12P Do retroperitoneal extragonadal germ cell tumours (pR-GCT) exist? Evaluation of treatment outcomes of an international retrospective collaboration (PRIMERE study-IGG05)

Primitive Retroperitoneal Germinal Cell Tumors (pR-GCT) corresponds to the clinical case of retroperitoneal ascertained GCT without evidence of primary testicular tumor and accounts for up to 40 % of extragonadal GCTs. An occult primary testicular tumor is often missed at diagnosis. Treatment modalities and outcomes have not been specifically addressed, preventing robust recommendations. We performed an international call to assess prognosis of treatment outcomes of these pts. Clinical, pathological and treatment data pts with pR-GCT between April 1988 and January 2022 were retrospectively collected across four referral centers. Kaplan Meier methods, univariable and multivariable Cox regression models (MCRMs) were used. Ninety-nine pts (median age 37 years - IQR: 29-45) were collected. Ninety-three (94%) had histological diagnosis by biopsy or primary retroperitoneal lymph-node dissection: 60 (62.5%) had non-seminomatous (pR-NSGCTs) and 33 (34.4%) had seminomatous (pR-SGCTs). IGCCCG prognostic allocation was possible in 91 pts: 34 (35.8%) were good, 23 (24.2%) intermediate and 34 (35.8%) poor-risk, respectively. All pts underwent cisplatin-based chemotherapy, usually BEP (94.9%). After chemotherapy, 25 (25.3%) pts underwent orchiectomy: viable tumor was present in 8 (32%), burn-out lesions in 6 (24%) and no lesion in 11 (44%). After a median FUP of 45 m (IQR:15-90), the 5-yrs OS was 58%, being 85% in case of pR-SGCT and 46% in case of pR-NSGCT. According to IGCCCG classification 5-yrs OS was 83.3% for good, 57% for intermediate, 42% for poor-risk pts, respectively. No difference in term of OS (60% vs 56%, p-value 0.81) was observed between patient who had or not radical orchiectomy. At MCRMs only IGCCCG poor risk category (HR 3.2, CI: 1.18-8.84, p-value 0.02) was independent predictor of a worse OS. Eventually, a significant proportion of patients with pR-GCT had a misclassified primary testicular tumor. 5 years-OS according to IGCCCG-classification is worse than expected. The role of a surgical exploration of the suspected primary tumor remains controversial and the existence of a real category of p-RGCT cannot be excluded.