At the University Hospital Essen intraocular tumors are treated by four radiotherapy modalities: eye plaque brachytherapy with 106Ru/106Rh and 125I for the uveal melanoma treatment (and some cases of retinoblastoma) external beam therapy at accelerators (X5-MV to X15-MV) for the retinoblastoma treatment and proton therapy for some selected cases of high risk (e.g. juxtapapillary uveal tumors). An average number of 150-200 melanoma patients per year are undergoing radiotherapy. Of these, 5-10% are treated with protons in cooperation with the Centre Antoine Lacassagne in Nice/France, 10-15% with 125Ι plaques and the rest with 106Ru/106Rh plaques.For uveal melanomae, the indication for either an 125 Ιor a 106Ru/106Rh plaque therapy depends on the prominence of the tumor. In the case of 106R Υ/ lo6 Rh applicators, a maximal sclera dose of 1,000-1,200 Gy and a minimal apex dose of 100 Gy is delivered for a maximal tumor prominence of 5-6 mm. The minimal scleral dosage is 700 Gy, sufficient for a safe occlusion of the uveal vessels in the tumor region. A brachytherapy with a 125 I plaque and a dosage of 80 Gy at the apex is a possible therapy concept for a tumor prominence from 5 up to 10 mm
A fast dosimetry system based on plastic scintillator detectors has been developed which allows three-dimensional measurement of the radiation field in water of beta-sources appropriate for application in cardiovascular brachytherapy. This system fulfills the AAPM Task Group 60 recommendations for dosimetry of cardiovascular brachytherapy sources. To demonstrate the use of the system, measurements have been performed with an 90Y-wire source. The dose distribution was determined with a spatial resolution of better than 0.2 mm, with only a few minutes needed per scan. The scintillator dosemeter was absolutely calibrated in terms of absorbed dose to water with a precision of +/-7.5%. The relative precision achievable is +/-2.5%. The response of the system is linear within +/-2% for dose rates from 0.5 mGy s(-1) to 500 mGy s(-1).
In 2008, the European project ‘T2.J06, Increasing cancer treatment efficacy using 3D brachytherapy’ was launched. One of the main goals of the Joint Research Project was the experimental determination of the dose rate constant Λ to allow the linkage between the air kerma strength or reference air kerma rate currently used for the source characterization and the ‘new’ absorbed dose rate to water with an uncertainty of <3% (k = 1) for some selected brachytherapy sources. The results obtained by five National Metrology Institutes (NMI) for four different types of brachytherapy sources are presented and compared with consensus data published in the literature. A further goal of the project was to develop a calibration chain for the transfer of the new reference quantity to the end user, minimizing the uncertainty. A first direct calibration in terms of absorbed dose rate to water of a secondary standard and the dissemination to the hospitals is presented.
The energy response was quantified relative to the response to (60)Co which is the common radiation quality for the calibration of therapy dosemeters. The observed energy dependence could be well explained with the assumption of ionization quenching as described by Birks' formula. Plastic scintillation detectors should be calibrated at the same radiation quality that they will be used at and changes of the spectrum within the application need to be considered. The authors results can be used to evaluate the range of validity of a given calibration.
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