Clinical development of a failure detection‐based online repositioning strategy for prostate IMRT—Experiments, simulation, and dosimetry study

Abstract

Purpose: To implement and evaluate clinic-ready adaptive imaging protocols for online patient repositioning ͑motion tracking͒ during prostate IMRT using treatment beam imaging supplemented by minimal, as-needed use of on-board kV. Methods: The authors examine the two-step decision-making strategy: ͑1͒ Use cine-MV imaging and online-updated characterization of prostate motion to detect target motion that is potentially beyond a predefined threshold and ͑2͒ use paired MV-kV 3D localization to determine overthreshold displacement and, if needed, reposition the patient. Two levels of clinical implementation were evaluated: ͑1͒ Field-by-field based motion correction for present-day linacs and ͑2͒ instantaneous repositioning for new-generation linacs with capabilities of simultaneous MV-kV imaging and remote automatic couch control during treatment delivery. Experiments were performed on a Varian Trilogy linac in clinical mode using a 4D motion phantom programed with prostate motion trajectories taken from patient data. Dosimetric impact was examined using a 2D ion chamber array. Simulations were done for 536 trajectories from 17 patients. Results: Despite the loss of marker detection efficiency caused by the MLC leaves sometimes obscuring the field at the marker's projected position on the MV imager, the field-by-field correction halved ͑from 23% to 10%͒ the mean percentage of time that target displacement exceeded a 3 mm threshold, as compared to no intervention. This was achieved at minimal cost in additional imaging ͑average of one MV-kV pair per two to three treatment fractions͒ and with a very small number of repositionings ͑once every four to five fractions͒. Also with low kV usage ͑ϳ2 / fraction͒, the instantaneous repositioning approach reduced overthreshold time by more than 75% ͑23% to 5%͒ even with severe MLC blockage as often encountered in current IMRT and could reduce the overthreshold time tenfold ͑to Ͻ2%͒ if the MLC blockage problem were relieved. The information acquired for repositioning using combined MV-kV images was found to have submillimeter accuracy. Conclusions: This work demonstrated with a current clinical setup that substantial reduction of adverse targeting effects of intrafraction prostate motion can be realized. The proposed adaptive imaging strategy incurs minimal imaging dose to the patient as compared to other stereoscopic imaging techniques.

IL-1 Generated Subsequent to Radiation-Induced Tissue Injury Contributes to the Pathogenesis of Radiodermatitis

“…We sought to use a model in which the skin would be more selectively exposed to radiation and thereby mimic the injury that would develop after exposure to β particles. To this end the flank skin of C57BL/6J mice was shaved and the right flank was exposed to an electron beam from a Varian II particle accelerator, commonly used in human radiotherapy (22). An advantage of this approach is that the energetic particles have a limited depth of penetration and therefore models damage from β particles.…”

Determinants of intrafraction variation during image-guided hypofractionated radiotherapy of the prostate

“…Multiple solutions have been presented to accommodate for IFV while maintaining reasonable target margins for hypofractionated schedules. Analysis of correction of IFV at each field delivery has found that this strategy reduces the time with IFV greater than 3 mm by more than 50 % (10 vs. 23 %) [17]. This is further justified by data from the University of Michigan which found that target margins were reduced by on-line correction between beams and further reduced by on-line correction during each individual beam [18].…”

Determination of action thresholds for electromagnetic tracking system‐guided hypofractionated prostate radiotherapy using volumetric modulated arc therapy

“…Adaptive strategies that track the motion signal, calculate any trend in position, predict the near-term motion trajectory, and make a decision whether to correct the shift, will be more powerful. [29][30][31] Future investigations are needed to incorporate dosimetric-driven criteria into the design of advanced management strategies and evaluating its clinical efficacy.…”