Under-grading of <4 cm renal masses on renal biopsy

What's known on the subject? and What does the study add? It is well documented that biopsy of small renal masses is inaccurate and tends to under-estimate tumour grade compared with surgical specimens. To our knowledge there has not been a study showing grading discrepancy between biopsy and surgical excision in a large population-based cohort.

To determine whether differences exist in tumour grade between patients who undergo partial nephrectomy (PN) and those who undergo ablation for renal tumours.

Data was obtained using the Surveillance, Epidemiology and End Results database. Patients with solitary renal tumours of <4 cm treated with ablation or PN and with renal cell carcinoma (RCC) histopathology were identified. Tissue diagnosis in the ablation specimens was obtained from biopsy reports, whereas tissue from PN specimens was determined from surgical pathology. Variables analysed included: year of diagnosis, age, sex, race/ethnicity, marital status, population density, education, poverty level, and tumour size. Stacked bar graphs were created to compare the distributions of grade and histology between the groups. Multinomial logistic regression was used to determine factors independently associated with grade.

In all, 7704 (87.4%) patients underwent PN and 1114 (12.6%) underwent either radiofrequency ablation or cryoablation. The PN patients were younger at diagnosis (59 vs 68 years, P < 0.001), more likely to be married (70% vs 64%, P < 0.001), and had smaller tumours (2.4 vs 2.6 cm, P < 0.001). There were no differences in the distribution of histology between the PN and ablation groups. Tumour grade was significantly lower in tumours treated with ablation. Compared with grade 1 disease, those undergoing ablation were 30% less likely to have grade 2 (P < 0.001), 30% less likely to have grade 3 (P < 0.001), and 92% less likely to have grade 4 disease (P < 0.01) than those having PN.

There is a strong association between grade and treatment type in patients with small renal masses after controlling for baseline characteristics. As grade is determined by different methods, we think that this shows systematic under-grading in biopsy of small renal masses.