Author: Joseph Barbiere, Joseph Hanley, Brett Lewis, Jay Mistry, Alois M. Ndlovu, Roland Teboh 👨🔬
Affiliation: Hackensack University Medical Center, HUMC, Jersey Shore University Medical Center 🌍
Purpose:
Quantitative PET-CT can evaluate clinical response to radiotherapy. For tumors less than approximately 3cm partial volume effects due to the poor PET resolution makes accurate measurement of metabolic parameters challenging. This work presents a unique solution with corresponding serial measurements during a course of lung SBRT.
Methods:
Four PET-CT data sets were acquired with RefleXion during BGRT treatment at 0, 1250, 2500, and 3750 cGy over a 7-day course. The fused images were analyzed using Eclipse tools and MATLAB. The PET Point Spread Function (PSF) was computed using an appropriate phantom. Since the convolution PSF matrix is singular, direct reconstruction of the true Activity Concentration (AC) is impossible. A uniform concentration AC model (TRUE) in varying backgrounds was constructed with optimized parameters such that the error between the computed AC equal to TRUE*PSF where * is the convolution operator and the measured AC was minimized. For comparison the maximum and means of the highest 80%, 1.5cm3 sphere, and PTV concentrations were also measured. All values were normalized to the injected activity.
Results:
The uniform concentration was statistically equal to the maximum AC (ACmax).
All serial data indicated a sequential increase with delivered dose.
ACmax showed the highest initial value with the others decreasing by as much as 60%.
ACmax showed an increase of 60% after treatment completion.
Conclusion:
Rapid convergence of the uniform model is facilitated by priori knowledge of the CT and PET parameters for amplitude, width, and background
The uniform model with amplitude ACmax explains why previous work has shown the best clinical correlations with maximum values. For example, maximum SUV can be a predictor of survival. The maximum is not just a single pixel value but indicative of the true distribution throughout the whole PTV.
There is a measurable serial increase in AC during lung SBRT.