Isolated pancreatic metastases of renal cell carcinoma (IsPMRCC) are a rare manifestation of metastatic, clear-cell renal cell carcinoma (RCC) in which distant metastases occur exclusively in the pancreas. In addition to the main symptom of the isolated occurrence of pancreatic metastases, the entity surprises with additional clinical peculiarities: (a) the unusually long interval of about 9 years between the primary RCC and the onset of pancreatic metastases; (b) multiple pancreatic metastases occurring in 36% of cases; (c) favourable treatment outcomes with a 75% 5-year survival rate; and (d) volume and growth-rate dependent risk factors generally accepted to be relevant for overall survival in metastatic surgery are insignificant in isPMRCC.
1. Introduction
The occurrence of isolated pancreatic metastases of clear-cell renal cell carcinoma (isPMRCC) is rare in the clinical course of clear-cell renal cell cancer (ccRCC). In this entity, the pancreas itself becomes—either definitively or for many years—the sole and only organ site of synchronous or metachronous distant metastases of a ccRCC. If the isolated occurrence of pancreatic metastases (PM) in ccRCC is to be regarded as extremely unusual, the clinical course reveals further peculiarities: (a) In metachronous PM, an unusually long interval from RCC surgery to the occurrence of the PM: from 855 case reports, a mean duration of 9.6 years could be calculated
[1], while large institutional reports (N > 20) indicate a time span of 6.9 to 11.2 years (median 9.0 years)
[2[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17],
3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], with the longest reported interval being 36 years
[18]; (b) The high frequency of multiple occurrences of PM: Of 733 casuistic observations, 36.4% concerned multiple PM
[1]. This is confirmed in single and multicentre reports with values of 19% to 70% (median 37%)
[2[2][4][6][8][10][12][13][14][16][17][19][20],
4,6,8,10,12,13,14,16,17,19,20], with a reported maximum of 15 foci
[21]; (c) The unusually protracted and favourable clinical course for metastatic ccRCC: For the spontaneous course
[22], a 3-year survival rate of 56% was calculated for the few reported, untreated patients (N = 19
[16,23,24,25,26,27,28,29,30,31,32,33,34][16][23][24][25][26][27][28][29][30][31][32][33][34]). In operated patients, a 5-year survival rate of 75.7% could be determined from 421 case reports
[22], and in the single and multicentre reports, the corresponding values are 50–88%
[3,4,6,7,8,9,10,11,12,13,14,15,19,20,35,36,37,38,39,40][3][4][6][7][8][9][10][11][12][13][14][15][19][20][35][36][37][38][39][40] with a median of 72%. Finally, in patients treated with antiangiogenetic vascular endothelial growth factor receptor tyrosine kinase inhibitors (TKI)
[41], a result not significantly different from the operative results was determined
[42]; (d) Volume and growth-rate-dependent risk factors generally accepted to be relevant for overall survival (OS) in metastatic surgery are insignificant in isPMRCC
[22]: In four large (N > 150) compilations of case reports, singular vs. multiple occurrences, size and number of PMs, as well as synchronous vs. metachronous occurrence and interval to PM occurrence, were not prognostically relevant
[22,32,43,44][22][32][43][44]. An identical lack of prognostic relevance of these risk factors was reported in five large (N > 20) institutional reports
[4,8,10,13,14][4][8][10][13][14] that analysed exclusively isPMRCC observations.
This overall favourable outcome cannot be explained by the single organ involvement per se, but is a specific feature of the isPMRCC, as evidenced by the significantly worse outcome of single organ metastases of the ccRCC in other organs. In a recent study on the impact of single organ metastases on the course of ccRCC, the median survival time of isPMRCC was three times longer than that of single organ metastases in other organs (8.8 vs. 2.8 years;
p < 0.001)
[45].
2. Genetic Characteristics and Peculiarities of the isPMRCC
2.1. Clear-Cell RCC Genome
The genome of the ccRCC was deciphered as early as 2013
[57][46]. It is characterized by the biallelic absence or functional inactivation of the
VHL tumour suppressor gene localized at 3p25 and the frequent inactivation of chromatin-modifying genes, such as
PBRM1,
BAP1 and
SETD2 [58][47] (
Table 1).
