2026 Expert Practice Recommendations: Optimizing Somatic and Germline HRR Mutation Testing in Metastatic Prostate Cancer

Source: Multidisciplinary expert panel (medical oncology, urology, pathology, clinical genetics, molecular diagnostics), published in Frontiers in Oncology, 2026. Disclaimer: This summary is for academic reference only and cannot replace clinical judgment. All testing decisions should be discussed within an MDT. Core message: HRR testing should be implemented early at metastatic diagnosis (preferably mHSPC stage). Tumor somatic testing guides PARP inhibitor eligibility; germline testing identifies hereditary cancer risk for patients and at‑risk relatives. Tissue NGS is gold standard; ctDNA is acceptable alternative when tissue is unavailable.

1. Testing Population & Timing

1. All patients with metastatic prostate cancer (mHSPC and mCRPC) are recommended for HRR panel testing.
   • Preferred timing: At initial diagnosis of metastatic disease, before progression to mCRPC.
   • Rationale: Waiting until mCRPC often results in insufficient archival tissue and delayed biomarker results, which limits access to PARP inhibitor therapy.
2. High-risk localized prostate cancer with strong family history of HRR-related cancers: germline panel testing may be offered even without metastasis.
3. Patients with only low-risk localized prostate cancer: routine HRR testing is not recommended.

2. Panel Gene List (Minimum Required Genes)

The HRR panel must include at minimum:

BRCA1, BRCA2, ATM, PALB2, CHEK2, FANCA, RAD51D, CDK12, BARD1, BRIP1, CHEK1, FANCL, RAD51B, RAD51C, RAD54L

• Pathogenic/likely pathogenic variants in these genes confer PARP inhibitor sensitivity.
• MSI‑H/dMMR testing should be performed alongside HRR testing for immunotherapy eligibility.

3. Somatic (Tumor) Testing

1. Sample priority: Archival formalin-fixed paraffin-embedded (FFPE) tumor tissue is first choice. Fresh metastatic biopsy preferred if archival tissue is low tumor cellularity / degraded.
2. ctDNA liquid biopsy: Valid alternative when tissue biopsy is inaccessible or inadequate.
   • Limitation: Low tumor fraction may yield false-negative results; repeat ctDNA can be considered at disease progression if prior result was non-informative.
3. Interpretation rule: Tumor NGS may detect variants that are actually germline. Any pathogenic HRR variant identified on tumor assay warrants reflex germline confirmation and genetic counselling.
4. Purpose of somatic testing: Guide PARP inhibitor/platinum chemotherapy selection for mCRPC.

4. Germline Testing

1. Indications for germline NGS:
   • All metastatic prostate cancer patients (pre-test counselling and informed consent required).
   • Positive pathogenic HRR variant found on tumor somatic testing.
   • Positive family history (prostate, breast, ovarian, pancreatic cancer).
2. Clinical implications of germline HRR mutation:
   • Patient: increased risk for second primary malignancies.
   • Family: at-risk relatives can undergo cascade genetic screening and cancer surveillance.
3. Pre-test counselling must cover: hereditary cancer risk, family implications, incidental findings, privacy issues.

5. Sequential Testing Algorithm

1. Option A (preferred): Germline panel at metastatic diagnosis + somatic tumor/ctDNA HRR testing
2. Option B: Tumor-first approach. If somatic assay detects pathogenic HRR variant → reflex germline confirmation + genetic counselling.

Do not rely solely on tumor testing to distinguish somatic vs germline origin.

6. Pre-Analytical & Assay Quality Requirements

1. Tumor tissue: Tumor cellularity ≥20% is ideal. Laboratories should report tumor content and assay sensitivity.
2. Variant classification: Follow ACMG/AMP criteria for pathogenic / likely pathogenic / variant of uncertain significance (VUS).
   • VUS should not be used to select PARP inhibitors.
3. Reporting standard: Clearly separate somatic variants from germline variants; provide clinical actionability annotation.

7. MDT & Genetic Counselling

• MDT members: Urologist, medical oncologist, pathologist, molecular biologist, genetic counsellor.
• Germline positive patients must be referred to genetic counselling for family cascade screening.
• VUS counselling: Avoid overinterpreting; repeat testing of family members is not routinely recommended for VUS.

8. Re-Testing Recommendations

• Re-testing may be considered:
  1. Prior tissue/ctDNA result non-informative due to low tumor fraction.
  2. Disease progression and treatment plan change, especially before PARP inhibitor/platinum therapy.
• If a pathogenic HRR variant is already confirmed, repeat HRR panel is unnecessary.

9. Recommendations NOT Supported

❌ Routine HRR testing for low-risk localized prostate cancer. ❌ Using VUS to prescribe PARP inhibitors. ❌ Omitting germline confirmation when tumor NGS identifies BRCA1/2 or PALB2 pathogenic variants. ❌ Relying only on ctDNA if high-quality tumor biopsy is obtainable. ❌ Delaying HRR testing until mCRPC progression without justification. ❌ Skipping genetic counselling for patients with confirmed germline HRR pathogenic variants.

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