Radiobiological Analysis of Isotoxic SBRT Simultaneous Integrated Boost of Multi-Parametric MR Defined Dominant Intraprostatic Lesions
M Dumas1*, M Leney2, J Kim1, E Mohamed1, N Wen1 (1) Henry Ford Health System, Detroit, MI (2) Northern Light Health, Brewer, ME
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TH-E-TRACK 6-6 (Thursday, 7/29/2021) 3:30 PM - 4:30 PM [Eastern Time (GMT-4)]
Purpose: To assess the utility of isotoxically boosting dominant intraprostatic lesions (DIL) by analyzing TCP and rectal NTCP with and without a DIL simultaneous integrated boost (SIB) using an MR-only workflow.
Methods: A dosimetric study was performed on 10 patients with biopsy-proven prostate cancer who underwent T2, mDixon, and diffusion-weighted imaging on a 3T MRI. A physician contoured the DIL based on T2 and apparent diffusion coefficient(ADC) images. For planning, synthetic CT images were generated from fat and water mDixon sequences using a k-means segmentation-based method for assigning HU values to predefined tissue types. SIB and non-boosted plans were created for each patient. The entire prostate (PTVprostate) was prescribed 36.25Gy in 5 fractions. The PTVDIL was isotoxically boosted to the maximum dose allowed while maintaining the non-boosted rectal NTCP. Prescribed dose and rectum constraints were based on the hypo-FLAME and 50-Gy SBRT DIL boost clinical trials. The linear-Poisson tumor control probability (TCP) radiobiological model and Lyman-Kutcher-Burman (LKB) model for normal-tissue complication probability (NTCP) were employed. Radiobiological calculations were done with BioSuite software (Uzan et al 2011) to determine individualized TCPs based on each patient’s ADC map and clonogen density.
Results: Clonogen density in the DIL ranged from 4.4E08-1.2E09 clonogens/cc. Mean dose and D95 to the SIBs were 54.5±1.9 Gy and 49.4±2.7 Gy. The average TCP was 99.6±0.6% with the SIB compared to 96.0±0.9% without the boost. The average NTCP for incidence of late rectal toxicity grade ≥2 was 4.4±2.9% (1.0-9.2%). Rectal doses to 1cc were within the protocol guidelines: 36.3±0.7 Gy and 36.6±0.9 Gy between the standard and SIB plans respectively.
Conclusion: We demonstrated the viability of MR-only SBRT treatment planning for prostate cancer with an isotoxically boosted DIL. DIL dose was escalated to an average D95 of 49.4Gy, while maintaining rectal toxicity limits and improving overall TCP by 4%.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a Research Scholar Grand from the American Cancer Society (RSG-15-137-01-CCE) and the National Cancer Institute (R01CA204189).
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