The protein encoded by the
VHL gene (pVHL) mediates its tumour-suppressive effect by binding to and mediating the proteasomal degradation of the hypoxia-inducible factor HIFα
[59,60][48][49]. Under physiological conditions, HIFα subunits are unstable and are regulated by cellular oxygen content
[61][50]. The loss or inactivation of
VHL with consecutive inactivation of pVHL, therefore, leads to the activation and enrichment of HIF despite normoxic conditions and irrespective of the cellular oxygen availability and triggers the subsequent up-regulation of numerous HIF target genes. The activation of these HIF target genes is crucial for the formation and progression of ccRCC due to their role in promoting angiogenesis, tumour cell survival, proliferation and progression. HIFα consists of the subunits 1α and 2α, both of which are involved in ccRCC initiation
[60,62][49][51]. During further ccRCC progression, however, HIF1α expression (located at chromosome 14q23
[63,64][52][53]) is lost in 30–40% since it can act as a tumour suppressor during the progression of ccRCC
[60,64][49][53]. However, HIF2α acts as an oncoprotein in ccRCC. Due to the behaviour of HIF, two forms of ccRCC can be distinguished: Those in which HIF1α and 2α are overexpressed, and those in which only HIF2α is overexpressed and which are associated with enhanced cell proliferation and unfavourable prognosis
[60][49]. HIF2α-triggered target factors include VEGF-α
[60,65][49][54], TGF α/EGFR
[66][55], c-Myc
[60,67,68][49][56][57], cyclin D1
[69[58][59],
70], SLC7A5-mTorC1
[60[49][60][61],
71,72], GLUT1
[73,74][62][63], antioxidant enzymes
[75][64], mitochondrial biogenesis factors
[76][65], GAS6/tyrosine kinase AXL
[77][66] and CXCR4/SDF1
[78][67], which control critical biological activities such as tumour angiogenesis, cell-autonomous proliferation, increasing glycolysis, resistance to oxidative damage, endoplasmic reticulum stress and metastatic ability
[60,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81][49][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70].
Further frequently altered genes in ccRCC are chromatin-modifying genes: polybromo-1
(PBRM1), BRCA1 associated protein 1 (
PAB1), SET domain containing 2 histone-lysine N-methytransferase (
SETD2), located on the same 3p chromosomal region
[82[71][72][73],
83,84], and less frequently, lysine demethylase 5C (
KDM5C) located on the X chromosome
[85][74] and telomerase reverse transcriptase (
TERT) promoter located on chromosome 5p
[86,87][75][76]. The frequency of detectable
VHL defects is estimated to be up to 90%
[58,86,88,89,90][47][75][77][78][79]. In contrast, the incidence of the other altered driver genes is significantly lower:
PBRM1 52.6–26.4%,
SETD2 35–7.6%,
BAP1 31–7.5%,
KDM5C 16–3.8%,
TERT 14–12.2% and mTor 13–5.7%
[58,84,86,88,89,91,92,93,94][47][73][75][77][78][80][81][82][83]. It was soon recognised that these gene alterations are associated with a different tumour biology, and thus, have an influence on the course of the disease and the outcome
[90,95][79][84].
PBRM1 is the most frequently mutated gene after
VHL [84,92][73][81] and mutations acquired in this gene largely do not overlap with loss of function mutations in
BAP1 [58,88,90,92,96][47][77][79][81][85]. PBRM1 mutations are associated with improved outcome in ccRCC
[95,97][84][86] and do not correlate with decreased survival
[88][77], whereas the absence of mutations of PBRM1 resulted in worse outcome
[90][79].
KDM5C mutations have also been associated with improved clinical outcome in clinical reports
[88,94][77][83]. In particular, the concurrent mutations of PBRM1 and KDM5C define a subgroup with increased angiogenesis associated with favourable prognosis, as Santos reports
[95][84]. The similar effects of
PBRM1 and
KDM5C mutations on outcome are consistent with the observation that the vast majority of up- and downregulated genes after suppression of PBRM1 or KDM5C were shared
[98][87]. Conversely,
PAB1 mutations in ccRCC have proved to be a driver of aggressiveness and correlated with reduced outcome
[58,84,88,89,90,92,99,100,101,102][47][73][77][78][79][81][88][89][90][91]. PAB1 mutations further tended to be associated with mTOR mutations
[92][81].
TERT and
TP53 were also identified as gene mutations associated with a poor prognosis
[58,86,90,99][47][75][79][88]. However, these gene changes are generally relevant to the occurrence and course of RCC, but none of these changes can be considered specific to the occurrence of metastases, let alone isPMRCC.
Table 1.
Altered driver genes in ccRCC, metastatic RCC and isPMRCC.