{'fraction_non_alphanumeric': 0.026394721055788842, 'fraction_numerical': 0.00599880023995201, 'mean_word_length': 4.541528239202658, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Tumor-to-Tumor Metastasis in the Central Nervous System: A 21-Year Single-Institutional Experience


 
 
 Tumor-to-tumor metastasis (TTM) has two distinct morphologic appearances. In strict terms, it is a phenomenon where one tumor (donor tumor) metastasizes to another tumor (recipient tumor). Another pattern of TTM is collision tumor, where two tumors occur in immediate proximity with infiltration at the border between the two. TTM is a rare event, particularly in the central nervous system (CNS). Herein, we present a series of cases from our institution encountered in the past 21 years.
 
 
 
 The institutional archives were searched for CNS tumors that bear metastatic deposits or are collision tumors. The hematoxylin and eosin and any available immunohistochemical slides were reviewed. Relevant demographic and clinical data were obtained from patient medical records.
 
 
 
 Four cases were identified. The findings are summarized in the Table. The median age at presentation was 67 years (range, 60-74 years). Three patients were male. All metastases were carcinomas. Three cases were TTM in strict terms and one was a collision tumor. All TTM cases were an unexpected finding with no prior clinical or radiological suspicion for primary or metastatic neoplasm. In the collision tumor case, the metastatic tumor was an expected finding and the primary CNS tumor was an incidental one.
 
 
 
 TTM in the CNS is a rare but important entity to recognize, as it often represents the presentation of an unknown primary malignancy. Further workup should aim at identifying the primary source of the malignancy, which warrants clinicopathologic correlation.
