Correlation of Delivery Reproducibility With Anatomical Modifications and Intrafraction Motion During Treatments Using Helical Tomotherapy (HT)-Integrated Detectors

Abstract

To investigate the correlation of the dose reproducibility acquired during treatment delivered, using integrated HT MVCT detectors, with the patient anatomy modifications that can occur during fractionated treatments. To assess the dosimetric effect of intra-fraction motion on the signal acquired by MVCT detectors during the HT treatment delivery (sinogram). First, the MVCT detector data variations related only to the delivery component were quantified. Ten HT plans, with increased level of complexity, were ranked using an extension of the Webb's Modulation Index (MI) for helical delivery. These plans were delivered several times with a phantom positioned between the radiation source and the detectors. Then, five H&N plans, acquired by MVCT detectors during clinical treatments delivery, were studied and correlated with patient anatomic variations evaluated by daily MVCT imaging. As a last step, an anthropomorphic phantom placed on a respiratory gating platform was used to simulate breathing movement during treatments. Setting different ranges of motion and respiratory amplitudes, a clinical HT plan was delivered several time and acquired with MVCT detectors. Two indices were introduced: the “Reproducibility Index” (RI) based on the definition of M.S. Padro (the RIDELIV was used to evaluate data acquired with the phantom while the RITREAT for patient data) and the SIN_DEV index, that quantify the differences on the acquired sinograms respect to the reference one. This indexes were related to the treatment complexity, the patient's anatomy variation and the intra-fraction motions. A dependence of the RIDELIV with the MI factor was observed (R2 = 0.6367), pointing out higher RIDELIV values for the most complex plan. Our analysis confirm the existence of a correlation between the RITREAT and the patient anatomy reductions in the neck diameter (R2 = 0.687) and parotids shrinkage (R2 = 0.521). The analysis of SIN_DEV index related to the anatomical modifications was assessed by means of Spearman correlation (SC), indicate higher SIN_DEV indexes at the end of the treatment that correspond to a larger parotids shrinkage (SC = -0.78, p<0.0001) and neck diameter reduction (SC = -0.75, p<0.0001). The preliminary results concerning the sinogram variations related to the respiratory amplitude and frequency, have showed SIN_DEV indexes up to 5% respect to no movement MVCT data, with a 5,4% of the sinogram area over 5% of difference. This work indicates the potential of using the HT-MVCT detectors data, acquired during the treatment delivery, to evaluate treatment reproducibility, patient's anatomy modifications and intra -fraction movements.