International Journal of Radiation Oncology*Biology*Physics
Physics ContributionInitial Clinical Experience Performing Patient Treatment Verification With an Electronic Portal Imaging Device Transit Dosimeter
Introduction
Dosimetric verification during patient treatment is an effective method of discovering errors. A recent analysis at the Netherlands Cancer Institute identified 17 serious errors in a cohort of 4337 patients (0.4%). Nine of the 17 (53%) would have been missed if verification had not been performed during treatment. These 9 were broken down into patient anatomic changes (7 of 9) and accidental modification of treatment parameters in the record and verify system after the plan was checked but before it was delivered (2 of 9) (1). Point detectors, such as diodes, thermoluminescent dosimeters, or metal-oxide semiconductor field effect transistors, have historically been used to perform these treatment time dose verification measurements. However, verification at a single point is insufficient when evaluating an intensity-modulated radiation therapy (IMRT) plan, owing to the high level of modulation across the treatment field (2). Therefore, planar or volumetric methods are necessary, which is why dosimetric applications for electronic portal imaging devices (EPIDs) have been an active area of research and development 3, 4.
A few institutions have implemented transit or in vivo EPID dosimetry using in-house research applications 3, 4. “Transit” specifically refers to measuring the dose at a plane behind the patient. One disadvantage of research software is that its use is generally constrained to a few academic or research centers. A recent publication describes a 2-dimensional transit dosimetry (2DTD) method that extends a commercial vendor's widely distributed through-air pretreatment EPID dosimetry algorithm to transit verification during treatment (5). That study presented verification results through phantom materials of increasing complexity. However, patients differ from phantoms in that they experience organ motion, deformation, treatment-related swelling, weight loss, and tumor regression. Therefore, an institutional review board (IRB)-approved study was initiated at Columbia University Medical Center to prospectively evaluate the 2DTD algorithm's performance on a patient population and to analyze the issues that would arise in a widespread clinical adoption of transit EPID dosimetry.
Section snippets
Methods and Materials
All portal dose images (PDIs) were acquired on a Varian (Varian Medical Systems, Palo Alto, CA) aSi1000 amorphous silicon EPID managed by the Image Acquisition System 3 software package. The EPID was attached to a Varian TrilogyTx linear accelerator via the Varian Exact Arm. The EPID detection layer consists of a 30 × 40-cm2 array of 768 × 1024 photodiodes, resulting in a pixel size of 0.39 mm. No extra buildup material was applied to the imager. The EPID pixel sensitivity matrix and dosimetric
Results
Eleven patients were enrolled on the IRB-approved transit dosimetry protocol before planning. Two patients were subsequently removed because of the use of an insufficient number of coplanar beams in their treatment plans. Therefore, there were a total of 9 patients available for analysis. The study was terminated before the targeted accrual of 20 patients, owing to the replacement of Varian's treatment planning and record and verify systems with those of a different vendor within the department.
Discussion
One purpose of this prospective study was to identify challenges that may arise in the course of a widespread clinical adoption of EPID transit dosimetry. One geometric challenge that became clear was the restriction of verifiable beams to coplanar beam angles, to avoid collisions between the EPID and the patient or couch. Two of 11 patients (18%) initially enrolled in this study were rejected owing to an insufficient number of coplanar beams. Alternate verification techniques may be necessary
Conclusions
The 2DTD algorithm, previously described and verified in a phantom study (5), is feasible for use in treatment delivery verification for real patients. On average, 95.7% of the pixels in the mPDIts pass a 5%, 3-mm γ comparison with the corresponding pPDIts when interference from the linear accelerator couch is excluded. A clinical case study featuring a patient who had bilateral pleural effusion during simulation and planning that had resolved by treatment time was used to illustrate the
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Conflict of interest: none.