COMP report: CPQR technical quality control guidelines for Gamma Knife radiosurgery.

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.

Aerococcus urinae: a possible reason for malodorous urine in otherwise healthy children.

UNLABELLED: Recently, Aerococcus urinae, primarily recognized as a common pathogen in elderly women, has been reported to cause an extremely unpleasant odour of the urine in paediatric patients similar to trimethylaminuria (fish odour syndrome). Herein, we present a case of A. urinae urinary tract colonization in a 12-year-old otherwise healthy boy, who finally refused micturition outside from his home environment as a result of the unpleasant odour. Within the last year, three cases (including our own) of A. urinae colonization causing foul-smelling urine in healthy children have been published, suggesting that this condition might be as frequent as trimethylaminuria. In case of polymicrobial growth in a urine specimen, A. urinae as the leading pathogen will usually be missed by routine bacteriological investigation. Novel bacteriological techniques such as MALDI-TOF MS provide a rapid tool to recognize this pathogen in urine. CONCLUSION: 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.

Effect of skull shape approximations in Gamma Knife dose calculations.

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 approximately 5% can result.

Measurement of Gamma Knife helmet factors using MOSFETs.

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.

An algorithm for independent verification of Gamma Knife treatment plans.

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.