Fluence field modulated CT on a clinical TomoTherapy radiation therapy machine

Abstract

Purpose: The multi-leaf collimator (MLC) assembly present on TomoTherapy (Accuray, Madison WI) radiation therapy (RT) and mega voltage CT machines is well suited to perform fluence field modulated CT (FFMCT). In addition, there is a demand in the RT environment for FFMCT imaging techniques, specifically volume of interest (VOI) imaging. Methods: A clinical TomoTherapy machine was programmed to deliver 30% imaging dose outside predefined VOIs. Four different size ROIs were placed at varying distances from isocenter. Projections intersecting the VOI received "full dose" while those not intersecting the VOI received 30% of the dose (i.e. the incident fluence for non VOI projections was 30% of the incident fluence for projections intersecting the VOI). Additional scans without fluence field modulation were acquired at "full" and 30% dose. The noise (pixel standard deviation) was measured inside the VOI region and compared between the three scans. Results: The VOI-FFMCT technique produced an image noise 1.09, 1.05, 1.05, and 1.21 times higher than the "full dose" scan for ROI sizes of 10 cm, 13 cm, 10 cm, and 6 cm respectively within the VOI region. Conclusions: Noise levels can be almost unchanged within clinically relevant VOIs sizes for RT applications while the integral imaging dose to the patient can be decreased, and/or the image quality in RT can be dramatically increased with no change in dose relative to non-FFMCT RT imaging. The ability to shift dose away from regions unimportant for clinical evaluation in order to improve image quality or reduce imaging dose has been demonstrated. This paper demonstrates that FFMCT can be performed using the MLC on a clinical TomoTherapy machine for the first time.Keywords: CT, FFMCT, dynamic bowtie, volume of interest imaging DESCRIPTION OF PURPOSEAs radiation therapy (RT) dose delivery systems have progressed, they have allowed for better healthy tissue sparing due to their ability to deliver higher dose gradients. In order to take advantage of high dose gradients, however, the location of the tumor and healthy tissue must be confirmed prior to treatment. TomoTherapy (Accuray, Madison WI) is a commercially available RT system that combines pretreatment imaging with intensity modulated delivery (IGRT). 1 Prior to an IGRT treatment, the patient is given a computed tomography (CT) scan so the patient's position can be adjusted to ensure proper dose delivery. In addition, the radiation oncologist can monitor any gross changes in the patient's morphology due to weight loss/gain or changes in body positioning. The focus of this paper is on the imaging involved in IGRT; we propose that since radiation oncologists only need good image quality for a small volume of interest within a patient's cross section, we should only create a high quality image within that VOI. Furthermore, we propose that if we can enhance the image quality in that small VOI without raising the patient's imaging dose, the quality of patient care should increase. Poor image quality has plagued RT since IGRT w...

Realization of fluence field modulated CT on a clinical TomoTherapy megavoltage CT system

“…FFMCT actually takes a page from the book of intensity modulated radiation therapy (IMRT) by modulating the imaging dose incident onto the patient as a function of rotation angle. Through the use of FFMCT applied to TomoTherapy CT (FFMCT-MVCT) acquisitions, it should be possible to increase the image quality within a pre-determined volume of interest while decreasing the image quality outside the VOI (Kolditz et al 2010, Chen et al 2011, Heuscher and Noo 2011, Oktay and Noo 2014, Szczykutowicz and Hermus 2015. The integral imaging dose can be adjusted to remain the same as for a non-FFMCT-MVCT acquisition.…”

Creation of an atlas of filter positions for fluence field modulated CT

“…[6][7][8] In addition, the technique described in this work should have applications in VOI imaging. [9][10][11][12][13][14] Recently, the use of 2D and 1D arrays of attenuating fluids (liquids and gases) has been proposed 15,16 for FFMCT/VOI imaging and these approaches will likely face similar challenges as previously discussed. All of these techniques rely on imaging with a fluence field that changes from view to view or changes on a patient to patient basis.…”