Assessment and management of radiotherapy beam intersections with the treatment couch

Monique Van Prooijen,Robert K Heaton,Thilakshan Kanesalingam,Mohammad K Islam

doi:10.1120/jacmp.v11i2.3171

Monique Van Prooijen, Robert K Heaton + Show 2 more

Open Access

https://doi.org/10.1120/jacmp.v11i2.3171

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Abstract

The impact of the treatment couch on a radiotherapy plan is rarely fully assessed during the treatment planning process. Incorporating a couch model into the treatment planning system (TPS) enables the planner to avoid or dosimetrically evaluate beam‐couch intersections. In this work, we demonstrate how existing TPS tools can be used to establish this capability and assess the accuracy and effectiveness of the system through dose measurements and planning studies. Such capabilities may be particularly relevant for the planning of arc therapies.Treatment couch top models were introduced into a TPS by fusing their CT image sets with the patient CT dataset. Regions of interest characterizing couch elements were then imported and assigned appropriate densities in the TPS. Measurements in phantom agreed with TPS calculations to within 2% dose and 1° gantry rotation. To clinically validate the model, a retrospective study was performed on patient plans that posed difficulties in beam‐couch intersection during setup. Beam‐couch intersection caused up to a 3% reduction in PTV coverage, defined by the 95% of the prescribed dose, and up to a 1% reduction in mean CTV coverage. Dose compensation strategies for IMRT treatments with beams passing through couch elements were investigated using a four‐field IMRT plan with three beams passing through couch elements. In this study, ignoring couch effects resulted in point dose reductions of 8±3%.A methodology for incorporating detailed couch characteristics into a TPS has been established and explored. The method can be used to predict beam‐couch intersections during planning, potentially eliminating the need for pretreatment appointments. Alternatively, if a beam‐couch intersection problem arises, the impact of the couch can be assessed on a case‐by‐case basis and a clinical decision made based on full dosimetric information.PACS numbers: 87.53.Bn;87.55.Gh;87.55.de;87.56.nk

Highlights

Radiation therapy techniques are becoming more complex at the same time as they are becoming more precise
The evaluation of beam intersection with couch components using the edge of the light field showed good agreement between the TPS and setup at the treatment unit
Had the couch model been used during the initial planning stage, the setup problems could have been identified and perhaps avoided through a modification of the initial beam angles

Summary

Introduction

Radiation therapy techniques are becoming more complex at the same time as they are becoming more precise. Investigators concentrated on ensuring couch avoidance and basic characterization of treatment couch top attenuation.[1,2,3] Muthuswamy[3] reports on a general analytical equation that could be used to determine whether a beam would intersect components of the treatment couch, while De Ost et al[1] and Meara et al[2] were concerned with identifying significant increases in skin dose with otherwise negligible attenuation for beam angles of 180° On this same topic, Meydanci and Kemikler[4] published a more detailed study of increased skin dose due to a carbon fiber tabletop for both 6 and 18 MV photon beams. Njeh et al[7] used a similar strategy for the evaluation of dose delivered by IMRT and found expected dose reductions of 1.6% and 3% for a seven-field prostate and a nine-field head and neck plan, respectively

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