[Establishment of the Commissioning Procedure for Modifying the Beam Model of Helical Tomotherapy]

Although much evidence about the helical tomotherapy system are available, there is not a document about the procedure of quality assurance (QA) for changing the beam model. This study establishes the commissioning procedure for modifying the beam model of helical tomotherapy. Firstly, some intensity-modulated radiotherapy (IMRT) plans were created, and compared them with the calculated dose and the measured dose. Secondly, the absorbed doses to water in the machine-specific reference field and the plan-class specific reference field with a protocol in Japan; Standard Dosimetry of Absorbed Dose to Water in External Beam Radiotherapy (Standard Dosimetry 12) were measured. Thirdly, we reconfirmed patient-specific quality assurance. The recommended commissioning procedure after the change of the beam model was shown through three verification processes. This report would be helpful for not only changing the beam model of helical tomotherapy but also introducing Standard Dosimetry 12 to a clinic.

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.

Postirradiation effects in alanine dosimeter probes of two different suppliers

The measurand relevant for the dosimetry for radiation therapy is the absorbed dose to water, DW. The Physikalisch-Technische Bundesanstalt (PTB) is establishing a secondary standard for DW for high-energy photon and electron radiation based on electron spin resonance (ESR) of the amino acid alanine. For practical applications, like, for example, intercomparison measurements using the ESR/alanine dosimetry system, the temporal evolution of the ESR signal of irradiated probes is an important issue. This postirradiation behaviour is investigated for alanine pellets of two different suppliers for different storage conditions. The influence of the storage conditions on the temporal evolution may be dependent on the type of probes used. The measurement and analysis method developed at the PTB is able to circumvent the apparent difficulties in the case of alanine/paraffin probes. Care has to be taken in case this method cannot be applied.