Is Stereotactic Body Radiation Therapy (SBRT) indicated for a patient with a history of high-risk metastatic prostate adenocarcinoma to bone, currently on enzalutamide, with localized tumor and good response to systemic therapy?

SBRT Indications for Prostate Cancer

SBRT is indicated for clinically localized low- and intermediate-risk prostate cancer as definitive primary treatment, but is NOT indicated for metastatic disease such as this patient's case. 1

Primary Indication: Localized Disease Only

SBRT is specifically recommended for organ-confined prostate cancer in patients with appropriate risk stratification 1. The 2022 AUA/ASTRO guidelines explicitly state that SBRT should be utilized as part of advanced treatment planning and delivery procedures to optimize the therapeutic ratio of external beam radiation therapy for localized prostate cancer 1.

Appropriate Candidates for SBRT:

• Low-risk disease (PSA <10 ng/mL, Gleason <7, clinical stage T1-T2a): 7-year biochemical control rates of 95.6% 2
• Intermediate-risk disease (PSA 10-20 ng/mL or Gleason 7): 7-year biochemical control rates of 89.6% 2
• Favorable intermediate-risk specifically (Gleason 3+4 with PSA <10): biochemical disease-free survival of 93.5% 2

The NCCN 2014 guidelines note that SBRT delivers highly conformal, high-dose radiation in 5 or fewer fractions with excellent biochemical progression-free survival rates of 95%, 84%, and 81% for low-, intermediate-, and high-risk patients respectively at 5 years 1.

Why SBRT is NOT Indicated for This Patient

This patient has high-risk metastatic castration-resistant prostate cancer with bone metastases, which falls completely outside SBRT indications 3, 4. The evidence base for SBRT is exclusively in clinically localized disease without metastases 1, 2, 5.

Appropriate Management for Metastatic CRPC:

For this patient currently on enzalutamide with bone metastases, the correct approach includes:

• Continue systemic therapy with enzalutamide (already initiated) 3
• Mandatory bone-protective therapy with denosumab 120 mg subcutaneously every 4 weeks or zoledronic acid 4 mg IV every 3-4 weeks 3, 4
• Palliative radiation (single 8 Gy fraction) for symptomatic bone lesions only, NOT SBRT 1, 4
• Radium-223 if symptomatic bone-predominant disease without visceral metastases 1

Role of Radiation in Metastatic Disease

For metastatic prostate cancer, conventional palliative external beam radiation therapy (not SBRT) is used for:

• Painful bone metastases: Single 8 Gy fraction provides equivalent pain relief to multi-fraction regimens 1, 4
• Spinal cord compression: Urgent treatment with dexamethasone and radiation 1, 6
• Impending pathological fractures: Postoperative fractionated RT after surgical stabilization 1

SBRT to bone metastases is mentioned only in the context of postoperative treatment for spinal metastases to improve local control, not as primary treatment for metastatic disease 1.

Critical Contraindications to SBRT

Absolute contraindications include 1:

• Prior pelvic irradiation
• Active inflammatory disease of the rectum
• Permanent indwelling Foley catheter

Metastatic disease is an implicit contraindication as all SBRT studies and guidelines specifically address organ-confined, localized prostate cancer 1, 2, 5.

Monitoring if SBRT Were Appropriate (Localized Disease)

For patients who ARE appropriate SBRT candidates with localized disease, follow-up includes 7:

• PSA monitoring with median nadir of 0.11 ng/mL at 7 years expected 2
• Multiparametric MRI and/or biopsy for rising PSA after achieving long-term nadir 7
• Median time to isolated local failure (if occurs) is 72 months 7

In summary, SBRT has no role in metastatic prostate cancer management and should not be considered for this patient. The focus should remain on systemic therapy optimization, bone-protective agents, and palliative measures as clinically indicated 3, 4.