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Abstract
The influence of changes in the thickness of the irradiated tissue block during radiotherapy on the error in dose delivery was assessed by dosimetry in vivo. In 55 cases of head and neck cancers, entrance and exit doses and the irradiated volume sizes were measured during the first and the twentieth fractions. The real and hypothetical midline doses (i.e. midline doses calculated when the entrance dose is considered as a planned one) were calculated. The average size of irradiated volume was smaller after 20 fractions. The mean of midline dose dosimetric errors during the twentieth fraction was larger (overdosage) than that during the first fraction. These means were compared and statistically significant differences were found. Our results confirm the value of in vivo dosimetry as an appropriate tool for detecting errors in delivered dose provoked by changes in irradiated volume dimensions and show the necessity to check irradiated volume thickness during the treatment.