This study showed that all 140 complications detected after the biopsy resolved or relieved, and 8 new complications (i.e., 6 small hematomas, 1 pseudoaneurysm, and 1 AVF) occurred on the initial follow-up CT or MRI. The initial follow-up image with the hematoma was performed at an earlier time, with a biopsy performed on a larger renal mass. The mean diameter of the renal masses did not significantly differ between the pre-biopsy and initial follow-up imaging; however, three masses (1.6%) shrank by more than 25% after the biopsy.

A high level of safety must be ensured for aggressive renal mass biopsies. In a systematic review of 5,228 patients, dissemination was observed in only two cases, and other complications were mainly back pain and hematoma/hematuria, most of which did not require any treatment [5]. The most common complication of a biopsy is hematoma, and its reported frequency varies (i.e., 8.5–67.9%) [8,9,10]. The CT was more sensitive than the US for detecting small and post-biopsy hematomas [11]. Our initial follow-up imaging showed six small new hematomas. These might not be visible on US or were negligible immediately after the biopsy and may have subsequently enlarged. Most biopsy-related hematomas are minor and asymptomatic; therefore, follow-up imaging may not usually be possible. In our study, such small hematomas spontaneously improved without treatment (hematomas were less likely to be observed over time), suggesting that strict imaging follow-up may not be necessary.

Pseudoaneurysms and AVF are rare vascular complications of renal mass biopsies. Pseudoaneurysm is generally related to renal parenchymal biopsy, nephrectomy, renal transplantation, or percutaneous procedures [12]. Renal AVFs can also result from renal parenchymal biopsy, trauma, inflammation, surgery, masses, and atherosclerosis [13]. Sosa-Barrios et al. reported that observation using color Doppler US after renal parenchymal biopsy of native kidneys showed AVF in 5.2% of patients, and 95% of them were asymptomatic [14]. The gold standard for diagnosis is angiography [15], and only one of our three AVFs could be detected on dynamic CT. In patients with renal AVF, possible symptoms include hematuria, hypertension, flank pain, vascular murmur in the renal arteries, heart failure, and decreased renal function due to hemodynamic changes [13, 14]. If patients are asymptomatic after renal mass biopsy, no follow-up imaging (e.g., color Doppler US and angiography) will be performed to diagnose the pseudoaneurysm and AVF, and it may be difficult to detect these completely on CT and MRI. All three patients with AVF were asymptomatic, and it is possible that there were other unnoticed AVFs. In our study, the frequencies of pseudoaneurysms and AVF were 1.1% and 3.3%, respectively, in patients who underwent angiography. More AVFs could have been detected if all patients had undergone angiography, because only 48.4% (92/190 patients) underwent angiography after the biopsy. In addition, it is important to perform color Doppler US for the diagnosis of renal AVF before angiography.

In our study, none of the 190 masses were enlarged (> 25%) on the initial follow-up imaging within 90 days. One active surveillance from multicenter research showed that the mean annual RCC growth rate for the entire cohort was 0.25 cm/year: masses that are ≥ 2.45 cm in the largest diameter at diagnosis grow faster than smaller masses [16]. The incidence of spontaneous regression of primary RCC is unknown; however, some English case reports have been published [17,18,19]. Hypothetical mechanisms for its spontaneous regression were suggested as following: humoral, immunological, and vascular factors (e.g., autoinfarction) [17]. The incidence of mass shrinkage after renal biopsy is unknown and was 1.6% (3/190 masses) in this study. One of the three masses showed a reduced contrast effect. Possible reasons for renal mass shrinkage after the biopsy include the follows: i) the mass is an inflammation-related change, not a neoplasm; ii) biopsy causes injury to feeding arteries resulting in ischemia/infarction; or iii) spontaneous regression of the mass.

This retrospective study conducted at a single institution has several limitations. First, the timing of the initial imaging follow-up was not uniform. Second, the protocol for the initial imaging follow-up was not standardized (e.g., CT or MRI or plain, contrast-enhanced, or dynamic study). Prospective follow-up with a more unified protocol may resolve these limitations; however, excessive imaging follow-up would be unnecessary because most complications are minor, asymptomatic, and improve spontaneously. Third, unenhanced CT or MRI may have overlooked pseudoaneurysms and AVFs. Finally, although 92 angiographies showed 1 pseudoaneurysm and 3 AVFs, angiography was not performed before the biopsy. Therefore, biopsy-related changes were not accurately assessed.

In conclusion, the initial follow-up imaging after the biopsy of small renal masses showed improvements in most of the complications, a few new complications, and an unchanged mass diameter.