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

Laure Vieillevigne,Victor Hernandez,Jordi Saez,Catherine Khamphan

doi:10.1002/acm2.12656

Abstract

The dosimetric leaf gap (DLG) and tongue‐and‐groove (T&G) effects are critical aspects in the modeling of multileaf collimators (MLC) in the treatment planning system (TPS). In this study, we investigated the dosimetric impact of limitations associated with the T&G modeling in stereotactic plans and its relationship with the need for tuning the DLG in the Eclipse TPS. Measurements were carried out using Varian TrueBeam STx systems from two different institutions. Test fields presenting MLC patterns with several MLC gap sizes (meanGap) and different amounts of T&G effect (TGi) were first evaluated. Secondly, dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) deliveries of stereotactic cases were analyzed in terms of meanGap and TGi. Two DLG values were used in the TPS: the measured DLG (DLGmeas) and an optimal DLG (DLGopt). Measured and calculated doses were compared according to dose differences and gamma passing rates (GPR) with strict local gamma criteria of 2%/2 mm. The discrepancies were analyzed for DLGmeas and DLGopt, and their relationships with both TGi and meanGap were investigated. DCA arcs involved significantly lower TGi and larger meanGap than VMAT arcs (P < 0.0001). By using DLGmeas in the TPS, the dose discrepancies increased as TGi increased and meanGap decreased for both test fields and clinical plans. Dose discrepancies dramatically increased with the ratio TGi/meanGap. Adjusting the DLG value was then required to achieve acceptable calculations and configuring the TPS with DLGopt led to an excellent agreement with median GPRs (2%/2 mm) > 99% for both institutions. We also showed that DLGopt could be obtained from the results of the test fields. We demonstrated that the need for tuning the DLG is due to the limitations of the T&G modeling in the Eclipse TPS. A set of sweeping gap tests modified to incorporate T&G effects can be used to determine the optimal DLG value.

Highlights

Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) treatments are valuable modalities for treating relatively small lesions with high delivered doses
A Varian TrueBeam STx equipped with the high‐definition multileaf collimator (HDMLC) for a 6 MV WFF photon beam energy with flattening filter (WFF) and at a dose rate of 600 monitor units (MU)/min was used for dose delivery
The multileaf collimator (MLC) transmission ratio was 1.25%. Both institutions independently determined an DLGopt of 1.1 mm, each institution using its own set of stereotactic clinical plans

Summary

Introduction

Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) treatments are valuable modalities for treating relatively small lesions with high delivered doses. Some SBRT protocols, such as RTOG 02361 and 08132 required a minimum field size, encouraging multiple static beams or DCA This requirement may be difficult to fulfill with VMAT as multileaf collimator (MLC) apertures do not strictly follow the projection of the planning target volume (PTV). The use of VMAT in SRS and SBRT is becoming increasingly widespread.[3] Since the target volumes are typically small, so are the radiation field sizes involved This can be challenging for the accuracy of the treatment planning system (TPS) calculations.[4] the ICRU 91 report[5] recently recommended rigorous testing of the TPS dose calculation accuracy in stereotactic treatments because lesions can be in proximity to vital sensitive structures

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