Are All Cancers Tumours?
No, not all cancers are tumours—some cancers exist as dispersed malignant cells without forming discrete masses, most notably liquid cancers like leukemias and certain lymphomas.
Understanding the Distinction
The term "cancer" refers broadly to malignant neoplastic processes, while "tumour" specifically describes an abnormal mass or growth of tissue. This distinction becomes clinically important when considering different cancer presentations:
Solid Tumours vs. Non-Tumour Cancers
Most cancers do form solid tumours, including carcinomas (epithelial origin), sarcomas (mesenchymal origin), and many lymphomas that present as discrete masses 1.
Hematologic malignancies often do not form tumours in the traditional sense—leukemias circulate as individual malignant cells in blood and bone marrow without creating a discrete mass 1.
Some lymphomas present without solid masses, existing as dispersed malignant lymphocytes rather than forming cohesive tumour structures 1.
Clinical Classification Framework
The medical literature consistently categorizes malignancies by their tissue of origin and growth pattern:
Carcinomas (>90% of pancreatic cancers, for example) form solid tumours with characteristic features including lymph node metastases, perineural infiltration, and vascular invasion 1.
Non-epithelial ovarian cancers including germ cell tumours and sex cord-stromal tumours account for approximately 10% of ovarian malignancies and present as discrete tumour masses 1.
Cancers of unknown primary (CUP) represent 3-5% of malignancies and by definition present with metastatic tumour deposits, though the primary site remains unidentified 1.
Important Pathologic Considerations
Benign tumours exist and are not cancers—they do not metastasize and lack the invasive characteristics of malignancy, though they can still cause significant morbidity through local compression 2, 3.
The distinction matters for treatment: immunohistochemistry must be applied to exclude chemosensitive tumours like lymphomas from solid tumour categories, as treatment approaches differ fundamentally 1.
Morphologically benign lesions can rarely metastasize, blurring traditional boundaries between benign tumours and cancers 4.
Clinical Pitfall to Avoid
Do not assume all malignancies require biopsy of a mass—hematologic cancers are diagnosed through blood smears, bone marrow aspirates, and flow cytometry rather than tissue biopsy of a discrete tumour 1. Conversely, when evaluating solid organ malignancies, tissue diagnosis from the tumour mass is essential for proper classification and treatment planning 1.