[Results of quality controls for individual doses to patients in radiation oncology].

BACKGROUND

Along with the treatment equipment and the planning system the individual doses to the patient also have to be part of quality assurance. To make sure that the dose application to the patient is correct, a two-step concept is used: the check of the planned dose and the in vivo dosimetry.

METHODS

In contrast to the treatment planning where the necessary parameters for a desirable dose distribution and dose are determined in the target volume, the dose in the ICRU point is calculated from the individual setup parameters. This is done by using an Excel Macro program. Additionally, on the occasion of the first treatment session, the entrance dose on the patient surface is measured for each separate treatment field with a semiconductor diode. This dose is compared to the reference value from the treatment planning.

RESULTS

Given a tolerance level of 4% for the correspondence of the doses in the ICRU point, 2.1% of the photon treatment plans showed a bigger deviation. The tolerance of the deviation of the measured entrance dose from the dose calculated in the treatment planning was set to +/- 5% for photons without wedge and to +/- 10% for photons with wedge or for electrons. 14% of all treatment fields exceeded the given tolerances without taking possible repeated measurements into account. The corresponding figures for photon fields without wedge, with wedge and for electron fields were 19%, 6% and 9%, respectively.

CONCLUSION

Patient-related checks are essential since transcription and setup errors cannot be excluded. The checks have to be simple, meaningful and unambiguous. One must assume their independence of the individual and the method. If the tolerances are exceeded, actions have to be taken. This has to be taken into account when the tolerances are established.