ASi-EPID transit signal calibration for dynamic beams: a needful step for the IMRT in vivo dosimetry

Francesca Greco,Rocchina Caivano,Savino Cilla,Andrea Fidanzio,Vincenzo Fusco,Lorenzo Placidi,Luigi Azario,Angelo Piermattei

doi:10.1007/s11517-013-1094-x

Abstract

This work reports a method based on correlation functions to convert EPID transit signals into in vivo dose values at the isocenter point, D iso, of dynamic IMRT beams supplied by Varian linac. Dose reconstruction for intensity-modulated beams required significant corrections of EPID response, due to the X-ray component transmitted through multileaf collimator. The algorithm was formulated using a set of simulated IMRT beams. The beams were parameterized by means of a fluence inhomogeneity index, FI, introduced to describe the degree of beam modulation with respect to open beams. This way, all dosimetric parameters involved in D iso reconstruction algorithm, such as the correlation functions, the correction factor for EPID to phantom distance and the modulated tissue maximum ratios, were determined as a function of the FI index. Clinical IMRT beams were used to irradiate a homogeneous phantom, and for each beam, the agreement between the reconstructed dose, D iso, and the dose computed by TPS, D iso,TPS, was well within 5 %. Moreover, the average ratios, R, between the D iso, and D iso,TPS, resulted equal to 1.002 ± 0.030. Thirty-five IMRT fields of 5 different patients undergoing radiotherapy for head–neck tumors were tested and the results were displayed on a computer screen after 2 min from the end of the treatment. However, 350 in vivo tests supplied an average ratio R equal to 1.004 ± 0.040. The in vivo dosimetry procedure here presented is among the objectives of a National Project financially supported by the Istituto Nazionale di Fisica Nucleare for the development of in vivo dosimetry procedures (Piermattei et al. in Nucl Instrum Methods Phys Res B 274:42–50, 2012) connected to the Record–Verify system of the radiotherapy center.

Highlights

As pointed out by the Radiotherapy Risk Profile report by WHO [25] the major causes of severe radiotherapy incidents are due to human errors
Using square uniform beam, delivered in dynamic multileaf collimators (MLC) modality, the kdy(w,L,MUc) ratios (Eq 2) were obtained averaging the values determined by the three EPIDs
We have demonstrated the feasibility of an in vivo dosimetry (IVD) method based on correlation functions to reconstruct Diso for dynamic IMRT beams by means of a software connected to the Record–Verify system of the center

Summary

Introduction

As pointed out by the Radiotherapy Risk Profile report by WHO [25] the major causes of severe radiotherapy incidents are due to human errors. Med Biol Eng Comput (2013) 51:1137–1145 reports experiences for dose reconstruction in a single point, in many points on planes or in 3D [18, 19, 22, 24, 29, 32] These methods generally require commissioning measurements in a water-equivalent phantom and many manual analysis that are time consuming. The most common EPIDs use the amorphous silicon (a-Si) as an indirect detector [1] that presents a response dependent on the change of the low-energy X-ray component [13] This component can change with the distance from the central axis [20] with the field size and the interaction with the multileaf collimators (MLC) [11]. A software package connected with the Record–Verify system of the center was realized to reconstruct the isocenter dose to compare with the planned one

Objectives

Methods

Results

Conclusion