Germ Cell & Testicular Tumors

Quick Answer

Germ cell tumors are the most common solid malignancy in young men and are highly curable. Nuclear medicine's signature role is narrow but important: FDG-PET evaluates a post-chemotherapy residual mass in pure seminoma (best for masses > 3 cm, imaged ≥ 6 weeks after chemotherapy) — an FDG-positive residuum suggests viable tumor (SEMPET). In non-seminomatous tumors (NSGCT), FDG-PET is not reliable, because teratoma is FDG-negative yet requires resection.

Definition

Testicular germ cell tumors (GCT) arise from germ cells and divide into seminoma and non-seminomatous germ-cell tumors (NSGCT) — the latter including embryonal carcinoma, yolk-sac tumor, choriocarcinoma, and teratoma (often mixed). Extragonadal (mediastinal/retroperitoneal) primaries occur.

Synonyms

Germ cell tumor (GCT); testicular cancer; seminoma; non-seminomatous germ-cell tumor (NSGCT); mixed germ cell tumor.

Epidemiology

GCT is the most common solid malignancy in men aged ~15–35, with excellent cure rates even when metastatic. Seminoma peaks slightly later than NSGCT. Incidence is higher in certain populations and has risen over decades.

Etiology & Risk Factors

• Cryptorchidism (undescended testis), prior GCT or family history, intratubular germ-cell neoplasia (precursor), and testicular dysgenesis. Infertility/subfertility is associated.

Pathophysiology & Tumor Markers

Serum markers are central to diagnosis, staging, and monitoring:

• AFP — elevated by yolk-sac and embryonal components; never by pure seminoma (an elevated AFP means the tumor is not pure seminoma).
• β-hCG — elevated by choriocarcinoma/syncytiotrophoblasts and some seminomas.
• LDH — bulk/prognosis.
• Teratoma is marker-negative, FDG-negative, and chemo-resistant — it must be resected, the crux of the NSGCT imaging problem.

Genetics & Molecular Biology

The hallmark is isochromosome 12p [i(12p)], present in most GCTs. Molecular subtyping distinguishes seminoma from embryonal/other elements; teratoma's differentiated somatic tissue underlies its chemo-resistance.

Histopathology

• Seminoma: uniform cells, "fried-egg" cytoplasm, lymphocytic stroma.
• NSGCT: embryonal carcinoma, yolk-sac (Schiller–Duval bodies), choriocarcinoma (syncytio/cytotrophoblast), and teratoma (somatic tissues). Most NSGCT are mixed.

Clinical Presentation

A painless testicular mass/swelling (occasionally pain from hemorrhage). Metastatic spread is lymphatic to retroperitoneal (para-aortic) nodes first, then lung; choriocarcinoma spreads hematogenously and early. Gynecomastia with high β-hCG.

Laboratory Findings

AFP, β-hCG, LDH at diagnosis, after orchiectomy, and through therapy (kinetics matter). Marker normalization vs plateau/rise guides management and defines marker-negative residual masses.

Imaging Findings by Modality

• Scrotal ultrasound: the primary tool for the testicular mass.
• CT (chest/abdomen/pelvis): staging (retroperitoneal nodes, lung) and residual-mass assessment.
• FDG-PET: post-chemotherapy residual seminoma (> 3 cm, ≥ 6 weeks post-chemo) — viable tumor vs fibrosis/necrosis (SEMPET).
• Brain MRI for choriocarcinoma/advanced disease as indicated.

Radiopharmaceutical Uptake Mechanisms

FDG reflects glucose metabolism — seminoma is typically avid, so a residual mass that remains FDG-positive after chemotherapy suggests viable tumor. Teratoma is not FDG-avid despite being viable and chemo-resistant, which is precisely why FDG cannot clear an NSGCT residual mass.

Typical PET Tracers

Typical SPECT Tracers

• None with a routine role.

Therapy Indications

• Orchiectomy (radical inguinal) — diagnostic and therapeutic.
• Seminoma: surveillance, carboplatin, or (historically) radiotherapy for early stages; BEP chemotherapy for advanced.
• NSGCT: surveillance vs retroperitoneal lymph-node dissection (RPLND) and BEP chemotherapy; resection of residual masses (teratoma/viable tumor).
• Salvage chemotherapy ± high-dose therapy for relapse.

Theranostics

No radioligand therapy — GCT is managed by surgery, chemotherapy, and (historically) radiotherapy. The nuclear-medicine contribution is the FDG residual-seminoma decision.

Differential Diagnosis (of a residual mass)

• Viable tumor vs necrosis/fibrosis vs teratoma — the central post-chemotherapy question.
• In seminoma, FDG helps (positive → viable); in NSGCT, FDG cannot exclude teratoma (resection needed).
• Post-chemotherapy inflammation can cause false-positive FDG (respect the ≥ 6-week interval).

Reporting Checklist

• For seminoma residual mass, state size (> 3 cm) and interval (≥ 6 weeks); interpret FDG-positive as suspicious for viable tumor.
• Explicitly note that FDG is not reliable for NSGCT residuals (teratoma FDG-negative).
• Correlate with marker kinetics (AFP/β-hCG/LDH).

Prognosis

GCT is among the most curable solid cancers, even when metastatic, stratified by the IGCCCG risk model (marker levels, primary site, and non-pulmonary visceral metastases). Seminoma has an excellent prognosis; NSGCT outcomes are strong with proper chemotherapy and residual-mass surgery.

Board Pearls

FDG-PET's role is post-chemotherapy residual seminoma — best for masses > 3 cm imaged ≥ 6 weeks after chemotherapy; a persistently FDG-positive residuum suggests viable tumor (SEMPET). Scanning too early risks inflammatory false positives.

FDG-PET is not reliable for NSGCT residual masses because teratoma is FDG-negative yet viable and chemo-resistant — it must be resected regardless of PET.

Markers anchor the disease: elevated AFP excludes pure seminoma (means non-seminomatous elements), β-hCG rises with choriocarcinoma/some seminoma, and teratoma is marker-negative and FDG-negative. Spread is lymphatic to retroperitoneal nodes first; the hallmark cytogenetic finding is i(12p).

Related Pages

• Tracer: FDG (residual-mass timing, inflammation caveat).
• Related studies: lymphoscintigraphy & sentinel node (not routine here); response: oncology response criteria.
• Pitfalls: Pearls, pitfalls & normal variants.

Figure / Diagram Suggestions

• A residual-mass decision flow: seminoma (FDG helps) vs NSGCT (resect — teratoma FDG-negative).
• A marker map (AFP/β-hCG/LDH → histology implications).
• A retroperitoneal nodal spread schematic.

Common Pitfalls

• Growing teratoma syndrome — an enlarging post-chemotherapy mass with normalizing/normal markers and no FDG uptake is teratoma requiring resection, not chemotherapy failure; do not mistake growth for progressive malignancy.
• FDG is unreliable for small residual masses (< 3 cm) — microscopic viable tumor and partial-volume effect make a negative scan uninformative below the size threshold.
• Post-chemotherapy thymic rebound and sarcoid-like granulomatous nodal reactions are common in these young patients and are FDG-avid — a classic false-positive mimicking relapse; correlate with markers and morphology.
• A negative FDG scan does not clear rising markers — marker-positive relapse can be FDG-occult (teratoma or small-volume disease), so marker kinetics still drive management.

Self-Check (Board-Style)