Simplifying EPID dosimetry for IMRT treatment verification

Journal Article · Tue Feb 15 04:00:00 EST 2011 · Medical Physics

DOI:https://doi.org/10.1118/1.3547714· OSTI ID:22098538

Pecharroman-Gallego, R.; Mans, Anton; Sonke, Jan-Jakob; Stroom, Joep C.; Olaciregui-Ruiz, Igor; Herk, Marcel van; Mijnheer, Ben J. ^[1]

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1. Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam (Netherlands)

Purpose: Electronic portal imaging devices (EPIDs) are increasingly used for IMRT dose verification, both pretreatment and in vivo. In this study, an earlier developed backprojection model has been modified to avoid the need for patient-specific transmission measurements and, consequently, leads to a faster procedure. Methods: Currently, the transmission, an essential ingredient of the backprojection model, is estimated from the ratio of EPID measurements with and without a phantom/patient in the beam. Thus, an additional irradiation to obtain ''open images'' under the same conditions as the actual phantom/patient irradiation is required. However, by calculating the transmission of the phantom/patient in the direction of the beam instead of using open images, this extra measurement can be avoided. This was achieved by using a model that includes the effect of beam hardening and off-axis dependence of the EPID response on photon beam spectral changes. The parameters in the model were empirically obtained by performing EPID measurements using polystyrene slab phantoms of different thickness in 6, 10, and 18 MV photon beams. A theoretical analysis to verify the sensitivity of the model with patient thickness changes was performed. The new model was finally applied for the analysis of EPID dose verification measurements of step-and-shoot IMRT treatments of head and neck, lung, breast, cervix, prostate, and rectum patients. All measurements were carried out using Elekta SL20i linear accelerators equipped with a hydrogenated amorphous silicon EPID, and the IMRT plans were made using PINNACLE software (Philips Medical Systems). Results: The results showed generally good agreement with the dose determined using the old model applying the measured transmission. The average differences between EPID-based in vivo dose at the isocenter determined using either the new model for transmission and its measured value were 2.6{+-}3.1%, 0.2{+-}3.1%, and 2.2{+-}3.9% for 47 patients treated with 6, 10, and 18 MV IMRT beams, respectively. For the same group of patients, the differences in mean {gamma} analysis (3% maximum dose, 3 mm) were 0.16{+-}0.26%, 0.21{+-}0.24%, and 0.02{+-}0.12%, respectively. For a subgroup of 11 patients, pretreatment verification was also performed, showing similar dose differences at the isocenter: -1.9{+-}0.9%, -1.4{+-}1.2%, and -0.4{+-}2.4%, with somewhat lower mean {gamma} difference values: 0.01{+-}0.09%, 0.01{+-}0.07%, and -0.09{+-}0.10%, respectively. Clinical implementation of the new model would save 450 h/yr spent in measurement of open images. Conclusions: It can be concluded that calculating instead of measuring the transmission leads to differences in the isocenter dose generally smaller than 2% (2.6% for 6 MV photon beams for in vivo dose) and yielded only slightly higher {gamma}-evaluation parameter values in planes through the isocenter. Hence, the new model is suitable for clinical implementation and measurement of open images can be omitted.

• OSTI ID: 22098538
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 2 Vol. 38; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
62 RADIOLOGY AND NUCLEAR MEDICINE
COMPUTER CODES
DOSIMETRY
HEAD
IMAGES
IN VIVO
IRRADIATION
LINEAR ACCELERATORS
LUNGS
MAMMARY GLANDS
NECK
PATIENTS
PHANTOMS
PHOTON BEAMS
POLYSTYRENE
PROSTATE
RADIATION DOSES
RADIOTHERAPY
RECTUM
VERIFICATION