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In modern radiotherapy modalities, such as the so-called intensity modulated radiotherapy (IMRT) or in tomotherapy, a dose distribution is generated, the contours of which are as close to the target volume (such as the tumour tissue) as possible. Such a distribution is produced by a superposition of many fields, which are irradiated with different cross-sectional dimensions and shapes, and from different directions. The fields may be very small; particularly in tomotherapy, individual fields are only 5 mm wide. Conventional dosimeters such as ionization chambers are calibrated in fields that are much larger, typically 10 cm x 10 cm. The response of the dosimeter may be significantly different from its value at calibration for the small fields. Therefore a dosimeter with the following properties would be highly desirable: a weak dependence of the response on the radiation quality (the latter changes slightly with the field size), a probe which is as small as possible, and with a minimal field disturbance due to its material properties.

The objective to specify the dose in the target tissue with an uncertainty of less than 2.5 % which was issued by the ICRU as long ago as in the seventies is - even today - often not achieved for conventional radiotherapy. With an ideal dosimeter, the entire dosimetric chain could be checked - from the calibration of the ionization chambers used in the therapy centres via the commissioning of accelerators up to the calculation of the delivered dose - also for the modern forms of therapy. The properties of the alanine dosimeter come quite close to the ideal.