WE-A-134-02: Combined MV+kV Beam Optimization for Enabling Real-Time KV Tumor Tracking.
PURPOSE
Despite the existence of real-time kV intrafractional tumor tracking strategies for many years, clinical adoption has been held back by concern over excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this specific problem by performing a first time investigation into the use of convex optimization tools to best integrate this excess kV imaging dose into the MV therapeutic dose as to make real-time kV tracking clinically feasible.


METHODS
Phase space files for both a 125 kVp kV beam and a 6 MV treatment beam were generated with BEAMNRC, and used to make dose influence matrices in DOSXYZNRC for a lung cancer patient. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional kV imaging constraints as required for real-time kV fluorescent tracking. The MOSEK optimization toolkit was used to solve the modified optimization problem.


RESULTS
Compared to conventional IMRT optimization, combined MV+kV optimization leads to a 2.4-5.2% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose. When using an open kV aperture, combined MV+kV optimization allows for a reduction of up to 50% of the excess kV dose compared to standard IMRT with kV fluoro tracking. For all kV field sizes considered, combined MV+kV optimization provided PTV coverage equal to standard IMRT without kV imaging and superior to standard IMRT plus kV fluoro tracking. Skin dose with combined MV+kV imaging was only slightly higher than the no-imaging case.


CONCLUSION
The use of MV+kV optimization allows for greatly reduced imaging dose to the skin, especially in cases of large kV aperture imaging. Combined MV+kV optimization demonstrates potential for real-time tumor tracking without excessive imaging dose, paving the way for clinical implementation.