True collision renal tumour of oncocytoma and papillary

Oncocytoma is the most common benign renal tumour. Its incidence is less common and represents 3 – 7 % of all renal tumours [1]. It originates from the intercalated cells of the collecting duct [2]. It occurs in people of all ages but most commonly in men above 50 [2]. In general, oncocytoma is associated with almost 100 % disease-specifi c survival rate [3]. Histologically, oncocytoma consists of round-to polygonalshaped cells with an abundant fi nely granular cytoplasm [4].

Renal tumours with hybrid features such as oncocytoma and chromophobe renal cell carcinoma are well documented in the literature [9], whereas collision renal tumours, defi ned as the coexistent but independent tumours that are histologically distinct [10], such as PRCC and oncocytoma, are very rare.
Herein, we present a case of a true collision renal tumour of an oncocytoma and a PRCC together with a review of the literature.

Case presentation
We present a 53-year-old male without signifi cant past medical history and no family history of renal cell carcinoma. As such, no genetic survey was carried out. He presented with painless macroscopic haematuria and nocturia. A digital rectal examination revealed a slight asymmetry of the prostate with no suspicion of malignancy. A urethrocystoscopy showed apart from a somewhat long supracollicular distance and an easily bleeding mucosa of the prostatic urethra, no pathological changes in the bladder´s mucosa. A CT-scan of the abdomen and thorax revealed 12×17×16 cm mass on the superior lobe of the left kidney, with a radiological preliminary diagnosis of a renal tumour with central necrosis. On the right kidney, a 1 cm lesion was described as a nodule of unclear nature. Several aortic and supradiaphragmatic nodes up to 1, 5 cm in diameter were suspected of being metastatic. With no signs of metastases to the lungs ( Figure 1).
The patient underwent a subcostal transperitoneal radical left sided nephrectomy. No sign of venous thrombosis was found. The procedure was completed successfully without complications, a 2 kg en-bloc specimen and additional adipose tissue suspected of containing numerous metastatic lymph nodes was sent to pathology for analysis. No adrenelectomy was performed. The patient was discharged after fi ve days of postoperative care with no signs of complications.
The pathology report had initially revealed a mixed type of tumour where the majority consisted of oncocytoma that measured 17 cm with multifocal PRCC. No metastatic lymph nodes were found but only healthy adipose tissue.
Re-examination of the pathological slides was done at the department of pathology, Gothenburg University Hospital. This revealed a true collision tumour composed of oncocytoma and papillary renal cell carcinoma, type 1, Fuhrman grade 2 (pT1a tumour + oncocytoma).
Eighteen days postoperatively both PET-CT and scintigraphy showed no metastatic disease. Because of the rarity of this tumour, the recommendation of the multidisciplinary conference was to follow-up the patient initially with a sixmonth timeframe using CT-scan for signs of local recurrence or metastases. The lesion in the right kidney insignifi cantly reduced in size and a renal ultrasound with contrast revealed a 1 cm lesion interpreted as angiomyolipoma alternatively oncocytoma with diffi culty to rule out malignancy. Therefore, the case was discussed once again in a multidisciplinary conference that recommended a yearly follow up with a CT thorax and abdomen.
During the 9-year follow-up, the CT examinations have not shown signs of local recurrence or metastases and no change in the size of the lesion in the right kidney.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images.

