C. Quintero Mateus, M. Grams, and M. Mashayekhi; Department of Radiation Oncology, Mayo Clinic, Rochester, MN
Purpose/Objective(s):To perform a test assessing the dosimetric impact of simulated patient motion during minibeam radiotherapy delivery (MBRT). MBRT delivers narrow and closely spaced regions of high and low doses, so patient motion which may blur the dose is a concern. We evaluated the robustness of delivery in terms of peak dose (PD), valley dose (VD), and peak-to-valley ratio (PVR). Materials/
Methods: The 180 kVp output from a clinical orthovoltage unit was used. The field was defined by a 3cm circular cone and further collimated by a custom tungsten block having 0.5 mm wide slits separated by 0.6 mm. Layers of bolus simulated a lesion on the skin of a head-and-neck phantom. A custom holder was 3D printed to hold the tungsten collimator in place on the phantom. Radiochromic films were placed at the surface and at 1 cm depth, and 5 Gy peak dose at 1cm depth was prescribed. Dose profiles from films were used to calculate PD, VD, and PRV along an axis perpendicular to the collimator slits. Additional measurements were made to simulate five types of intrafraction motion relative to the orthovoltage unit: linear displacement parallel (LD1) and perpendicular (LD2) to the collimator slits, normal to the collimator surface (LD3), angular displacement on the plane parallel to the collimator slits (AD1), and on the plane perpendicular to the collimator slits (AD2). The range of motion was [0-10 mm], [0-6 mm], [0-10 mm], [0-1°], and [0-1°] respectively. Data was compared to measurements taken with no motion relative to the phantom. Results: Compared to data with no motion, the largest effect on PD at surface (-3.8%±2.2% -µ difference ±2s) and at 1cm depth (-30.8%±3.9%) was observed for LD2. AD2 also had a considerable effect on PD at 1cm depth (-8.8%±3.2%) whereas the rest of motions kept PD values within ±2.0% at surface and ±3.0% at 1cm depth. Considerable changes in VD at 1cm depth were observed across all types of motions, LD1 having the smallest effect (-4.2%±10.0%) and AD2 the largest effect (-12.0%±15.4%) Changes within ±8% in PVR at surface were observed for all types of motions. The largest effect on PVD at 1cm depth occurred for LD2 (-33.7%±3.1%) and AD2 (5.7%±9.3%). The rest of the motions kept PVR within ±3.0% at 1cm depth. Conclusion: Discrepancies were observed due to simulated motion, especially for displacements perpendicular to the collimator slits (LD2 and AD2). The effect of intrafraction motion on PVR may be considerable, but distinct regions of high and low doses were always maintained. This preliminary experiment emphasizes the importance of proper patient immobilization and monitoring for eventual clinical transition of MBRT.
