Evaluation of margining algorithms in commercial treatment planning systems

Pooler, Alistair M.; Mayles, Helen M.; Naismith, Olivia F.; Sage, John P.; Dearnaley, David P.; on behalf of CHHIP collaborators,

doi:10.1016/j.radonc.2007.11.006

« Previous Next »Radiotherapy & Oncology
Volume 86, Issue 1 , Pages 43-47, January 2008

Evaluation of margining algorithms in commercial treatment planning systems

Alistair M. Pooler
- Physics Department, Clatterbridge Centre for Oncology NHS Foundation Trust, Wirral, UK
- Corresponding author. A.M. Pooler, Physics Department, Clatterbridge Centre for Oncology, Clatterbridge Road, Bebington, Wirral CH63 4JY, UK.
Helen M. Mayles
- Physics Department, Clatterbridge Centre for Oncology NHS Foundation Trust, Wirral, UK
Olivia F. Naismith
- Department of Physics, The Royal Marsden NHS Foundation Trust, London, UK
John P. Sage
- Department of Physics, Arden Cancer Centre, Walsgrave Hospital, Coventry, UK
- Present address: Department of Radiotherapy, University Hospitals of Leicester, Leicester, UK.
David P. Dearnaley
- Department of Academic Radiotherapy, Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Surrey, UK
on behalf of CHHIP collaborators

Received 20 September 2007; received in revised form 31 October 2007; accepted 2 November 2007. published online 30 November 2007.

Abstract

Introduction

During commissioning of the Pinnacle (Philips) treatment planning system (TPS) the margining algorithm was investigated and was found to produce larger PTVs than Plato (Nucletron) for identical GTVs. Subsequent comparison of PTV volumes resulting from the QA outlining exercise for the CHHIP (Conventional or Hypofractionated High Dose IMRT for Prostate Ca.) trial confirmed that there were differences in TPS’s margining algorithms. Margining and the clinical impact of the different PTVs in seven different planning and virtual simulation systems (Pinnacle, Plato, Prosoma (MedCom), Eclipse (7.3 and 7.5) (Varian), MasterPlan (Nucletron), Xio (CMS) and Advantage Windows (AW) (GE)) is investigated, and a simple test for 3D margining consistency is proposed.

Methods

Using each TPS, two different sets of prostate GTVs on 2.5mm and 5mm slices were margined according to the CHHIP protocol to produce PTV3 (prostate+5mm/0mm post), PTV2 (PTV3+5mm) and PTV1 (prostate and seminal vesicles+10mm). GTVs and PTVs were imported into Pinnacle for volume calculation. DVHs for 5mm slice plans, created using the smallest PTVs, were recalculated on the largest PTV dataset and vice versa. Since adding a margin of 50mm to a structure should give the same result as adding five margins of 10mm, this was tested for each TPS (consistency test) using an octahedron as the GTV and CT datasets with 2.5mm and 5mm slices.

Results

The CHHIP PTV3 and PTV1 volumes had a standard deviation, across the seven systems, of 5% and PTV2 (margined twice) 9%, on the 5mm slices. For 2.5mm slices the standard deviations were 4% and 6%. The ratio of the Pinnacle and the Eclipse 7.3 PTV2 volumes was 1.25. Rectal doses were significantly increased when encompassing Pinnacle PTVs (V₅₀=42.8%), compared to Eclipse 7.3 PTVs (V₅₀=36.4%). Conversely, fields that adequately treated an Eclipse 7.3 PTV2 were inadequate for a Pinnacle PTV2. AW and Plato PTV volumes were the most consistent (0.3%) and (−0.4%). However, the 1×50mm margin in Pinnacle produced a 15.9% larger volume than 5×10mm margins, while for Eclipse 7.3 the single margined volume was 14.3% smaller. These inconsistencies were reduced to ∼5% by adjusting the superior/inferior margins.

Conclusions

Accurate margin algorithms are necessary to ensure that volume expansion does not add extra uncertainty to the radiotherapy planning process. We have found significant differences in the 3D margining algorithms of TPSs, devised a simple test to predict inconsistency and suggested corrective action to minimise the variation.

Keywords: Margins, Margining algorithms, Treatment planning systems, Quality assurance