Although multiple synchronous renal tumors are rare, the incidence of sporadic multifocal renal tumors at the time of diagnosis as reported in the literature is 4-20% [8-10]. This implies that observing multiple tumors in the kidney is relatively common. This is the first case to present synchronous clear cell RCC and PRNRP in the same kidney. PRNRP was first described by Al-Obaidy et al in 2019 . This study identified a PRNRP with a KRAS mutation and a co-occurring clear cell RCC with a PIK3CA mutation for the first time in the literature. The different mutation results of NGS analysis supports the biologically different diagnoses of these two synchronous renal masses.
KRAS mutations are frequently found in adenocarcinomas of the lung, colon, and pancreas [11, 12]. Varied papillary lesions, including intraductal papillary mucinous neoplasms of the pancreas and urothelial papillomas of the bladder, also have frequent KRAS mutations [13, 14]. KRAS mutations are rare events in kidney tumors, however, it has recently been found to be a characteristic feature of PRNRP as reported by multiple studies, including the current reported cases [15-17].
PIK3CA mutations are rare in clear cell RCC and are present only in 2-5% of tumors [18, 19]. PIK3CA codes for the catalytic subunit of phosphoinositide-3-kinase (PIK3), a key enzyme of the mTOR pathway; therefore, the mTOR inhibitor, everolimus, may be effective for this type of mutation.
The most common symptoms reported in RCC are hematuria in 90% of cases, flank pain in 19%, and mass effect in 14% . Our patient presented with the first two symptoms. Regarding aggressiveness, clear cell RCC presents the greatest malignant potential and a 5-year survival rate of 70%, while papillary and chromophobe RCCs are associated with less metastatic potential and an overall 5-year survival of 88% and 94% .
Radical nephrectomy is considered the standard procedure for treating malignant renal tumors. However, recent studies show that patients with sporadic single or multiple ipsilateral renal tumors may undergo nephron-sparing surgery, with oncologically comparable results with low morbidity and recurrence rates.
Awareness of the coexistence of multiple synchronous tumors of different pathologic neoplasms in the same kidney is important for managing such cases, and nephron-sparing surgery or active surveillance may be warranted for some renal masses . The fact that the pathological concordance rate is as low as 67.3% and the grade concordance rate is 62.5%  suggests that if a biopsy is indicated preoperatively, each nodule should be biopsied for diagnosis . Different tumors will have different prognoses and degrees of aggressiveness.
There were seven cases of multiple kidney masses in Pusan National University Yangsan Hospital from March 2010 to January 2020, and Table 1 shows the characteristics of these ipsilateral multiple renal masses according to subtype. While five cases involved multiple clear cell RCCs, one presented papillary and clear cell RCCs, whereas the other had papillary RCC with metanephric adenoma. Two reports of large numbers of patients concluded that 5-6% of multiple ipsilateral renal tumors develop a contralateral metachronous recurrence and this risk is 5 times that of patients with a sporadic single tumor [23, 24]. We suggest that multiple ipsilateral synchronous RCCs of different histologic subtypes need to be followed closely and operations for each mass are necessary. The frequency of clinical multifocality is consistent with reported local recurrence rates following partial nephrectomy. The impact of tumor multifocality on patient survival is controversial; however, each nodule should be evaluated for an accurate prognosis.
In summary, we report the first unusual case of unilateral synchronous PRNRP with a KRAS mutation (c.35G>T/p.G12V in exon 2) and clear cell RCC with a PIK3CA mutation (c.1624G>A/p.E542K in exon 10).