We have developed dosemeters based on plastic scintillators for a variety of applications in radiation therapy. The dosemeters consist basically of a tissue-substituting scintillator probe, an optical fiber light guide, and a photomultiplier tube. The background light generated in the light guide can be compensated by a simultaneous measurement of the light from a blind fiber. Plastic scintillator dosemeters combine several advantageous properties which render them superior to other dosemeter types for many applications: minimal disturbance of the radiation field because of the homogeneous detector volume and the approximate water equivalence; no dependence on temperature and pressure (under standard clinical conditions) and angle of radiation incidence; no high voltage in the probe; high spatial resolution due to small detector volumes; direct reading of absorbed doses; and a large dynamical range. The high spatial resolution together with direct reading make these detectors suitable for real-time 3-D dosimetry using multi-channel detector systems. Such a system has been developed for eye plaque dosimetry and successfully employed for dosimetric treatment optimization. The plaque optimization can be performed by dosimetric measurements for the individual patient ("dosimetric treatment planning"). The time consumption for this procedure is less than for a physically correct computer-based therapy planning, e.g., by means of a Monte Carlo simulation.
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
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