Combined MV + kV inverse treatment planning for optimal kV dose incorporation in IGRT

Abstract

Despite the existence of real-time kV intra-fractional tumor tracking strategies for many years, clinical adoption has been held back by concern over the excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this problem by investigating, for the first time, the use of convex optimization tools to optimally integrate this excess kV imaging dose into the MV therapeutic dose in order to make real-time kV tracking clinically feasible. Phase space files modeling both a 6 MV treatment beam and a kV on-board-imaging beam of a commercial LINAC were generated with BEAMnrc, and used to generate dose influence matrices in DOSXYZnrc for ten previously treated lung cancer patients. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional constraints as required for real-time kV intra-fractional tracking. An interior point optimizer was used to solve the modified optimization problem. It was found that when using large kV imaging apertures during fluoroscopic tracking, combined MV + kV optimization lead to a 0.5%-5.17% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose over the ten patients. This was accompanied by a reduction of up to 42% of the excess kV dose compared to standard MV IMRT with kV tracking. For all kV field sizes considered, combined MV + kV optimization provided prescription dose to the treatment volume coverage equal to the no-imaging case, yet superior to standard MV IMRT with non-optimized kV fluoroscopic tracking. With combined MV + kV optimization, it is possible to quantify in a patient specific way the dosimetric effect of real-time imaging on the patient. Such information is necessary when substantial kV imaging is performed.

Imaging dose from cone beam computed tomography in radiation therapy

“…In this case, it may be warranted to include the imaging dose in the planning process. It has been shown that this approach is feasible and that including imaging dose during plan optimization reduces the overall dose delivered to patient [54,78,88]. This, however, is not a trivial task and requires either use of Monte Carlo or commissioning of imaging beams in treatment planning systems.…”

“…Another method to reduce imaging dose proposed by Roxby et al [87] introduces a Copper filter to the beam hence increasing its half value layer and decreasing the delivered dose. More recently, Grelewicz and Dzierma [88] proposed a method to reduce kV imaging dose by combining MV and kV inverse planning using MC. As for the megavoltage beams, the use of low accelerating potential for the imaging beam and a Carbon target, the IBL, has reduced the dose commonly encountered when using the 6 MV therapeutic beam for imaging [56].…”

Quality assurance for the clinical implementation of kilovoltage intrafraction monitoring for prostate cancer VMAT

“…13 • Incorporating the imaging dose into the optimization framework to reduce delivery time where beneficial. 29 It should also be noted that if KIM replaces daily cone beam CT imaging, then the total imaging dose to the patient would be reduced.…”

Use of a laser‐guided collimation system to perform direct kilovoltage x‐ray spectra measurements on a linear accelerator onboard imager

“…Modern commercial linear accelerator (linac) systems are increasingly outfitted with onboard‐imaging (OBI) devices in order to provide additional information on patient positioning for radiation therapy. In the case of kV x‐ray imaging beams, it has been demonstrated that accurate dose simulations requires full knowledge of the kV beam energy spectrum . The recently released AAPM TG180 report on image guidance doses delivered during radiotherapy provides a review of some of the literature on the dose delivered by the OBI during treatment.…”

“…Many research applications, however, require more detailed information. To fill this gap, Monte Carlo (MC) methods have been used extensively in the literature to generate the necessary kV beam spectroscopic information needed for accurate OBI beam modeling . However, direct experimental verification is still needed as beam spectroscopic quality can vary substantially based on parameters such as the degree of collimation, filtration, and other sources that may have not have been properly modeled in simulations .…”