Is metastatic prostate cancer always associated with a high prostate-specific antigen (PSA) level?

Is Metastatic Prostate Cancer Always Associated with High PSA?

No, metastatic prostate cancer is not always associated with high PSA levels—approximately 10-22% of patients with metastatic disease present with low or even undetectable PSA, particularly those with neuroendocrine differentiation or small cell variants. 1, 2

Understanding PSA-Discordant Metastatic Disease

While elevated PSA typically accompanies metastatic prostate cancer, clinicians must recognize important exceptions:

When Metastases Occur Despite Low PSA

• Small cell/neuroendocrine prostate cancer is present in approximately 17% of metastatic castration-resistant prostate cancer (mCRPC) patients who undergo metastatic biopsies, and these tumors characteristically produce low PSA levels despite extensive metastatic burden and visceral involvement. 1

• In one series, 22% of patients (10 of 46) with metastatic progression had undetectable PSA levels (0.1-2 ng/mL), and 8 of these 10 patients had small cell carcinoma. 2

• Atypical histologic variants (ductal, sarcomatoid, or small cell) were observed in 46% of patients who progressed with low PSA, and in 80% of those with undetectable PSA at progression. 2

Clinical Features That Should Raise Suspicion

Even with low or normal PSA, metastatic disease should be suspected when:

• High-grade tumors at diagnosis (Gleason score ≥8) are present—85% of patients with PSA-discordant metastases had Gleason scores ≥7. 2

• Locally advanced disease (clinical T3-T4) is identified—63% of PSA-discordant cases had T3 or T4 tumors. 2

• Grade Group 5 disease is present at initial pathology, as these patients are at especially high risk for neuroendocrine differentiation and PSA-discordant progression. 1

Imaging Considerations in Low-PSA Settings

When Standard Imaging May Miss Disease

• Conventional imaging (bone scan, CT, MRI) infrequently detects metastases when PSA is <5 ng/mL, even in the setting of biochemical recurrence. 3

• Bone scans are generally unnecessary for clinically localized disease when PSA is <20 ng/mL, as positivity is <5% even when post-radical prostatectomy PSA levels reach 40-45 ng/mL. 3, 1

• However, bone scans should be considered for Gleason 8 or greater disease, or stage T3 prostate cancer, even if PSA is <10 ng/mL, due to the risk of high-grade, PSA-discordant metastases. 3

Role of Advanced Imaging

• Novel PET tracers (fluciclovine, choline, PSMA) show greater sensitivity for detecting prostate cancer recurrence and metastases at low PSA values (<2.0 ng/mL) compared to conventional imaging. 3

• In patients with high clinical suspicion of metastasis despite negative conventional imaging and low PSA, novel PET-CT imaging should be contemplated, especially in the setting of a clinical trial. 3

Diagnostic Algorithm for Suspected PSA-Discordant Disease

When to pursue tissue diagnosis despite low PSA:

1. Perform biopsy of accessible metastatic lesions in mCRPC to identify small cell/neuroendocrine histomorphology when any of the following are present: 1

   • Lack of response to androgen-deprivation therapy (ADT)
   • Low PSA despite radiographic evidence of metastasis
   • Presence of visceral metastases
   • Initial Grade Group 5 disease

2. In 67% of PSA-discordant cases, patients were asymptomatic at the time metastases were detected, emphasizing the need for routine imaging in high-risk patients rather than relying solely on PSA or symptoms. 2

3. Alternative tumor markers may be elevated when PSA is not—during disease progression, 75% of PSA-discordant cases had elevated CEA, CA19-9, CA125, neuron-specific enolase, or pro-gastrin releasing peptide. 4

Critical Pitfalls to Avoid

• Never assume absence of metastases based solely on low or normal PSA in patients with high-grade (Gleason ≥8) or locally advanced (T3-T4) disease. 2

• Do not delay imaging in patients with Grade Group 5 disease even if PSA remains low, as these patients require early imaging and tissue confirmation per NCCN guidance. 1

• In 41% of PSA-discordant cases, progression to metastasis occurred without any increase in PSA from the nadir level, meaning stable PSA does not exclude progression. 2

• Complete physical evaluation and imaging studies may be indicated in surveillance of patients with high-grade, locally advanced tumors, especially when atypical histologic variants are present, rather than PSA-based surveillance alone. 2