Purpose: RefleXion™ X1 is a novel therapy machine designed to support both biology-guided radiotherapy (BgRT) and SBRT. To determine the dosimetric effects of motion during RefleXion SBRT treatment delivery, a lung phantom film dosimetry study was performed under multiple motion conditions using the first clinical installation of RefleXion X1.
Methods: CIRS Dynamic Thorax Motion Phantom with a 3.0cm diameter spherical tumor film insert was used to program respiratory waveforms with varying peak-to-peak amplitude (A) and period (T). 4DCT scans were acquired for the following 3 dynamic conditions: A=0.6cm with T=5s, A=1.2cm with T=4s, and A=2cm with T=6s to simulate human respiratory motion of lung tumours. Phase-based binning was performed, and GTVs were contoured on each respiratory-phase. ITVs were then generated, and PTVs were created with 2mm and 5mm expansions. To determine the effects of different motion and delivery parameters, treatment plans were created for both PTVs using 1 and 2cm jaw sizes with a prescription of >=95% coverage of PTV with 5.0Gy. Plans were delivered to a phantom under planned dynamic conditions. Two-dimensional dose was measured using Gafchromic EBT-3 film along the sagittal plane of the target center. The film measurements were used to calculate the 2D cumulative DVH, percentage of ITV receiving the prescription dose, and the minimum dose received by 95% of the ITV.
Results: The larger motion amplitude resulted in more under-dosing of the ITV, and this effect was greater for 1cm-jaw plans. The 2cm-jaw plans provided adequate coverage to the ITV for smaller amplitudes of motion (i.e., 0.6cm/1.2cm) with ITV-to-PTV margins of 2mm or 5mm.
Conclusion: The interplay of RefleXion SBRT delivery and target motion was measured for several clinically-relevant lung cases. Clinically acceptable ITV coverage was obtained for smaller amplitudes of motion with 2cm-jaws whereas significant under-dosing was observed for larger motion amplitudes with 1cm-jaws.