Source position verification and dosimetry in HDR brachytherapy using an EPID

• Published in: Medical physics (Lancaster) Vol. 40; no. 11; pp. 111706 - n/a
• Main Authors: Smith, R. L., Taylor, M. L., McDermott, L. N., Haworth, A., Millar, J. L., Franich, R. D.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-11-2013
• Subjects: Algorithms; ANATOMY; biomedical equipment; Biomedical imaging; BRACHYTHERAPY; Brachytherapy - methods; Calibrating of instruments or apparatus; CALIBRATION; Catheters; Digital computing or data processing equipment or methods, specially adapted for specific applications; DOSE RATES; DOSIMETRY; Dosimetry/exposure assessment; electronic portal imaging device; EPID; Equipment Design; ERRORS; high dose rate brachytherapy; Humans; Image data processing or generation, in general; Image guided radiation therapy; IMAGE PROCESSING; Infrared sources; IRIDIUM 192; Iridium Radioisotopes - chemistry; Medical Errors - prevention & control; medical image processing; Medical imaging; Medical X‐ray imaging; Movement; PHANTOMS; Photons; QUALITY ASSURANCE; Quality assurance in radiotherapy; Quality Control; RADIATION DOSE DISTRIBUTIONS; RADIATION PROTECTION AND DOSIMETRY; Radiation therapy; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - methods; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; Reproducibility of Results; Scintigraphy; Standards and calibration; Testing or calibrating of apparatus or arrangements provided for in groups G01D1/00 to G01D15/00; Therapeutic applications, including brachytherapy; treatment verification; VERIFICATION; X‐ray imaging; X‐rays; high dose rate brachytherapy; electronic portal imaging device; dosimetry; treatment verification; EPID
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4823758

Purpose:
Accurate treatment delivery in high dose rate (HDR) brachytherapy requires correct source dwell positions and dwell times to be administered relative to each other and to the surrounding anatomy. Treatment delivery inaccuracies predominantly occur for two reasons: (i) anatomical movement or (ii) as a result of human errors that are usually related to incorrect implementation of the planned treatment. Electronic portal imaging devices (EPIDs) were originally developed for patient position verification in external beam radiotherapy and their application has been extended to provide dosimetric information. The authors have characterized the response of an EPID for use with an192Ir brachytherapy source to demonstrate its use as a verification device, providing both source position and dosimetric information.
Methods:
Characterization of the EPID response using an192Ir brachytherapy source included investigations of reproducibility, linearity with dose rate, photon energy dependence, and charge build-up effects associated with exposure time and image acquisition time. Source position resolution in three dimensions was determined. To illustrate treatment verification, a simple treatment plan was delivered to a phantom and the measured EPID dose distribution compared with the planned dose.
Results:
The mean absolute source position error in the plane parallel to the EPID, for dwells measured at 50, 100, and 150 mm source to detector distances (SDD), was determined to be 0.26 mm. The resolution of the z coordinate (perpendicular distance from detector plane) is SDD dependent with 95% confidence intervals of ±0.1, ±0.5, and ±2.0 mm at SDDs of 50, 100, and 150 mm, respectively. The response of the EPID is highly linear to dose rate. The EPID exhibits an over-response to low energy incident photons and this nonlinearity is incorporated into the dose calibration procedure. A distance (spectral) dependent dose rate calibration procedure has been developed. The difference between measured and planned dose is less than 2% for 98.0% of pixels in a two-dimensional plane at an SDD of 100 mm.
Conclusions:
Our application of EPID dosimetry to HDR brachytherapy provides a quality assurance measure of the geometrical distribution of the delivered dose as well as the source positions, which is not possible with any current HDR brachytherapy verification system.