Abstract
We sought to validate new couch modeling optimization for tomotherapy planning and delivery. We constructed simplified virtual structures just above a default setting couch through a planning support system (MIM Maestro, version 8.2, MIM Software Inc, Cleveland, OH, USA). Based on ionization chamber measurements, we performed interactive optimization and determined the most appropriate physical density of these virtual structures in a treatment planning system (TPS). To validate this couch optimization, Gamma analysis and these statistical analyses between a three‐dimensional diode array QA system (ArcCHECK, Sun Nuclear, Melbourne, FL, USA) results and calculations from ionization chamber measurements were performed at 3%/2 mm criteria with a threshold of 10% in clinical QA plans. Using a virtual model consisting of a center slab density of 4.2 g/cm3 and both side slabs density of 1.9 g/cm3, we demonstrated close agreement between measured dose and the TPS calculated dose. Agreement was within 1% for all gantry angles at the isocenter and within 2% in off‐axis plans. In validation of the couch modeling in a clinical QA plan, the average gamma passing rate improved approximately 0.6%–5.1%. It was statistically significant (P < 0.05) for all treatment sites. We successfully generated an accurate couch model for a TomoTherapy TPS by interactively optimizing the physical density of the couch using a planning support system. This modeling proved to be an efficient way of correcting the dosimetric effects of the treatment couch in tomotherapy planning and delivery.
Highlights
The American Association of Physicists in Medicine (AAPM) task group report 176 recommends that the beam intensity attenuation by the couch should be taken into account by the treatment planning system (TPS).[8]
Our modeling decreased the dose difference to < 1.0% when the center slab of the virtual structure was assigned a physical density of 4.2 g/cm[2] and both side slabs were 1.9 g/cm[2] (Fig. 4)
5.8% of the largest discrepancy was observed at a gantry angle of 150° at Point (A), where the path length of the beam through the couch is longest. These discrepancies decreased to ≤ 2.0% after using the couch modeling optimization
Summary
Carbon‐fiber flat top couches are widely used for radiotherapy.[1]. These couches have heterogeneous absorption properties when beams pass through the couch before entering the patient.[2–18] Several authors have reported that the failure to factor in couch attenuation for beams sent in the posteroanterior direction can cause a reduction in target volume coverage.[10–12] The American Association of Physicists in Medicine (AAPM) task group report 176 recommends that the beam intensity attenuation by the couch should be taken into account by the treatment planning system (TPS).[8]. TomoTherapy (Accuray, Sunnyvale, CA, USA) planning software has implemented this such that this virtual couch has appropriate predefined physical densities and is commissioned sufficiently. Kong et al reported that for AP treatment in TomoTherapy, with about 50% rear dose contribution, the passing rate of gamma analysis deteriorated to 91.28% (3%/3 mm) even when using a two‐dimensional array ion chamber device with angular dependence correction They concluded that in pretreatment plan verification, the greater the dose contribution from the rear, the poorer the agreement between the measured dose and TPS.[19]. We developed and validated a new couch modeling optimization for tomotherapy planning and delivery
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