Why are four PET/CT (Positron Emission Tomography/Computed Tomography) scans performed after each Lutetium-177 (LU-177) infusion for prostate cancer?

Multiple Post-Therapy Scans After LU-177 Infusions for Prostate Cancer

Four post-therapy scans after each Lutetium-177 infusion for prostate cancer are performed to conduct essential dosimetry calculations that optimize treatment efficacy while minimizing toxicity to critical organs. 1

Purpose of Post-Therapy Scans

The multiple scans serve several critical purposes:

1. Dosimetry Calculations:

   • Scans are typically performed at 4,24, and 96 hours post-infusion to calculate the radiation absorbed dose to both tumors and normal tissues 2
   • These measurements allow for quantitative assessment of the pharmacokinetic uptake and clearance at the voxel level
   • The data is translated into absorbed dose using voxel S values, which is crucial for treatment planning

2. Treatment Efficacy Assessment:

   • Post-therapy scans document the distribution of the radiopharmaceutical in the body
   • They help evaluate the functional response to treatment 1
   • Higher tumor absorbed doses correlate with better PSA responses - patients receiving >14.1 Gy show significantly better PSA decline compared to those receiving <10 Gy 2

3. Safety Monitoring:

   • Monitoring radiation exposure to critical organs (kidneys, salivary glands, liver, spleen, bone marrow)
   • Mean absorbed doses to kidneys (0.39 Gy/MBq), submandibular and parotid glands (0.44 and 0.58 Gy/MBq), liver (0.1 Gy/MBq), spleen (0.06 Gy/MBq), and bone marrow (0.11 Gy/MBq) must be carefully tracked 2
   • This helps prevent exceeding threshold doses for radiation-related toxicity

Scan Protocol and Timing

The EANM/IAEA guidelines recommend:

• Multiple scans over several days after each LU-177 infusion
• Planar images to derive biokinetics over time
• SPECT and SPECT/CT fused images to provide three-dimensional activity distribution 1
• Input data should include blood, urine, and whole-body scans scheduled up to at least 3 days after treatment 1

Clinical Importance of Post-Therapy Imaging

1. Personalized Treatment Planning:

   • Significant correlations exist between screening 68Ga-PET/CT parameters and tumor/normal tissue dose 2
   • This provides rationale for patient-specific dosing in subsequent cycles
   • Patients with higher tumor burden show reduced salivary and kidney doses 2

2. Response Assessment:

   • Whole-body tumor-absorbed dose correlates with PSA response at 12 weeks 2
   • Post-therapy scans help determine if treatment modifications are needed for subsequent cycles

3. Safety Considerations:

   • Monitoring for potential toxicity to critical organs
   • Particularly important for patients with pre-existing risk factors for delayed renal toxicity 1
   • Helps guide decisions about dose adjustments for subsequent cycles

Treatment Protocol Context

Standard LU-177 PSMA therapy involves:

• Typical administered activity of 7.4 GBq (200 mCi) per cycle 3
• Treatment cycles every 6 weeks 3
• Total of 4-6 cycles in a complete treatment course 3
• Regular assessment of hematological parameters before each cycle 3

Conclusion

The four post-therapy scans after LU-177 infusions are essential components of the treatment protocol that enable precise dosimetry calculations, treatment efficacy assessment, and safety monitoring. This imaging-based approach allows for personalized treatment planning and optimization of the therapeutic ratio between tumor control and normal tissue toxicity.