Evaluation of the Physiological Response to Fractionated Radiotherapy
Sharifi H, Glide-Hurst C, Gopal A, Brown S, Chetty IJ, Movsas B, and Zhong H. Evaluation of the physiological response to fractionated radiotherapy. Med Phys 2017; 44(6):2813.
Purpose: To perform a preliminary exploration of radiation-induced changes in pulmonary function for lung cancer patients undergoing fractionated radiotherapy using 4DCT imaging. Methods: Serial 4DCT images of four lung cancer patients treated with 30 fractions of 2 Gy/fxn were acquired studied before, during, and after treatment. For each 4DCT image, multi-resolution B-Spline deformable image registrations (DIRs) were performed from the end-inhale to the end-exhale. The resultant displacement vector fields were used to generate Jacobian-based ventilation and lung volume-based compliance images. A finite element-based method was used to calculate volumetric changes. The lung excluding PTV was segmented. Isodose lines were used to divide the lung volume into 10 dose regions (0.5% to 90% of Dmax). Ventilation and compliance changes from the pre- to the mid- and the post-treatment were calculated. The changes with respect to dose were expressed as the slope of their linear fit. Positive slopes indicate the changes are increasing as function of dose. Results: Landmark based evaluations of DIRs showed average displacement errors of 3.0 mm. The ventilations at the pre, mid, and post-treatments were correlated to the differences of the lung volume between the end-inhale and end-exhale phases with the correlation coefficients equal to 0.99, 0.94, 0.46, and 0.97 for the 4 patients. For Jacobian-based ventilation, the slope of the linear fit increased by an average of 0.4 from the pre- to the mid-treatment, and 0.5 increase from the pre- to the post-treatment. For lung compliance, the average slope change was 0.6 from pre- to mid-treatment and 0.9 from pre- to post-treatment. Conclusion: These preliminary results showed that the ventilation changes continuously increase from pre to the post treatment, with slightly higher changes occurring after treatment. Since compliance changes mainly appeared three months after treatment, this indicates that mechanical changes occur at this time, a few months after treatment.