Beam Hardening Effects on IMRT QA
Abstract
Purpose
Measurement based QA is required in clinical practice due to the delivery complexity of IMRT. Many factors can affect measurement accuracy and QA passing rate. In this study, we aimed to investigate the impact of beam hardening and energy dependency of detector response on IMRT QA.
Methods
IMRT QA with portal dosimetry and ArcCheck were employed in this study. The response of amorphous silicon detectors utilized by EPID have strong energy dependency. Because of the hardening effect, the average energy of MLC transmission is higher than the nominal beam. To consider this, MLC transmission and dosimetry leaf gap (DLG) were measured with EPID and used for portal dosimetry calculation. The response of the Diode detectors utilized by ArcCheck is also energy dependent. The detector response at the entry and exit side of the phantom is different because of beam hardening caused by the phantom attenuation. The detectors at the entry and exit sides shall be both calibrated and applied respectively according to the beam direction. One practical way to compensate for the beam hardening effect is to change the material density in the center region of the phantom. Twenty IMRT and VMAT plans for beam energies from 6MV to 15MV were used for this investigation.
Results
The measured MLC transmission and DLGs with EPID are significantly different from those measured with an ionization chamber. The IMRT QA passing rate for criteria of 2%/2mm and 3%/2mm were both improved by up to 10% when beam hardening effects were considered. The average QA passing rate increases from 89% to 97% for portal dosimetry and from 87% to 95% for ArcCheck.
Conclusion
Beam hardening and energy dependency of detector response has significant effects on IMRT QA. To achieve more consistent QA results, their effects shall be considered in calculation or measurement calibration.