Historically, the nomenclature of “carcinoid” was describing the tumor entity of neuroendocrine cells with low malignancy in the gastrointestinal tract (GI) [
7]. However the histopathogenesis of the primary testicular carcinoid remains under debate suggesting respective hypothesis: originating from the same precursor cell as the Leydig cells do or resulting from chromosomal irregularity [
8,
9]. The diagnosis of primary testicular carcinoid is based on histopathology. The detailed histopathologic assessment is very important for the classification and the prognosis of the primary testicular carcinoid tumor [
3]. Mitotic activity and vascular or tunica albuginea invasion were found not to increase the malignancy of the carcinoid tumors. However the occurrence of a low-degree of tumor differentiation, symptoms of a carcinoid syndrome as well as a tumor size correlate with increased metastatic potential [
10,
11]. A precise detection of biomarkers assessed by immunohistochemistry (IHC) enables the subtyping and accurate classification of PTCT [
12]. In general it is considered and it was described by Abbosh et al. that caudal type homeobox 2 (Cdx-2), a marker for neuroendocrine tumor cells originating from GI tract, is negative in PTCT [
9,
13]. Lee et al. and Bing et al. described malignant germ cell tumors of the testes stained positive for Cdx-2 suggesting precaution in making a diagnosis in metastatic tumor with unknown origin and thereby considering an occult testicular malignancy rather than of GI origin [
14,
15]. Albeit Atalay et al. demonstrated a strong significant Cdx-2 staining, as it is observed in tumor cells with GI origin, merely in testicular teratoma [
16]. To the best of our knowledge, there is in the literature only one reported case of Cdx-2 positve PTCT [
17]. In our case, the exceptionally rare Cdx-2 stain positivity was found as well, albeit no GI origin of carcinoid tumor cells was detected. This data suggests that the analytic specificity of Cdx-2 marker needs to be assessed critically and distinct in each case. The standard treatment of PTCT remains the radical inguinal orchiectomy [
6]. The recurrence of PTCT may occur up to 17 years after the initial diagnosis and treatment requiring close clinical and imaging aftercare [
18]. Since the data is based on single cases there is no consensus about general follow-up guidelines [
6,
19]. In summary PTCT has a good prognosis with the 5-year overall survival rate of 78.7% and the 5-year specific survival rate of 84.3% and should be alternatively considered in Cdx-2 positive metastatic tumor of unknown origin [
19].