Commissioning of the tongue-and-groove modelling in treatment planning systems: from static fields to VMAT treatments

Abstract

Adequate modelling of the multi-leaf collimator (MLC) by treatment planning systems (TPS) is essential for accurate dose calculations in intensity-modulated radiation-therapy. For this reason modern TPSs incorporate MLC characteristics such as the leaf end curvature, MLC transmission and the tongue-and-groove. However, the modelling of the tongue-and-groove is often neglected during TPS commissioning and it is not known how accurate it is. This study evaluates the dosimetric consequences of the tongue-and-groove effect for two different MLC models using both film dosimetry and ionisation chambers. A set of comprehensive tests are presented that evaluate the ability of TPSs to accurately model this effect in (a) static fields, (b) sliding window beams and (c) VMAT arcs. The tests proposed are useful for the commissioning of TPSs and for the validation of major upgrades. With the ECLIPSE TPS, relevant differences were found between calculations and measurements for beams with dynamic MLCs in the presence of the TG effect, especially for the High Definition MLC, small gap sizes and the 1 mm calculation grid. For this combination, dose differences as high as 10% and 7% were obtained for dynamic MLC gaps of 5 mm and 10 mm, respectively. These differences indicate inadequate modelling of the tongue-and-groove effect, which might not be identified without the proposed tests. In particular, the TPS tended to underestimate the calculated dose, which may require tuning of other configuration parameters in the TPS (such as the dosimetric leaf gap) in order to maximise the agreement between calculations and measurements in clinical plans. In conclusion, a need for better modelling of the MLC by TPSs is demonstrated, one of the relevant aspects being the tongue-and-groove effect. This would improve the accuracy of TPS calculations, especially for plans using small MLC gaps, such as plans with small target volumes or high complexities. Improved modelling of the MLC would also reduce the need for tuning parameters in the TPS, facilitating a more comprehensive configuration and commissioning of TPSs.

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

“…The original sweeping gap test involves uniformly extended leaves for different MLC gaps without exposing any of the leaf sides and consequently without any T&G effect. In a previous work, we designed a set of test fields based on the sweeping gap test that incorporated well‐defined amounts of T&G by applying different shifts to the adjacent leaves. These test fields were: (a) the asynchronous sweeping gap (aSG) for sliding window beams and (b) the asynchronous oscillating sweeping gap (aOSG) for VMAT arcs.…”

“…These test fields were: (a) the asynchronous sweeping gap (aSG) for sliding window beams and (b) the asynchronous oscillating sweeping gap (aOSG) for VMAT arcs. The detailed characteristics of these test fields are given in Hernandez et al and similar tests have also been proposed by other investigators . For each beam of aSG and aOSG tests, a tongue‐and‐groove index (TGi) was defined as the quotient of the distance between adjacent leaf ends “ s ” and the MLC gap size (meanGap) used: TGi = s /meanGap.…”

“…Dose measurements were performed with a PTW ion chamber model T31013. This chamber is smaller than the typical Farmer chamber used for measuring the DLG, but its active length (16 mm) still spanned several leaves, providing an estimate of the average impact of the T&G effect . The chamber was positioned at the isocenter, 10 cm depth, in a water phantom for the aSG test and in a cylindrical phantom for the aOSG test.…”

“…A similar procedure was followed for TGi, which was defined as the ratio of the difference between adjacent leaf positions and their MLC gap, averaged over all the leaves in the beam and all control points. TGi was set to a maximum value of 1 in case of interdigitation (i.e., for s > gap) because the dosimetric impact due to the T&G effect increases linearly with s only for s < gap . The relationships between the dose agreement (in terms of GPR and dose differences at the isocenter) with both meanGap and TGi were investigated.…”

“…In arc treatments, T&G effects are typically smoothed out due to the gantry rotation, but they can produce a reduction in average doses of up to 5%–7% . In a recent study we used a set of test fields that demonstrated inadequate modeling of the T&G in the Eclipse TPS, remarkably for the HDMLC and small dynamic MLC gaps. SRS or SBRT VMAT arcs may lead to highly irregular MLC patterns and small MLC apertures with individual leaves repeatedly extending into the radiation field and thus presenting high T&G effects.…”