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Towards Integrated Treatment Planning of Combined External Beam Radiotherapy and Targeted Radionuclide Therapy

I Marsh

I Marsh*, J Grudzinski , A Besemer , J Weichert , P Harari , B Bednarz , University of Wisconsin, Madison, WI


SU-K-FS1-16 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: Four Seasons 1

Purpose: Combining external beam radiotherapy (EBRT) with supplemental and complimentary cycles of targeted radionuclide therapy (TRT) can enhance the dose to the tumor, reduce local radiation toxicity, and target metastatic disease throughout the body. This work describes the integration of Monte Carlo personalized TRT dosimetry with a commercial treatment planning system to evaluate the dosimetric impacts on PTV coverage and metastatic disease sites.

Methods: CLR1404, a small molecule, phospholipid ether analog exhibits selective sequestration in multiple solid tumors. Patients proven to have metastatic cancer were administered ¹²⁴I-CLR1404 and imaged with PET/CT 5-6 times over 120-168 hours post-injection. Therapeutic ¹³¹I-CLR1404 absorbed dose rate (ADR) distributions were estimated for each time point using patient-specific Geant4 (v9.6) Monte Carlo simulations, accounting for the difference in decay rates. Voxel-level ADR distributions were integrated over time to predict the absorbed dose in temporally co-registered regions of interest. The EBRT dose distribution was determined using PinnacleTM treatment planning system to achieve typical clinical treatment planning objectives (TPOs) for uniformity and normal tissue constraints. The dose distributions from each treatment plan were combined as a weighted sum while maintaining TPOs from the EBRT.

Results: Injected activities achieving a mean dose of 2 Gy to the PTV were prescribed for TRT and the EBRT prescription doses were reduced to maintain TPOs. Increases of 2% in D₉₈/D₂ PTV heterogeneity as well as a 2.18 Gy boost to systemic metastatic regions were observed in a representative case.

Conclusion: This work demonstrates that personalized TRT treatment planning can be used in conjunction with EBRT to tailor specific doses to target tissues, maintain standard TPOs, and quantify the stimulating boost to systemic metastatic sites.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by 1 R21CA198392-01

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