Background. Success of radiotherapy relies on accurate dose delivery. In vivo dosimetry improves control of treatment quality.
Patients and methods. In vivo dosimetry with commercial diodes was performed in 209 rectal cancer patients treated with four-field box technique. The diodes measured either entrance or exit dose in each treatment field. The results were compared to the planned values and the dose delivered to the isocenter was calculated. Tolerance levels were set to 5% for entrance dose and 8% for exit dose.
Results. 421 entrance dose and 415 exit dose measurements were performed. The average difference from expected values was 0.9% for entrance dose (SD 2.1%) and -0.5% for exit dose (SD 3.3%). In 209 patients, the average absorbed dose in the isocenter differed from the planned values by 0.2% (SD 1.4%). Measurement results exceeded the tolerance levels in two patients.
Conclusion. Smaller standard deviation of absorbed dose to the isocenter (1.4%), compared to those of entrance (2.1%) and exit dose measurements (3.3%), confirms a correlation between the entrance and exit dose deviations of pairs of opposed fields. The fact that during this study in vivo dosimetry exposed two cases of potentially inaccurate treatments proves its necessity.
