An incidental papillary renal cell carcinoma in a unilocular renal cyst

A case report

Abstract

Cystic renal cell carcinoma (RCC) is a malignant epithelial tumor arising in a pre-existing renal cyst. The renal cyst maybe multilocular or unilocular with the latter being an even rarer occurrence. Cystic renal malignancy arising in a unilocular cyst is quite rare with only a few cases reported till date, most of which are clear cell RCCs. Literature available with regard to staging tumor nodule in a benign unilocular cyst is limited. We report a case of a 52-year-old male presenting with a single large unilocular cyst which had a small solid nodule of papillary RCC in its posterior wall which was detected on microsopy. On pathologic examination, all cases of clinically/ radiologically benign renal cysts should be examined with a high index of suspicion as a small tumor focus may be present in the form of a nodule or in the free wall of the cyst which was initially missed.

BACKGROUND

Conventional renal cell carcinoma (RCC) may arise in a preexisting simple renal cyst. This is a relatively rare (<5% of all RCC) occurrence, however, carries a better prognosis for the patient when compared to solid RCC.[¹] The renal cyst maybe multilocular or unilocular; latter being an even rarer occurrence with only a few case reports described in literature. Most of the cases reported earlier were clear cell RCC and only one case of papillary RCC has been documented. We present a case of a unilocular renal cyst, the wall of which contained a small nodule of papillary RCC; detected incidentally on microscopy.

CASE PRESENTATION

A 52-year-old male presented to surgery OPD with complaints of swelling and pain in his left flank since 1 month. There was no history of hematuria, fever, or weight loss. On per-abdomen examination, a soft, nontender, immobile mass was palpated in the left flank. On diethylenetriamine penta-acetic acid scan, left kidney was found to be nonfunctional while right kidney showed normal function.

Contrast-enhanced computed tomography abdomen revealed a well-defined cyst with fluid-filled density occupying almost whole of the left kidney measuring 14.7 × 11.4 × 13.5 cm [Figure 1a]. Right kidney was normal in size, shape, and position. Patient was taken up for left nephrectomy with a clinical diagnosis of nonfunctioning kidney. Grossly, the specimen of left kidney showed a single large cyst measuring 12 × 10 × 1 cm which was filled with reddish-brown hemorrhagic contents [Figure 1b]. A friable, detached, small nodule measuring 1 × 0.5 × 0.4 cm identified on the posterior wall of the cyst was sampled.

Figure 1: (a) CT scan of left kidney with a large unilocular cyst and (b) gross image of kidney cyst filled with red-brown friable material

On microscopy, sections from the nodule showed a tumor with single layer of cells arranged in tubulo-alveolar to papillary pattern having monomorphic nuclei, fine chromatin, conspicuous nucleoli, and moderate clear to eosinophilic cytoplasm [Figure 2a-d]. The tumor cells were diffusely positive for CK7 and vimentin [Figure 3a and 3b] and focally positive for CD10. A final diagnosis of papillary RCC was given (pT1aNxMx). Rest of the cyst wall showed thick fibrocollagenous tissue, ulcerated lining, exudates with cholesterol clefts, and lymphohistiocytic infiltration. On review of radiology, a small enhancing lesion was detected in the posterolateral wall of the cyst. No additional treatment was rendered as tumor was confined to the kidney and <4 cm in greatest dimension (pT1a). The patient is well at 6 months' follow-up.

Figure 2: (a) Tumor in the cyst wall (hematoxylin and eosin, 40×), (b) high power view of tumor cells with monomorphic nuclei and clear cytoplasm arranged in a tubule-alveolar pattern (hematoxylin and eosin, 200×), (c) cyst wall with normal kidney (hematoxylin and eosin, 40×), and (d) adjacent kidney with many cholesterol clefts (hematoxylin and eosin, 100×)

Figure 3: (a and b) Tumor cells are positive for CK7 and vimentin immunohistochemistry (IHC, 200×)

DISCUSSION

RCCs have an increasing prevalence worldwide. The most common subtypes are clear cell, papillary, and chromophobe constituting about 90% of all RCCs.[¹] RCCs may arise from a cyst or may undergo cystic degeneration as enummerated[²]:

1. Multilocular cystic neoplasm of low malignant potential
2. Conventional RCC with cystic degeneration
3. Acquired cystic disease-associated RCC
4. Adult cystic nephroma
5. Tubulocystic RCC
6. Unilocular cyst with mural tumor nodule.

