An institutional experience of quality assurance of a treatment planning system on photon beam

Using beam profile inflection point in process of treatment planning system verification

Background

The comparison between profiles during the commissioning of the treatment planning system is an essential procedure. It is impossible to designate a field size for off-axis, wedged, and FFF beams directly by using the definition of the on-axis symmetric field size. This work proposes the use of different characteristic points as indicators of the field size for commissioning and QA purposes. This work aimed to search for the beam profile's characteristic points and use them for the TPS commissioning purposes.

Materials and methods

The proposal is to use profile inflection points as the beam profile characteristic points. The usage of dedicated software allowed for comparing distances between inflection points and between points of 50% intensity. For the off-axis, wedged, and FFF fields, comparisons were made to the nominal field sizes.

Results

Distances between inflection points proved to be different by less than 1 mm from nominal field sizes for all kinds of investigated beams.

Conclusions

Inflection points are convenient for comparing the off-axis, wedged, and FFF field sizes because of their independence from profile normalization. With finite accuracy, the inflection points could be used for the above kind of beam sizes designation.

Evaluation of dose calculations accuracy of a commercial treatment planning system for the head and neck region in radiotherapy

AIM

The objective was to quantify dose calculation accuracy of TiGRT TPS for head and neck region in radiotherapy.

BACKGROUND

In radiotherapy of head and neck cancers, treatment planning is difficult, due to the complex shape of target volumes and also to spare critical and normal structures. These organs are often very near to the target volumes and have low tolerance to radiation. In this regard, dose calculation accuracy of treatment planning system (TPS) must be high enough.

MATERIALS AND METHODS

Thermoluminescent dosimeter-100 (TLD-100) chips were used within RANDO phantom for dose measurement. TiGRT TPS was also applied for dose calculation. Finally, difference between measured doses (D_meas) and calculated doses (D_calc) was obtained to quantify the dose calculation accuracy of the TPS at head and neck region.

RESULTS

For in-field regions, in some points, the TiGRT TPS overestimated the dose compared to the measurements and for other points underestimated the dose. For outside field regions, the TiGRT TPS underestimated the dose compared to the measurements. For most points, the difference values between D_calc and D_meas for the in-field and outside field regions were less than 5% and 40%, respectively.

CONCLUSIONS

Due to the sensitive structures to radiation in the head and neck region, the dose calculation accuracy of TPSs should be sufficient. According to the results of this study, it is concluded that the accuracy of dose calculation of TiGRT TPS is enough for in-field and out of field regions.

Determination of boundaries between ranges of high and low gradient of beam profile

AIM

This work addresses the problem of treatment planning system commissioning by introducing a new method of determination of boundaries between high and low gradient in beam profile.

BACKGROUND

The commissioning of a treatment planning system is a very important task in the radiation therapy. One of the main goals of this task is to compare two field profiles: measured and calculated. Applying points of 80% and 120% of nominal field size can lead to the incorrect determination of boundaries, especially for small field sizes.

MATERIALS AND METHODS

The method that is based on the beam profile gradient allows for proper assignment of boundaries between high and low gradient regions even for small fields. TRS 430 recommendations for commissioning were used.

RESULTS

The described method allows a separation between high and low gradient, because it directly uses the value of the gradient of a profile. For small fields, the boundaries determined by the new method allow a commissioning of a treatment planning system according to the TRS 430, while the point of 80% of nominal field size is already in the high gradient region.

CONCLUSIONS

The method of determining the boundaries by using the beam profile gradient can be extremely helpful during the commissioning of the treatment planning system for Intensity Modulated Radiation Therapy or for other techniques which require very small field sizes.