Design and implementation of a "cheese" phantom-based Tomotherapy TLD dose intercomparison.

BACKGROUND: The unique beam-delivery technique of Tomotherapy machines (Accuray Inc., Sunnyvale, Calif.) necessitates tailored quality assurance. This requirement also applies to external dose intercomparisons. Therefore, the aim of the 2014 SSRMP (Swiss Society of Radiobiology and Medical Physics) dosimetry intercomparison was to compare two set-ups with different phantoms. MATERIALS AND METHODS: A small cylindrical Perspex phantom, which is similar to the IROC phantom (Imaging and Radiation Oncology Core, Houston, Tex.), and the "cheese" phantom, which is provided by the Tomotherapy manufacturer to all institutions, were used. The standard calibration plans for the TomoHelical and TomoDirect irradiation techniques were applied. These plans are routinely used for dose output calibration in Tomotherapy institutions. We tested 20 Tomotherapy machines in Germany and Switzerland. The ratio of the measured (Dm) to the calculated (Dc) dose was assessed for both phantoms and irradiation techniques. The Dm/Dc distributions were determined to compare the suitability of the measurement set-ups investigated. RESULTS: The standard deviations of the TLD-measured (thermoluminescent dosimetry) Dm/Dc ratios for the "cheese" phantom were 1.9 % for the TomoHelical (19 measurements) and 1.2 % (11 measurements) for the TomoDirect irradiation techniques. The corresponding ratios for the Perspex phantom were 2.8 % (18 measurements) and 1.8 % (11 measurements). CONCLUSION: Compared with the Perspex phantom-based set-up, the "cheese" phantom-based set-up without individual planning was demonstrated to be more suitable for Tomotherapy dose checks. Future SSRMP dosimetry intercomparisons for Tomotherapy machines will therefore be based on the "cheese" phantom set-up.

Surface dose characterisation of the Varian Ir-192 HDR conical surface applicator set with a vertically orientated source.

BACKGROUND: Conical surface applicators with an Ir-192 high-dose-rate brachytherapy source are a common modality for the treatment of non-melanomatous skin cancer with high tumour control rates. Surface dose characterisation of the Varian Varisource GammaMed+ IX afterloader vertical type surface applicators is performed two dimensionally using high-resolution film dosimetry. AIM: The focus of this study was to determine if Varian surface applicators with a vertical source suffer from the dose distribution irregularities reported for comparable applicators. Our goal was to evaluate if the irregularities found affected treatment and dose output verification procedures. METHODS: Ionisation chamber-based verification of applicator output was established according to guidelines provided by the manufacturer. For additional measurement of surface dose Gafchromic EBT3 film dosimetry was used. The term "therapeutic dose" was defined as 85% of the prescribed dose level. RESULTS: For the 10 different applicator inserts evaluated, cold spots were observed. Mean cold spot size was 2.0 mm × 3.6 mm (± 0.6 mm). The cold spots were dosimetrically well below 85% of the prescribed dose. The cold spot was situated 2.2 mm (1.4-2.7 mm) unilaterally from the central axis and caused general asymmetry in the dose profiles intersecting the cold spot area. A source tilt of approximately 8° (± 1°) was determined for the source used for irradiation. CONCLUSIONS: A central underdosed area exceeding 15 % of the prescribed dose has not been previously reported. Source tilt was observed and found to affect clinical use and possibly treatment outcome in applicators using a vertically arranged source. Surface applicators with a vertically orientated source were subject to dose irregularities that could impact on chamber-based applicator output verification procedures. We recommend film dosimetry-backed applicator commissioning to avoid systematic errors.

GAFCHROMIC EBT photospectral dose response dependence on temperature and implications for flat bed scanning.

PURPOSE: Various forms of GAFCHROMIC film have been used for several years as radiographic media for measuring dose distributions. GAFCHROMIC EBT (GC-EBT) film is particularly useful for clinical dose ranges. This thin film dosimeter develops a blue color (lambdamax approximately 635 nm) when irradiated with ionizing radiation. METHODS: Temperature controlled photospectrometry was used to assess temperature related readout changes in GC-EBT type film dosimetry. For observing clinical impact of findings, multiple scan studies with rising flat bed scanner temperature were performed. RESULTS: The whole optical spectrum in the observed wavelength range of 450-700 nm shows a distinct spectrum shift linear with temperature toward lower wavelengths when readout temperature is increased. In addition, absorption decreases in maximum regions and increases in minimum regions of the absorbance spectrum. The most pronounced wavelength dependent readout differences occur in the slopes of the spectrum curves. Absorption readout differences of -1%/degree C for a 2.7 Gy irradiated film piece at a readout wavelength of approximately 650 nm can be found. The readout difference is strongly dependent on readout light spectral characteristic, irradiation dose, and temperature. Readout difference can be positive or negative. Characteristic temperature behavior patterns are present for each color channel of a flat bed scanner. All described effects are reversible within the measurement accuracy of this study. CONCLUSIONS: When using unsuitable readout light, careful control of the readout temperature is necessary in order to obtain consistent and accurate results. Adapted GC-EBT type film dosimetry guidelines are presented. Temperature dependent readout differences on a flat bed scanner can be avoided when using scanner bed temperature as a fixed dosimetry parameter.