Abstract
PurposeWe introduce a technique that employs a 2D detector in transmission mode (TM) to verify dose maps at a depth of dmax in Solid Water. TM measurements, when taken at a different surface‐to‐detector distance (SDD), allow for the area at dmax (in which the dose map is calculated) to be adjusted.MethodsWe considered the detector prototype “MP512” (an array of 512 diode‐sensitive volumes, 2 mm spatial resolution). Measurements in transmission mode were taken at SDDs in the range from 0.3 to 24 cm. Dose mode (DM) measurements were made at dmax in Solid Water. We considered radiation fields in the range from 2 × 2 cm2 to 10 × 10 cm2, produced by 6 MV flattened photon beams; we derived a relationship between DM and TM measurements as a function of SDD and field size. The relationship was used to calculate, from TM measurements at 4 and 24 cm SDD, dose maps at dmax in fields of 1 × 1 cm2 and 4 × 4 cm2, and in IMRT fields. Calculations were cross‐checked (gamma analysis) with the treatment planning system and with measurements (MP512, films, ionization chamber).ResultsIn the square fields, calculations agreed with measurements to within ±2.36%. In the IMRT fields, using acceptance criteria of 3%/3 mm, 2%/2 mm, 1%/1 mm, calculations had respective gamma passing rates greater than 96.89%, 90.50%, 62.20% (for a 4 cm SSD); and greater than 97.22%, 93.80%, 59.00% (for a 24 cm SSD). Lower rates (1%/1 mm criterion) can be explained by submillimeter misalignments, dose averaging in calculations, noise artifacts in film dosimetry.ConclusionsIt is possible to perform TM measurements at the SSD which produces the best fit between the area at dmax in which the dose map is calculated and the size of the monitored target.
Highlights
Conformal radiotherapy techniques such as intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc radiotherapy (VMAT)[1,2] require accurate verification of treatment plans
Pretreatment quality assurance (QA)[3] considers point‐dose measurements performed with an ionization chamber[4] and dose distribution measurements performed with an electronic portal imaging device (EPID),[5,6,7] a phantom‐based electronic array[8,9,10,11] or films
Using the averaged M (0.0196) and the BA0 value corresponding to any given field size, the dose in Dose mode (DM) at dmax was calculated using the transmission mode (TM) measurement at a given surface‐to‐detector distance (SDD) as: DM 1⁄4 TMSDD Â ðBA0 À M Â SDDÞ
Summary
Conformal radiotherapy techniques such as intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc radiotherapy (VMAT)[1,2] require accurate verification of treatment plans. An in vivo verification approach validates, in real time, accuracy, and integrity of treatment plans; parameters monitored include, for instance, the output of a medical linear accelerator (linac) and the position and/or movement of the leaves of a multileaf collimator (MLC).[12,13,14,15] Solutions for in vivo monitoring include[16] the use of transit and transmission detectors Transit detectors such as EPIDs are placed so that the beam penetrates the patient first, and the detector.[17,18,19] QA with transit EPIDs is challenging; their response is energy dependent and there is additional scatter from the patient; they are not able to discriminate between changes in signal due to changes in fluence incident on the patient from changes in signal due to anatomical variations within the patient.[20]
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