Verification of photon beam data calculated by a treatment planning system based on pencil beam model

الملخص EN

A new three-dimensional treatment system was installed in the Oncology Center, Jeddah.

This system is based on the pencil beam model.

The aim of this study is to verify the validity of photon beam data as calculated by this treatment planning system by comparing it with measured photon beam data.

To do this verification, a quality assurance program, consisting of six tests, was performed.

In this program, both the calculated output factors and dose at different conditions were compared with the measured data.

As a result of that comparison, we found that the calculated output factor was in excellent agreement with the measured factors.

Doses at depths beyond the depth of maximum dose calculated on-axis or off-axis in both the fields or penumbra region were found in good agreement with the measured dose under all conditions of energy, SSD and field size, for open and wedged fields.

In the buildup region, calculated and measured doses only agree (with a difference £ 5%) for field sizes > 7 x 7 cm2.

For smaller fields, the difference was higher than 5% because of the difficulty in dosimetry in that region.

In conclusion, dose calculation using treatment planning system based on the pencil beam model is accurate enough for clinical use except when calculating dose at depths above maximum dose for small field size.

Accurate dosimetry should be done to overcome this problem.