Discussion
Collision renal tumours have been previously described consisting mainly of oncocytoma and chromophobe renal cell carcinoma. The coexistence of oncocytoma and PRCC is rare.
While our patient presented with painless macroscopic haematuria, other reported symptoms at presentation varied from asymptomatic to abdominal pain and/or isolated macroscopic haematuria (Table 1). Interestingly, one case presented with B-symptoms of malignancy (such as lethargy, anorexia and hypercalcaemia) [15] (Table 1).
Painless macroscopic haematuria and fl ank/abdominal pain are two classic symptoms of renal malignancy. The mentioned B-symptoms of malignancy are suggested to be secondary to hypercalcemia, which was the case in that patient [15]. It has been documented that 13-20% of patients with RCC are associated with hypercalcemia [20]. In other words, symptoms at presentation of renal mixed tumours did not differ from patients with other renal tumours.
With the exception of two cases [18,19] (Table 1), our patient was relatively younger (53-years old) compared to other cases [2,[11][12][13][14][15][16][17] (Table 1). Overall, the median age of men at diagnosis was 68 years, in accordance with reported median age of other renal tumours [21]. Regarding the gender, it shows tendency toward male predominance with reported 11 (92 %) male cases including our case and one female (8%), in disagreement with sex distributions of known renal tumours [22] (Table   1). However, it is unfair to make a conclusion regarding sex distribution in cases with mixed oncocytoma and PRCC due to the shortage in the number of registered patients.
The diagnosis of collision renal tumour is based on histological and immunohistochemical studies. In fact, our case and previous cases have pathological evidence of coexisting mixed oncocytoma and PRCC ( Table 2). The decision of active surveillance versus active treatment is depending mostly on the results of histological studies. The size of oncocytoma varies from 1.5 cm to 17 cm with a median size of 4,7cm. PRCC confi guration varies from singular foci varying in size from 0.7 cm to 3.4 cm to multifocal, nest-like confi gurations with a median size of 1 cm. Previous reports indicating increased risk of recurrence with tumour size above 4 cm [23]. The subtype of PRCC was given in 8/12; being type I in fi ve cases (63%), and type II in 3 cases (37 %). Previous studies did not identify the subtype of PRCC as a strong prognostic factor [24]. The Fuhrman grade was available for 6/12 cases including our own,  where fi ve are Fuhrman grade 2 (83 %) and Fuhrman grade 1 in one case (17 %). There is insuffi cient data regarding the prognostic signifi cance of Fuhrman grading although in two series grading did not achieve a signifi cant relationship with outcome on multivariate analysis that included tumour stage [25,26]. Data on staging was available only in 3/12 cases where all are pTa1 (Table 3). Altogether, mixed oncocytoma and PRCC seem to have overall good prognosis.
A collision tumour contains two distinct tumours of different topographical origins [27], with the oncocytoma being the largest component, most often engulfi ng the PRCC component, which may be a singular lesion or nest-like/ blended [12,18] including our case. Thus, it was suggested that oncocytoma of any larger size may have had small PRCC contained within but were not detected when sectioned prior to paraffi n embedding [2]. The exact mechanisms behind collision renal tumours are not known, however, theories such as that the lesions either arise from a common precursor that at a later stage of tumorigenesis differentiate into topographically different tumours or that two unrelated cell lines develop topographically different tumours, either synchronously or metachronously, have been suggested [28].
The use of a predicting prognosis nomogram or a staging system such as the University of California Los Angeles Integrated Staging System or UISS [29] could have been useful and interesting to allow a comparison of more parameters of this type of tumors with existing data from two separate entities (Oncocytoma and PRCC). However, unfortunately none of the authors included these parameters in the cases studied, making it diffi cult to obtain any form of prognostic value. In our case, after following the UISS staging system, our patient was classifi ed as a *low risk* and has a 91.1% chance for a 5-year survival.
The reported follow-up period in previous cases was up to 5-years. Our case was followed-up for 9-years. The follow up was according to the standard program for PRCC with CTscanning of the thorax and abdomen twice/year during the fi rst two years then once yearly. No signs of local recurrence or metastases were reported during the 9-years follow-up period ( Figure 2). Our results are in accordance with previous reports [11,12,[15][16][17], indicating that no additional follow-up manner is required in such cases with good prognosis of oncocytoma mixed with PRCC.

Conclusion
The possibility of presence of malignant components should be considered when treating benign renal tumours.
Renal collision tumour of oncocytoma and PRCC seems to have a good prognosis and doesn't require a specifi c follow-up programme rather than the standard program for PRCC.

Authors contributions
All authors should have made substantial contributions to all of the following: the conception and design of the study, or acquisition of data, or analysis and interpretation of data; drafting the article or revising it critically for important intellectual content; and fi nal approval of the version to be submitted.