The application of transit dosimetry to precision radiotherapy

Abstract

A method of using electronic portal imaging (EPI) for transit dosimetry is described. In this method, a portal image of the treatment field is first aligned with a digitally reconstructed radiograph (DRR) to geometrically relate the computed tomography (CT) scan, used to generate the DRR, with the EPI. Then the EPI is corrected for scatter within the patient to yield a map of primary fluence striking the detector. This is backprojected through the planning CT data set to yield a distribution of primary fluence within the patient. This distribution is then convolved with dose deposition kemels to yield a map of dose delivery within the patient. Such a distribution may be compared with the dose distribution resulting from the original treatment plan in order to evaluate the adequacy of the treatment. This method has been evaluated using a humanoid phantom. We find the transit dosimetry relative dose distribution when compared with film and thermoluminescent dosimeter (TLD) measurements and compared with our planning system to agree within 2% in the pelvic region of a humanoid phantom.

Sensitivity study of an automated system for daily patient QA using EPID exit dose images

“…In vivo dose verification has been performed to verify delivered dose, typically only during the first fraction using point dose detectors such as diode, thermoluminescent dosimeters and optically stimulated luminescent dosimeters, and metal‐oxide semiconductor field effect transistor 3, 4, 5, 6. However, a point dosimeter can easily miss the errors that affect the area outside of the measurement point and can be insensitive to small errors because of placement uncertainty and movement due to patient breathing.…”

Guide to clinical use of electronic portal imaging

“…Efforts to determine and quantify ( in vivo ) dose in two and three dimensions are underway. The earliest works investigated the characteristics of the various EPIDs for transmission dose measurement 90 – 94 . These studies indicate that with the proper calibration and care, the EPID can be used to generate an exit dose image within 2–5 % of expected values.…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…33,84 Other studies converted EPID images to dose images at the detector plane, in some cases behind a phantom. 24,42,48,60,73,87,95,129 Alternatively, EPID images were used to reconstruct the dose in a plane within the patient or phantom 2,11,45,65,94,115,131 . Of these studies, some have explored the possibility of using an a-Si EPID for dose verification of IMRT fields.…”

“…79 Applications of portal dosimetry have been reported mainly for the type of EPIDs based on a liquid-filled (Li-Fi) ionisation chamber matrix 11,14,36,59,65,123 59,122 the CCD camera-based EPID 28,48,60,96,114,130 and a solid-state based EPID. 45 These studies compared the PDI with various combinations of film and ionisation chamber measurements. The accuracy in dose prediction of 3% for most of these studies has usually been limited to point doses on the central beam axis, low dose gradient regions and / or use of homogeneous phantoms.…”