterior resection, penile biopsy, partial amputation of the penis and liver core biopsy was reviewed by the