Feasibility of dose painting using volumetric modulated arc optimization and delivery

Abstract

Purpose. Dose painting strategies are limited by optimization algorithms in treatment planning systems and physical constraints of the beam delivery. We investigate dose conformity using the RapidArc optimizer and beam delivery technique. Furthermore, robustness of the plans with respect to positioning uncertainties are evaluated. Methods. A head & neck cancer patient underwent a [⁶¹Cu]Cu-ATSM PET/CT-scan. PET-SUVs were converted to prescribed dose with a base dose of 60 Gy, and target mean dose 90 Gy. The voxel-based prescription was converted into 3, 5, 7, 9, and 11 discrete prescription levels. Optimization was performed in Eclipse, varying the following parameters: MLC leaf width (5 mm and 2.5 mm), number of arcs (1 and 2) and collimator rotation (0, 15, 30 and 45 degrees). Dose conformity was evaluated using quality volume histograms (QVHs), and relative volumes receiving within ±5% of prescribed dose (Q_0.95–1.05). Deliverability was tested using a Delta4^® phantom. Robustness was tested by shifting the isocenter 1 mm and 2 mm in all directions, and recalculating the dose. Results. Good conformity was obtained using MLC leaf width 2.5 mm, two arcs, and collimators 45/315 degrees, with Q_0.95–1.05=92.8%, 91.6%, 89.7% and 84.6%. Using only one arc or increasing the MLC leaf width had a small deteriorating effect of 2–5%. Small changes in collimator angle gave small changes, but large changes in collimator angle gave a larger decrease in plan conformity; for angles of 15 and 0 degrees (two arcs, 2.5 mm leaf width), Q_0.95–1.05 decreased by up to 15%. Consistency between planned and delivered dose was good, with ∼90% of gamma values <1. For 1 mm shift, Q_0.95–1.05 was decreased by 5–15%, while for 2 mm shift, Q_0.95–1.05 was decreased to 55–60%. Conclusions. Results demonstrate feasibility of planning of prescription doses with multiple levels for dose painting using RapidArc, and plans were deliverable. Robustness to positional error was low.

Acknowledgements

This project was supported by CIRRO – The Lund-beck Foundation Center for Interventional Research in Radiation Oncology and The Danish Council for Strategic Research. The project was furthermore supported by the Danish Cancer Society and by Axel Muusfeldts Foundation. The work was furthermore partially supported by grant 1UL1RR025011 from the Clinical and Translational Science Award (CTSA) program of the National Center for Research Resources (NCRR), National Institutes of Health (NIH), and in part by the NCI, grant 2P30 CA 014520-34. Stine Korreman and Silke Ulrich are involved in a research agreement with Varian Medical Systems, Inc.

Declaration of interest: This study was partially supported by a research grant from Varian Medical Systems, Inc. The authors alone are responsible for the content and writing of the paper.