Investigation of the dose accuracy and benefits of gated IMRT and VMAT for clinical implementation

Abstract

Introduction To evaluate the dose accuracy and benefits of gated intensity modulated radiation therapy (IMRT) and gated volumetric modulated arc therapy (VMAT) for clinical implementation. Methods and materials The Dynamic Thorax Phantom from CIRS (DTP) was used for this study. The phantom mimics a human thorax and contains a moving and programmable rod enclosing a spherical tumor in the left lung. Thirteen different settings were tested combining three spheres of different diameters (1, 2 or 3 cm) and seven breathing patterns. One 4D-CT per setting was acquired using the Real-time Positioning Management system (Varian). Gated and non-gated IMRT and VMAT treatment plans were calculated with Eclipse 10.0. Absolute dose measurements were performed with thermoluminescent dosimeters (TLD, diameter 2 mm, thickness 0.1 mm) and 2D relative dose distributions were measured with radiochromic films (Gafchromic EBT3, ISP). Dose-volume histograms were compared. Furthermore, an irregular patient-based breathing pattern was applied to the DTP. Gated and non-gated 4DCT-image based VMAT plans were created. PTV coverage was compared using a moving gafchromic film inserted in the DTP. Results The absolute calculated and measured doses were within 3%. The passing rate of 95% (gamma criterion 3%/3 mm) was achieved in the majority of the 2D dose distribution comparison plans. The reduction in dose to OAR was important for high amplitude pulmonary movements (>5 cm). For amplitudes of 5 cm, 10.2% of healthy lung tissue receiving 20 Gy was spared with IMRT and 9.7% with VMAT compared to non-gated treatments. The size of the tumor appeared to be an important selection criterion: the volume of healthy tissue spared increased with tumors volume. Conclusion The commissioning of this new technique was satisfying. IMRT and VMAT gating should only be used clinically for large and mobile tumors to obtain the greatest dosimetric benefit and to counterbalance the decrease in patient comfort due to the longer treatment delivery time. To evaluate the dose accuracy and benefits of gated intensity modulated radiation therapy (IMRT) and gated volumetric modulated arc therapy (VMAT) for clinical implementation. The Dynamic Thorax Phantom from CIRS (DTP) was used for this study. The phantom mimics a human thorax and contains a moving and programmable rod enclosing a spherical tumor in the left lung. Thirteen different settings were tested combining three spheres of different diameters (1, 2 or 3 cm) and seven breathing patterns. One 4D-CT per setting was acquired using the Real-time Positioning Management system (Varian). Gated and non-gated IMRT and VMAT treatment plans were calculated with Eclipse 10.0. Absolute dose measurements were performed with thermoluminescent dosimeters (TLD, diameter 2 mm, thickness 0.1 mm) and 2D relative dose distributions were measured with radiochromic films (Gafchromic EBT3, ISP). Dose-volume histograms were compared. Furthermore, an irregular patient-based breathing pattern was applied to the DTP. Gated and non-gated 4DCT-image based VMAT plans were created. PTV coverage was compared using a moving gafchromic film inserted in the DTP. The absolute calculated and measured doses were within 3%. The passing rate of 95% (gamma criterion 3%/3 mm) was achieved in the majority of the 2D dose distribution comparison plans. The reduction in dose to OAR was important for high amplitude pulmonary movements (>5 cm). For amplitudes of 5 cm, 10.2% of healthy lung tissue receiving 20 Gy was spared with IMRT and 9.7% with VMAT compared to non-gated treatments. The size of the tumor appeared to be an important selection criterion: the volume of healthy tissue spared increased with tumors volume. The commissioning of this new technique was satisfying. IMRT and VMAT gating should only be used clinically for large and mobile tumors to obtain the greatest dosimetric benefit and to counterbalance the decrease in patient comfort due to the longer treatment delivery time.