Dosimetric impact of interplay effects inStereotactic Body Radiation Therapy (SBRT) lung cancer

Abstract

Lung cancer patient breathing induced interplay effects in the volumetric modulated arc therapy (VMAT) together with stereotactic body radiation therapy (SBRT), which lead to non-uniform doses within the target volume and unwanted dose to the surrounding tissues. The purpose of this study was to investigate the dosimetric impact of interplay effects in 6 MV FFF of photon beams for lung cancer and to calculate the novel interplay effects factor (IEF) for VMAT-SBRT lung cancer. This study performed the static and dynamic measurements using two dimensions (2D) robotic platform and diode array for 3D-conformal radiation therapy (3D-CRT) plans, while using three dimensions (3D) CIRS dynamic thorax phantom and EBT3 films, and CC01 ionization chambers for VMAT-SBRT plans. The 3D-CRT results, the 1%, 1mm of gamma passing rates decreased for higher amplitudes. However, gamma passing rates increased with larger field sizes. For other parameters, no interplay effect was detected. The VMAT-SBRT results showed that higher amplitudes, more dimensions, and smaller doses are more pronounced. Therefore, the IEF calculation parameters consisted of these three mentioned factors. The 10.13 was the suitable number to be used as the cut point of lung VMAT-SBRT for applying respiratory motion management. The interplay effects for 6 MV FFF photon beams in VMAT-SBRT lung cancer are more impacted for the higher amplitudes and dimensions, and the smaller doses. The breathing amplitudes above 5 mm lead to significant changes in the shape of dose distribution due to the interplay effects, especially for 3D movements. The novel interplay effects factor (IEF) is a parameter that can indicate the interplay effects directly for VMAT-SBRT lung cancer patients. The value of 1 is the ideal score of IEF, while a value more than 10.13 is the score that can imply the need of respiratory motion management. However, the score is lower than 10.13, but that case provides some tumor motion that exceeds 5 mm in all directions, the need for respiratory motion management was also recommended.