Primary GCTs of extragonadal origin comprise 3% to 5% of all germ cell tumors. Extragonadal GCTs arise from midline structures [
2,
3]. A case of extragonadal malignant teratoma of the extremities has been reported by Chinoy
et al. [
4]. Herein, we describe what is to the best of our knowledge the first reported case of a mixed non-seminomatous germ cell tumor (teratocarcinoma) located in the soft tissue of the right arm. The histogenesis of extragonadal GCTs is not clearly defined: two competing hypotheses have been proposed, but there are inadequate data to determine which, if either, is correct. The first hypothesis is that extragonadal GCTs are derived from primordial germ cells that fail to complete the normal migration along the urogenital ridge to the gonadal ridges during embryonal development. This may be due to an abnormality in the primordial germ cell itself or in its microenvironment [
5]. The second main hypothesis is that germ cells transformed in the testes undergo reverse migration [
6]. This hypothesis is supported by genetic data suggesting that extragonadal GCTs and testicular GCTs share a common cell of origin [
7,
8]. The etiology of extragonadal GCTs is unknown; in rare cases, they have been associated with Klinefelter syndrome [
9‐
12]. A biopsy is required for definitive diagnosis and treatment of extragonadal GCTs; the majority of patients have clear evidence of germ cell features or teratoma, while a small subset have a poorly differentiated tumor without distinctive germ cell features. In our patient’s case the diagnosis of teratocarcinoma was established on the basis of pathological examination, which showed the combination of teratoma and embryonal carcinoma [
13]. However it is important to note that the term teratocarcinoma has largely been abandoned, and these tumors are referred to as a malignant mixed GCT, with a description of the specific germ cell tumor elements present. Once the diagnosis of a germ cell tumor has been established, a primary testicular tumor must be excluded. Testicular palpation is insufficient to exclude a testicular primary; ultrasonography should be performed in all patients [
14]. It may be difficult to distinguish true extragonadal GCTs from metastatic tumors in which the primary testicular lesion has regressed [
15‐
17]. Extragonadal non-seminomatous GCT should be considered in the differential diagnosis of histologically poorly differentiated cancer and of neoplasms of unknown primary site, particularly in young men with midline disease. In our patient’s case another uncommon differential diagnosis must be evocated: it is a soft tissue metastasis from primary testicular cancer [
18].
Because an extragonadal GCT may be curable with cisplatin-based chemotherapy, many recommend that the diagnostic evaluation in such cases should include measurement of the serum tumor markers AFP and β-HCG, immunohistochemical assessment of the biopsy [
19], and cytogenetic analysis for abnormalities of chromosome arm 12p [
20]. However, isochromosome 12p is not pathognomonic of GCTs [
21,
22]. Mixed GCTs, mainly teratomas with additional malignant components (other germ cell tumors, carcinomas or sarcomas), have been reported to be more aggressive. More than 50% of those patients died within 2 years of follow-up due to local invasion or distant metastases (lymph nodes, liver, lung, heart, bone, and brain) [
23,
24]. A multimodality approach is generally used, utilizing chemotherapy initially followed by surgery for any residual mass. We recommend four cycles of bleomycin, etoposide and cisplatin (BEP) chemotherapy as the initial therapy, rather than surgery or radiation therapy; for patients with a residual mass following initial chemotherapy, we recommend complete surgical resection if technically feasible. If viable malignancy is identified, two additional cycles of chemotherapy should be given [
25,
26].