In vivo dosimetry during conformal radiotherapy - Requirements for and findings of a routine procedure

Purpose: Conformal radiotherapy requires accurate knowledge of the actual d ose delivered to a patient. The impact of routine in vivo dosimetry, includ ing its special requirements, clinical findings and resources, has been ana lysed for three conformal treatment techniques to evaluate its usefulness i n daily clinical practice. Materials and methods: Based on pilot studies, routine in vivo dosimetry qu ality control (QC) protocols were implemented in the clinic. Entrance and e xit diode dose measurements have been performed during two treatment sessio ns for 378 patients having prostate, bladder and parotid gland tumours. Dos e calculations were performed with a CT-based three-dimensional treatment p lanning system. In our QC-protocol we applied action levels of 2.5% for the prostate and bladder tumour group and 4.0% for the parotid gland patients. When the difference between the measured dose at the dose specification po int and the prescribed dose exceeded the action level the deviation was inv estigated and the number of monitor units (MUs) adjusted. Since an accurate dose measurement was necessary, some properties of the on-line high-precis ion diode measurement system and the long-term change in sensitivity of the diodes were investigated in detail. Results: The sensitivity of all diodes decreased by approximately 7% after receiving an integrated dose of 10 kGy, for 4 and 8 MV beams. For 34 (9%) p atients the difference between the measured and calculated dose was larger than the action level. Systematic errors in the use of a new software relea se of the monitor unit calculation program, limitations of the dose calcula tion algorithms, errors in the planning procedure and instability in the pe rformance of the accelerator have been detected. Conclusions: Accurate in vivo dosimetry, using a diode measurement system, is a powerful tool to trace dosimetric errors during conformal radiotherapy in the range of 2.5-10%, provided that the system is carefully calibrated. The implementation of an intensive in vivo dosimetry programme requires ad ditional staff for measurements and evaluation. The patient measurements ad d only a few minutes to the total treatment time per patient and guarantee an accurate dose delivery, which is a prerequisite for conformal radiothera py. (C) 1999 Published by Elsevier Science Ireland Ltd. All rights reserved .