Issues and challenges associated with classifying neoplasms in percutaneous needle biopsies of incidentally found small renal masses

Percutaneous needle core biopsy has become acceptable for classifying renal tumours and guiding patient management in the setting of an incidentally-detected small renal mass (SRM), defined as an asymptomatic, non-palpable mass <4 cm in maximum dimension. Long-held concerns preventing the incorporation of biopsies into routine patient care, including the perception of poor diagnostic yield and risks of complications such as bleeding or biopsy tract seeding, have largely been disproven. While needle biopsies for SRMs have traditionally been performed in academic centres, pathologists based in non-academic centres can expect to encounter these specimens as urologists and/or interventional radiologist trainees complete their training programs and begin work in non-academic centres. This review covers the rationale for performing these biopsies, the expected diagnostic yield, relevant differential diagnoses and an approach to classifying SRMs based on limited samples as well as the use of immunohistochemical (IHC) staining panels to aid in this process. There is also an undeniable learning curve for pathologists faced with reporting these biopsies and a number of issues and potential pitfalls attributable to sampling must be kept in mind by pathologists and clinicians alike.