A model-based 3D patient-specific pre-treatment QA method for VMAT using the EPID

Abstract

This study reports the development and validation of a model-based, 3D patient dose reconstruction method for pre-treatment quality assurance using EPID images. The method is also investigated for sensitivity to potential MLC delivery errors. Each cine-mode EPID image acquired during plan delivery was processed using a previously developed back-projection dose reconstruction model providing a 3D dose estimate on the CT simulation data. Validation was carried out using 24 SBRT-VMAT patient plans by comparing: (1) ion chamber point dose measurements in a solid water phantom, (2) the treatment planning system (TPS) predicted 3D dose to the EPID reconstructed 3D dose in a solid water phantom, and (3) the TPS predicted 3D dose to the EPID and our forward predicted reconstructed 3D dose in the patient (CT data). AAA and AcurosXB were used for TPS predictions. Dose distributions were compared using 3%/3 mm (95% tolerance) and 2%/2 mm (90% tolerance) γ-tests in the planning target volume (PTV) and 20% dose volumes. The average percentage point dose differences between the ion chamber and the EPID, AcurosXB, and AAA were 0.73 ± 1.25%, 0.38 ± 0.96% and 1.06 ± 1.34% respectively. For the patient (CT) dose comparisons, seven (3%/3 mm) and nine (2%/2 mm) plans failed the EPID versus AAA. All plans passed the EPID versus Acuros XB and the EPID versus forward model γ-comparisons. Four types of MLC sensitive errors (opening, shifting, stuck, and retracting), of varying magnitude (0.2, 0.5, 1.0, 2.0 mm), were introduced into six different SBRT-VMAT plans. γ-comparisons of the erroneous EPID dose and original predicted dose were carried out using the same criteria as above. For all plans, the sensitivity testing using a 3%/3 mm γ-test in the PTV successfully determined MLC errors on the order of 1.0 mm, except for the single leaf retraction-type error. A 2%/2 mm criteria produced similar results with two more additional detected errors.

FMEA of manual and automated methods for commissioning a radiotherapy treatment planning system

“…EPID‐based dosimetry has been investigated and developed by many groups. Lee et al compared EPID‐based planar dose map to MapCheck and radiographic film‐based dosimetry and concluded that the EPID‐based approach is appropriate for 2D dose verification, including complex intensity modulated fields; Mans et al, and McCowan et al developed model‐based 3D dose verification process using EPID for VMAT plan; a good review paper by Van Elmpt et al has summarized the utilization of EPID in nontransmission‐based (no phantom or patient) and the transmission‐based (with phantom or patient) measurement for both 2D and 3D dose verification. The major conclusions from those publications are that the use of EPID for dosimetry has matured and a variety of reliable and accurate EPID‐based dose verification programs have been developed and are being used in a growing number of clinics.…”

Utilising the Virtual Environment for Radiotherapy Training System to Support Undergraduate Teaching of IMRT, VMAT, DCAT Treatment Planning, and QA Concepts

“…With the increasing use of electronic portal imaging devices such as EPIDose (Sun Nuclear) and Portal Dosimetry (Varian Medical Systems, Palo Alto, CA) QA systems [22,23], it is possible that some private centres will change their clinical QA workflows potentially negating the need for setup dosimeters. The RTs working on the treatment machines are better able to utilise any gaps between patient treatments for the QA procedures.…”

Transit and non‐transit 3D EPID dosimetry versus detector arrays for patient specific QA

“…Commercially available detector arrays, such as Arc-CHECK (Sun Nuclear Corp., Melbourne, FL), Octavius 4D (PTW, Freiburg, Germany), and Delta4 (Scandidos AB, Upsala, Sweden), compare measured to planned 3D dose distributions. [18][19][20] In 3D transit EPID dosimetry, the dose is reconstructed within the patient or phantom based on EPID measurements acquired behind the patient or phantom. [7][8][9][10][11][12][13][14][15] Although initially developed for patient setup verification, Electronic Portal Imaging Devices (EPIDs) also have useful dosimetric characteristics as summarized in review articles.…”

“…In 3D non-transit EPID dosimetry, the dose is determined within the patient or phantom based on in air or fluence EPID measurements, that is, without an attenuating medium between the source and the detector. [18][19][20] In 3D transit EPID dosimetry, the dose is reconstructed within the patient or phantom based on EPID measurements acquired behind the patient or phantom. [21][22][23][24][25] A clear advantage of EPIDs is their availability which makes them suitable for large scale implementations.…”