It has been shown from an evaluation of the inverse reading of the dosemeter (1/M) against the inverse of the polarizing voltage (1/V), obtained with a number of commercially available ionization chambers, using dose per pulse values between 0.16 and 5 mGy, that a linear relationship between the recombination correction factor kS and dose per pulse (DPP) can be found. At dose per pulse values above 1 mGy the method of a general equation with coefficients dependent on the chamber type gives more accurate results than the Boag method. This method was already proposed by Burns and McEwen (1998, Phys. Med. Biol. 43 2033) and avoids comprehensive and time-consuming measurements of Jaffé plots which are a prerequisite for the application of the multi-voltage analysis (MVA) or the two-voltage analysis (TVA). We evaluated and verified the response of ionization chambers on the recombination effect in pulsed accelerator beams for both photons and electrons. Our main conclusions are: (1) The correction factor k(S) depends only on the DPP and the chamber type. There is no influence of radiation type and energy. (2) For all the chambers investigated there is a linear relationship between kS and DPP up to 5 mGy/pulse, and for two chambers we could show linearity up to 40 mGy/pulse. (3) A general formalism, such as that of Boag, characterizes chambers exclusively by the distance of the electrodes and gives a trend for the correction factor, and therefore (4) a general formalism has to reflect the influence of the chamber construction on the recombination by the introduction of chamber-type dependent coefficients.

No abstract

Electronic portal imaging device (EPID) plays an important role in radiation therapy portal imaging, geometric and dosimetric verification. Consistent image quality and stable radiation response is necessary for proper utilization that requires routine quality assurance (QA). A commercial ‘EPID QC’ phantom weighing 3.8 kg with a dimension of 25×25×4.8 cm3 is used for EPID QA. This device has five essential tools to measure the geometric accuracy, signal‐to‐noise ratio (SNR), dose linearity, and the low‐ and the high‐contrast resolutions. It is aligned with beam divergence to measure the imaging and geometric parameters in both X and Y directions, and can be used as a baseline check for routine QA. The low‐contrast tool consists of a series of holes with various diameters and depths in an aluminum slab, very similar to the Las Vegas phantom. The high‐resolution contrast tool provides the modulation transfer function (MTF) in both the x‐ and y‐dimensions to measure the focal spot of linear accelerator that is important for imaging and small field dosimetry. The device is tested in different institutions with various amorphous silicon imagers including Elekta, Siemens and Varian units. Images of the QA phantom were acquired at 95.2 cm source‐skin‐distance (SSD) in the range 1–15 MU for a 26×26 cm2 field and phantom surface is set normal to the beam direction when gantry is at 0° and 90°. The epidSoft is a software program provided with the EPID QA phantom for analysis of the data. The preliminary results using the phantom on the tested EPID showed very good low‐contrast resolution and high resolution, and an MTF (0.5) in the range of 0.3–0.4 lp/mm. All imagers also exhibit satisfactory geometric accuracy, dose linearity and SNR, and are independent of MU and spatial orientations. The epidSoft maintains an image analysis record and provides a graph of the temporal variations in imaging parameters. In conclusion, this device is simple to use and provides testing on basic and advanced imaging parameters for daily QA on any imager used in clinical practices.PACS number: 87.57 C‐, 87.57 N‐

Current dosimetry protocols from AAPM, DIN and IAEA recommend a cross-calibration for plane-parallel chambers against a calibrated thimble chamber for electron dosimetry. The rationale for this is the assumed chamber-to-chamber variation of plane-parallel chambers and the large uncertainty in the wall perturbation factor (p(wall)60Co)pp at 60Co for plane-parallel chambers. We have confirmed the results of other authors that chamber-to-chamber variation of the investigated chambers of types Roos, Markus, Advanced Markus and Farmer is less than 0.3%. Starting with a calibration factor for absorbed dose to water and on the basis of the three dosimetry protocols AAPM TG-51, DIN 6800-2 (slightly modified) and IAEA TRS-398, values for (p(wall)60Co)Roos of 1.024 +/- 0.005, (p(wall)60Co)Markus of 1.016 +/- 0.005 and (p(wall)60Co)Advanced Markus of 1.014 +/- 0.005 have been determined. In future this will permit electron dosimetry with the above-listed plane-parallel chambers having a calibration factor N(D, w)60Co without the necessity for cross-calibration against a thimble chamber.

The dosimetry protocols DIN 6800-2 and AAPM TG-51, both based on the absorbed dose to water concept, are compared in their theoretical background and in their application to electron dosimetry. The agreement and disagreement in correction factors and energy parameters used in both protocols will be shown and discussed. Measurements with three different types of ionization chambers were performed and evaluated according to both protocols. As a result the perturbation correction factor P(60Co)wall for the Roos chamber was determined to 1.024 +/- 0.5%.