gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Feasibility study of MLC based IMPT

Meeting Abstract

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  • J. Daartz - F.H.B. Proton Therapy Center, Massachusetts General Hospital, Boston, USA
  • A. Trofimov - F.H.B. Proton Therapy Center, Massachusetts General Hospital, Boston, USA
  • H. Paganetti - F.H.B. Proton Therapy Center, Massachusetts General Hospital, Boston, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog047

DOI: 10.3205/09ptcog047, URN: urn:nbn:de:0183-09ptcog0476

Published: September 24, 2009

© 2009 Daartz et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Background: We recently investigated the dosimetric properties of a commercially available micro-multileaf collimator (mMLC) in a proton therapy beamline for conformal therapy. Our current work studies the feasibility of MLC based delivery of intensity modulated proton therapy (IMPT).

Materials and methods: For a set of intra-cranial lesions, scanned-beam IMPT treatment plans were calculated using the KonRad software. The resulting optimized 3-dimensional intensity distributions were sequenced for delivery with a MLC on a layer-by-layer basis, while constraining the total number of required step-and-shoot segments. The minimum segment size was set to 1x1cm2. Dose recalculation was performed using the segmented intensity maps. These dose distributions were compared to the original pre-segmentation results by means of gamma index analysis and dose volume histograms.

The procedure was repeated for scanning layer spacing of 0.5 cm water equivalent depth (assuming delivery with unmodulated proton beams), and 1 cm (for delivery with spread-out Bragg peaks (SOBP)). For the plans which used 1cm layer spacing SOBPs of small modulation width of 6–8 mm were constructed from pristine peaks. The aim of this step was to evaluate the potential to reduce the total number of segments for each scanned layer (due to the more constrained optimization, with reduced number of degrees of freedom), in addition to the reduction in the required number of layers.

Results: Minimizing the total number of segments necessary to produce an acceptable dose distribution is of critical importance, since it affects both the treatment time and the accuracy of dose delivery. The result depends strongly on the size of the target lesion, and mainly on the number of range layers used. In our test cases we were able to significantly reduce the number of necessary segments by using small-modulation SOBPs for the optimization, while maintaining the quality of the dose distribution.

For example, using SOBP based optimization with 1cm layer spacing the dose distribution for an irregularly shaped target of ~10cm extent in depth direction could be segmented with 75 segments per beam (10 proton energies, four beams in total). Only 3% of voxels failed the 3%/3 mm gamma criteria. Differences in DVHs were only seen for the target, and remained small. To achieve similar quality of dose distributions with 5mm layer spacing of pristine peaks, 158 segments and 21 energies per beam are necessary.

Conclusion: The application of an MLC for IMPT is well feasible, but the quality of dose distribution, given a maximum number of segments, depends on the size of the target lesion. Plan optimization using increased layer spacing and small-modulation SOBPs, rather than pristine peaks, decreases the number of segments while maintaining the dose distribution. Depending on the beamline specific neutron dose additional restrictions to the total number of monitor units might be necessary.