In-vivo EPID dosimetry for IMRT and VMAT based on through-air predicted portal dose algorithm

Abstract

We have adapted the methodology of Berry et al. (2012) for Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) treatments at a fixed source to imager distance (SID) based on the manufacturer's through-air portal dose image prediction algorithm. In order to fix the SID a correction factor was introduced to account for the change in air gap between patient and imager. Commissioning data, collected with multiple field sizes, solid water thicknesses and air gaps, were acquired at 150 cm SID on the Varian aS1200 EPID. The method was verified using six IMRT and seven VMAT plans on up to three different phantoms. The method's sensitivity and accuracy were investigated by introducing errors. A global 3%/3 mm gamma was used to assess the differences between the predicted and measured portal dose images. The effect of a varying air gap on EPID signal was found to be significant - varying by up to 30% with field size, phantom thickness, and air gap. All IMRT plans passed the 3%/3 mm gamma criteria by more than 95% on the three phantoms. 23 of 24 arcs from the VMAT plans passed the 3%/3 mm gamma criteria by more than 95%. This method was found to be sensitive to a range of potential errors. The presented approach provides fast and accurate in-vivo EPID dosimetry for IMRT and VMAT treatments and can potentially replace many pre-treatment verifications.

A method for in vivo treatment verification of IMRT and VMAT based on electronic portal imaging device

“…However, for the CIRS thorax phantom, the small deviation of the position has no obvious effect on the change in equivalent thickness and little influence on the TI. These results agreed with the results reported by Najem et al [ 37 ]. For the same reason, the result does not change significantly when the gantry angle error is 5°.…”

“…The EPID needs to be moved for each beam to ensure that the air gap is constant, which is difficult to achieve in VMAT. Najem et al [ 37 ] improved Berry's method, which can be used in any exit gap. In this method, the open image was calculated by an empirical attenuation correction factor to obtain the TI.…”

Wprowadzenie opartego o obrazy transit-EPID rozszerzenia istniejącego protokołu IGRT

“…Żadne z wymienionych narzędzi kontroli jakości nie dostarcza jednak informacji na temat powtarzalności realizacji leczenia podczas napromieniania. Zarówno niezgodności geometryczne, które mogą być wychwycone, choć nie w pełni wyeliminowane, dzięki stosowaniu protokołu IGRT, jak i błędy w realizacji danej frakcji leczenia przez akcelerator (które mogą zostać niewykryte w pojedynczym pomiarze dawki in-vivo czy w weryfikacji przed rozpoczęciem leczenia) mogą rzutować na dawkę zdeponowaną w pacjencie, a więc na skuteczność jak i toksyczność terapii [5]. Coraz bardziej skomplikowane techniki napromieniania wymagają coraz bardziej zaawansowanych narzędzi kontroli ich realizacji.…”

“…W tym celu często stosowana jest rejestracja obrazów przez kasetę portalową (EPID) w trakcie napromieniania pacjenta. Proponowane koncepcje obejmują zarówno rekonstrukcję punktową dawki, jak i odtworzenie 2D lub 3D rozkładu dawki w pacjencie [5,7,8,9]. Pozwala to ocenić, nawet w czasie rzeczywistym [10], zgodność realizacji frakcji napromieniania z planem leczenia.…”

Comparison of calculation algorithms to predict the IQM detector response for various modulation degrees of VMAT treatment plans on linear accelerator equipped with the HD120 MLC

“…The prediction is obtained with a separate software program. 4 The transmission detector inserted between the radiation source and the patient is also a tool for on-line photon beam dose verification. This allows independent monitoring of radiation delivery, including multileaf collimator (MLC) leaf motion.…”