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Inverse-Planned Conformal Arc Therapy (iCAT): A Hybrid Solution for Limiting Interplay Effects in Stereotactic Body Radiation Therapy (SBRT)

E Pryser*, L Rankine, The University of North Carolina at Chapel Hill, Chapel Hill, NC


(Sunday, 7/29/2018) 4:30 PM - 5:00 PM

Room: Exhibit Hall | Forum 7

Purpose: Volumetric modulated arc therapy (VMAT) is increasingly used in Stereotactic Body Radiation Therapy (SBRT). However, highly mobile tumors treated with VMAT may be dosimetrically affected by the interplay of the multi-leaf collimator (MLC) and tumor motion. We demonstrate a novel treatment planning technique that combines the advantages of VMAT and DCAT: inverse-planned conformal arc therapy (iCAT).

Methods: iCAT is similar to DCAT, but employs the optimizer in a restricted capacity to decide PTV-to-MLC non-uniform block-margins and MU-weightings at each control point. This is especially useful in tumors located near heterogeneous boundaries (e.g. chest-wall) or proximal to organs-at-risk (OARs). Unlike VMAT, however, the ability for MLCs to cross in front of the ITV is minimized. In this study, we present three clinical SBRT patients originally treated with DCAT with retrospectively re-planned iCAT and VMAT plans. Optimization criteria remained consistent across plan technique within each case, and each tumor site was adjacent to an organ-at-risk (OAR) that was selected for high-priority optimization. Plans were normalized to the same prescription (Rx) coverage, V(Rx)=95%. We evaluated modulation factor (MF), conformity index (CI), adjacent OAR dose, and two in-house designed criteria: monitor unit (MU)-weighted average of both open control point area (Aᶜᵖ) and percent-area of blocked ITV (Rᵇˡᵒᶜ�). We assessed TPS calculation and delivery accuracy by measuring plans using a diode-array and calculating 2D-Γ passing rates.

Results: Compared to DCAT, average adjacent OAR dose was 12%(6-22%) lower for iCAT, and 15%(9-25%) lower for VMAT. Average CI were 0.80, 0.90, and 0.94 for the DCAT, iCAT, and VMAT plans, respectively. The Aᶜᵖ and Rᵇˡᵒᶜ� results indicated that iCAT provided better dosimetry than DCAT without substantially reducing the aperture area or blocking the ITV.

Conclusion: The iCAT technique is beneficial for highly mobile tumors in SBRT, combining advantages of DCAT and VMAT while minimizing interplay effects.


Conformal Radiotherapy, Optimization, Inverse Planning


TH- External beam- photons: Motion management (intrafraction)

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