Multilocular cystic RCC is composed of variably sized cysts with thin septae. A grossly identifiable tumor nodule is against this diagnosis. Diagnostic feature is presence of tumor cells in the fibrous septae. Clear cell RCC may show large areas of cystic degeneration. Papillary RCCs account for one-fourth of cases of RCCs having cystic degeneration.[²] The cystic space is filled with hemorrhagic/necrotic debris and is devoid of epithelial lining. Acquired cystic disease-associated RCC is the most common tumor occurring in end-stage renal disease and has a characteristic microcystic or sieve-like architecture. Adult cystic nephroma is a well-circumscribed multilocular tumor with noncommunicating cysts lined by flat cuboidal to columnar cells. Tubulocystic RCC is a well-circumscribed multicystic tumor having variably sized tubules admixed with large cysts.

Unilocular cyst with mural tumor nodule grossly shows a discrete nodule in an otherwise benign simple renal cyst. Such nodules may arise in a previous simple cyst or could be the result of asymmetric intrinsic growth of the cyst. Our case was that of RCC arising in a simple unilocular renal cyst. Cystic renal malignancies are low-stage, low-grade lesions bearing good prognosis.

Shicong et al.[³] described two cases with RCC arising in free wall of a simple renal cyst. Till now, only 14 such cases have been previously reported [Table 1]. The age of patients ranged from 25 to 75 years. Most patients were females with male-to-female ratio of 3.7:1. Our patient was a male. In most cases, the tumor was subcentimetric and thus missed on radiology. Of these previously reported cases, one case was of papillary RCC while five were clear cell RCC. Histology of tumor was not reported in the remainder cases. Our case showed features of papillary RCC (type 1). The risk of malignancy arising in a simple cystic lesion is reported to be 1.7%.[³] Thus, although rare, seemingly benign cystic lesions may harbor an occult malignant focus. It is not always possible to determine if the cyst is completely benign based on radiology. Thorough gross examination and sampling is vital to exclude small malignant focus.

Table 1: Table depicting the previously reported cases of RCC arising in a unilocular cyst

Ours was a case of a unilocular cyst in a nonfunctioning kidney which was radiologically characterized as a simple cyst. On gross examination, a friable nodule was identified in the wall of the cyst which turned out to be a papillary RCC on microscopy.

Williamson et al.[⁴] in their review article address the challenges in grading tumor nodules in a renal cyst. In a multicystic tumor where tumor cells are present throughout, the largest dimension of the cyst should be considered to determine tumor stage (pT). However, in cases with solid tumor nodule arising in a benign cyst, the size of tumor nodule is taken for staging.[⁴] RCC including clear cell RCC is treated by surgical resection and adjuvant therapy. Partial nephrectomy is the preferred option in tumors up to 7 cm in size, which are organ confined.[⁵]

CONCLUSIONS

Renal cysts may be simple or complex. A high index of suspicion is required while examining a case of renal cyst even if characterized as benign on radiology as a small solid malignant focus may have been missed.

Declaration of patient consent

Ethics approval and consent to participate: Written informed consent and verbal consent were taken from the patient. The institute's ethics committee was informed regarding the case report. They had approved of the writing of the manuscript and provided ethical waiver. However, a formal waiver had not been issued as all meetings were halted due to COVID-19 crisis in India through the months of April to June 2021. Ours was a tertiary care COVID center, so all man force was shifted toward COVID care during that period.

Consent for publication: Written informed consent was taken from the patient included in the study.

Availability of data and material: The data for current study are available from the corresponding author at reasonable request.

Author contributions: N. A. and S. M.—writing of manuscript and collection of case details, S. M.—taking microphotographs, and A. A.—a critical review of the manuscript.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.