ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
00:00 - 00:00
A method for determining dosimetric leaf separation of a multileaf collimator
Antti Kulmala, Finland


A method for determining dosimetric leaf separation of a multileaf collimator
Authors: Antti Kulmala.(Clinical Research Institute HUCH, Radiotherapy, Helsinki, Finland), Juho Pennanen.(Varian Medical Systems Finland, Radiotherapy, Helsinki, Finland), Anna Rintala.(Helsinki University Hospital, Cancer Center, Helsinki, Finland), Mikko Tenhunen.(Helsinki University Hospital, Cancer Center, Helsinki, Finland)
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Purpose or Objective

The purpose of this work is to demonstrate a robust method for determining an actual dosimetric leaf separation (DLS) of a multileaf collimator (MLC). Correct value of DLS is significant for dose estimation validity of intensity modulated radiotherapy, especially when applied to small target structures with the sweeping gap delivery technique. Conventional methods to determine DLS can be laborious. In this work a simple and more specifying method is presented, and in addition applied to a variety of therapy beam qualities and two MLC models.

Theoretical DLS expresses an opening of a hypothetical leaf pair with flat, focused tips, which integral radiation fluency throughput equals to integral fluency under a real closed leaf pair with three-dimensional, rounded tips. However, in clinical practice the DLS includes MLC gap calibration offset, which in spite of good attempts, might differ from negligible.

The demonstrated method presents the actual DLS as a combination of a semi-empirical DLS and a mechanically determined MLC gap calibration offset. Model results are compared to reference DLS results derived conventionally using dosimetrical measurements on uniform intensity fields created by dynamic sliding windows and high resolution detection of field gap widths.

Material and Methods

Theoretical DLS is estimated with a ray-tracing algorithm based on a known description of the MLC leaf geometry and a solved linear attenuation coefficient. The attenuation coefficient is derived from the measured MLC transmission and the known leaf thickness. MLC gap calibration error is determined by measuring a set of actual leaf gaps corresponding to gaps (widths: 6.4 mm, 3.2 mm, 1.6 mm and 0.8 mm) planned with the treatment planning software and extrapolating offset to zero leaf gap. Actual leaf gaps are measured on MLC level using a precise feeler gauge.

The proposed method is used to define actual DLSs for photon beams on TrueBeam therapy system equipped with standard definition MLC, and for four matched FFF 6 MV beams of which two are collimated with standard MLCs and two with high definition MLC


Measured transmission, gap calibration error and DLS, in addition to modelled theoretical DLS, derived actual DLS and deviation between measured and modelled actual DLS are presented in table 1: for studied five energies and in table 2: for studied four MLCs. The measured DLS data has a very close agreement, on average 0.0 mm difference, with DLS data obtained by the proposed method.

Measured and modelled MLC characteristics and deviation between measured and modelled DLS parameters for TrueBeam therapy system.

Measured and modelled MLC characteristics and deviation between measured and modelled DLS parameters for two standard MLCs and two high-definition MLCs.


We have demonstrated that the proposed method can be used to determine the actual DLS for several clinically relevant beams qualities and for standard and high definition MLC types. The agreement between the actual DLSs derived using proposed method and those derived conventionally has been found to be clinically acceptable.