A formalism for independent treatment verification has been developed for Gamma Knife radiosurgery in analogy to the second checks being performed routinely in the field of external beam radiotherapy. A verification algorithm is presented, and evaluated based on its agreement with treatment planning calculations for the first 40 Canadian Gamma Knife patients. The algorithm is used to calculate the irradiation time for each shot, and the value of the dose at the maximum dose point in each calculation matrix. Data entry consists of information included on the plan printout, and can be streamlined by using an optional plan import feature. Calculated shot times differed from those generated by the treatment planning software by an average of 0.3%, with a standard deviation of 1.4%. The agreement of dose maxima was comparable with an average of -0.2% and a standard deviation of 1.3%. Consistently accurate comparisons were observed for centrally located lesions treated with a small number of shots. Large discrepancies were almost all associated with dose plans utilizing a large number of collimator plugs, for which the simplifying approximations used by the program are known to break down.

The relative dose rate for the different Gamma Knife helmets (4, 8, 14, and 18 mm) is characterized by their respective helmet factors. Since the plateau of the dose profile for the 4 mm helmet is at most 1 mm wide, detector choices are limited. Traditionally helmet factors have been measured using 1 x 1 x 1 mm3 thermoluminescent dosimeters (TLDs). However, these are time-consuming, cumbersome measurements. This article investigates the use of metal-oxide-semiconductor field effect transistors (MOSFETs) (active area of 0.2 x 0.2 mm2) as a more accurate and convenient dosimeter. Their suitability for these measurements was confirmed by basic characterization measurements. Helmet factors were measured using both MOSFETs and the established TLD approach. A custom MOSFET cassette was designed in analogy to the Elekta TLD cassette (Elekta Instruments AB) for use with the Elekta dosimetry sphere. Although both dosimeters provided values within 3% of the manufacturer's suggestion, MOSFETs provided superior accuracy and precision, in a fraction of the time required for the TLD measurements. Thus, MOSFETs proved to be a reasonable alternative to TLDs for performing helmet factor measurements.

As treatment of A. urinae infection is simple, we recommend that in healthy children with malodorous urine, this pathogen is excluded before the initiation of costly metabolic investigations.

The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR), has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control testing for Gamma Knife radiosurgery.

Absfracf-Bracliytlierapy (BT) is a well-established cancer treatment modality thal requires accurate localization of source holders with respect to the tunior and nearby critical structures. Conipiited tomography (CT) is ideal Sir imaging the Source holders. In this paper, we describe a solid-state eight-channel detector designed for a fourth-generation CT scanner that uses a higli-dose-rate BT source to provide the photons needed to form an image. The detector conlprises eight 0.8 x 0.275 X 1.0 cm' CdW04 scintillation cryrtals, a linear photodiode array, gated integrator, low-pass-filter gain stage, sample-and-hold, and multiplexer electronics.The detector response to lighl is linear for light intensities between 8 x lo-" Wlcma and 3 x Wlcm'. Using a 5.3 Ci '"Ir source positioncd 82.5 cni from the detector, the signal-to. noise ratio (SNR) with no attenuation in the path nf the beam is 118; with 41 cni of Plexiglas placed hetween the source and the detector, the SNR is 12. In addition, the detector signal is mnch larger than the electronic noise floor for up to at least 43 cm of Plexiglas, demonstrating that this detector is snitable for w e in an '"Ir-based CT scanner. A preliminary image obtained wing the detector in a first-generation geometry is presented.

Doses in Leksell GammaPlan (Elekta, Stockholm, Sweden) are calculated for each shot by summing the contribution from the 201 radiation beams emitted by the Gamma Knife (Elekta), weighted for attenuation in the tissue traversed. The patient's head is modeled based on 24 skull measurements, from which the depth to the calculation point is determined for each beam. The limited number of measurement points associated with this approach can result in substantial discrepancies between the skull model used for dose calculations and the actual skull contour from computed tomography or magnetic resonance data. A review of 24 patients found that differences between the actual and approximated skull shape gave treatment time errors as large as 4.1%, although in most instances, the errors were less than 1%. The conclusion was that the Leksell GammaPlan head model provides a quick and convenient approach for specifying the shape of the patient's head in all but extreme cases, where discrepancies as large as ~5% can result.PACS number: 87.53.Ly

A new ionization chamber array developed by IBA Dosimetry (Germany) is evaluated at our centre. The dosimetric characteristics of this device such as dose and dose rate dependence, stability, energy dependence, profiles and error detection ability were studied. COMPASS is an array with 1600 parallel plate ion chambers. Each chamber is 3.8 mm in diameter and 2mm in height spaced 6.5 mm apart. The array is mounted onto the wedge slot of the Varian Clinac 2100iX (Varian Medical Systems, Palo Alto, CA) linear accelerator. Linearity of the detector with dose and response to dose rate were done using Clinac 2100iX 6 MV beam. The detector showed excellent linearity response with dose and a variation of <0.2% with dose rates. Short term and long term reproducibility results showed a standard deviation of 0.05%. Energy dependence of COMPASS was evaluated by response of the device to beam energies ranging from 60Co to 18 MV X‐rays. KQ, determined by COMPASS increases with energy as opposed to a decreased response with energy of a thimble ion chamber. Profiles measured with COMPASS were similar to Markus (plane parallel) ion chamber for field sizes ≤.2em20×20.2emcm2. The chamber array showed increased dose response off‐axis for larger fields. The penumbra measured by COMPASS was 3.14± 0.03 mm vs. 1.25± 0.02 mm for film measurements for a range of field sizes. A leaf gap error of 1 mm was detected by the device using an IMRT test field. Research partially sponsored by IBA Dosimetry.

No abstract