What is the recommended treatment approach for a patient with oligometastatic prostate cancer (Prostate Cancer with limited metastases) metastasized to the ischium and pubis, including definitive doses for stereotactic body radiation therapy (SBRT) or intensity-modulated radiation therapy (IMRT)?

Definitive SBRT Doses for Oligometastatic Prostate Cancer to Pelvic Bones

For oligometastatic prostate cancer with bone metastases to the ischium and pubis, SBRT should be delivered at doses ≥18 Gy in a single fraction or 30 Gy in 3 fractions, as these regimens provide optimal local control (88-95% at 2 years) with acceptable toxicity. 1

Evidence-Based Dose Recommendations

Single-Fraction SBRT Regimens

• 18-24 Gy in 1 fraction is the preferred single-fraction approach, with doses ≥18 Gy achieving 95% metastasis control at 2 years compared to only 58% with 16 Gy 1
• 20 Gy in 1 fraction was used successfully in the POPSTAR trial with >90% local progression-free survival at 2 years and only 14% grade 2 toxicity and 3% grade 3 toxicity 2, 3
• 24 Gy in 1 fraction demonstrated superior complete pain response (35% at 3 months, 32% at 6 months) compared to conventional radiotherapy in phase II/III trials 2

Multi-Fraction SBRT Regimens

• 30 Gy in 3 fractions provides excellent local control with no local failures reported in patients treated with this regimen 1
• 30-50 Gy in 3-5 fractions is the dose range used in ongoing prospective trials for oligometastatic prostate cancer 4
• 35 Gy in 5 fractions was evaluated in phase II trials for bone metastases with no grade 3 or 4 toxicity 2

Clinical Context and Patient Selection

Oligometastatic Disease Definition

• Patients should have ≤3-5 metastatic lesions identified on advanced imaging (PSMA-PET preferred over conventional imaging) 2, 4
• The 2024 AUA/ASTRO/SUO guidelines support SBRT for oligometastatic recurrence following primary therapy, with improved ADT-free survival demonstrated in randomized trials 2

Treatment Outcomes

• Local control rates: 82-93% at 2 years with appropriate dosing 3, 1
• Biochemical response: 75% of patients achieved biochemical response in prospective phase II trials 2
• ADT-free survival: STOMP trial demonstrated 21 months versus 13 months with observation alone (HR 0.60) 2
• Progression-free survival: Median 6.6 months, with castration-sensitive patients experiencing better outcomes than castration-resistant patients 5

Treatment Planning Considerations

Technical Requirements

• SBRT should be delivered using advanced techniques (CyberKnife, VMAT, or IMRT-based SBRT) with appropriate motion management for pelvic bone lesions 6, 4
• Planning target volume should account for setup uncertainty and any potential motion, though pelvic bones have minimal respiratory motion 6
• Dose calculation should use advanced algorithms (type B models) for accurate dose delivery 6

Toxicity Profile

• Grade 3 toxicity occurs in only 3% of patients with single-fraction SBRT 2, 3
• Pain flare is the most common acute toxicity: grade 1 in 9% and grade 2 in 3% of patients 1
• No grade 2 or greater late toxicities were reported in the largest single-institution series 1
• Vertebral compression fracture risk exists but is rare (one grade 4 event reported in phase II/III trials) 2

Critical Dose-Response Relationship

The dose threshold of 18 Gy in a single fraction is critical: lesions treated with 16 Gy had only 58% metastasis control versus 95% with ≥18 Gy at 2 years (P≤0.001) 1. This represents a clinically significant difference that should guide dose selection.

Factors Favoring Higher Doses

• Oligoprogressive disease (versus oligorecurrent) requires more aggressive dosing, as these patients have 9-fold worse local control 5
• Castration-resistant disease benefits from higher doses, though systemic therapy remains the backbone of treatment 5, 1
• Longer time from diagnosis to SBRT correlates with worse local control, suggesting more aggressive biology requiring higher doses 5

Integration with Systemic Therapy

• SBRT can be delivered with or without concurrent androgen deprivation therapy, though 48% of patients remained ADT-free at 2 years when SBRT was used as monotherapy 3
• For castration-sensitive oligometastatic disease, SBRT may delay ADT initiation and potentially reduce metabolic and cardiovascular adverse effects 2
• The role of combining SBRT with androgen receptor signaling inhibitors or chemotherapy in oligometastatic disease requires further study, though concomitant systemic therapy is permitted 2, 5

Common Pitfalls to Avoid

• Underdosing: Using 16 Gy or lower results in significantly inferior local control and should be avoided for definitive treatment 1
• Inappropriate patient selection: Patients with widespread polymetastatic disease or uncontrolled primary tumors should not receive SBRT as definitive therapy 7
• Ignoring imaging sensitivity: Conventional imaging may miss additional metastases; PSMA-PET is preferred for accurate staging 2
• Delaying treatment: Prolonged time from metastasis diagnosis to SBRT correlates with worse outcomes 5