Technical Note: Patient dose from kilovoltage radiographs during motion-synchronized treatments on Radixact®

Abstract

Purpose: Synchrony is a motion management system available on the Radixact linear accelerator that utilizes kilovoltage (kV) radiographs to track target motion and synchronize the delivery of radiation with the motion. Proper management of this imaging dose requires accurate quantification. The purpose of this work was to use Monte Carlo (MC) simulations to quantify organ-specific patient doses from these images for various patient anatomies.

Methods: Point doses in water were measured per TG-61 for three beam qualities commonly used on the Radixact. The point doses were used to benchmark a model of the imaging system built using the Monte Carlo N-Particle (MCNP) transport code. Patient computed tomography (CT) datasets were obtained for 5 patients and 100 planar images were simulated for each patient. Patient dose was calculated using energy deposition mesh tallies.

Results: The MCNP model was able to accurately reproduce the measured point doses, with a median dose difference of less than 1%. The median dose (D_50% ) to soft tissue from 100 radiographs among the 5 patient cases ranged from 2.0 to 4.6 mGy. The max dose (D_1% ) to soft tissue ranged from 6.2 to 31.0 mGy and the max dose to bony structures ranged from 20.2 to 71.7 mGy. These doses can be scaled to estimate total patient dose throughout many fractions.

Conclusions: Patient dose is largely dependent on imaging protocol, patient size, and treatment parameters such as fractionation and gantry period. Organ doses from 100 radiographs (an approximate number for one fraction) on the Radixact are slightly less than the doses from Tomo MVCT setup images. Careful selection of clinical protocols and planning parameters can be used to minimize risk from these images.

Keywords: Radixact; intrafraction imaging dose; tomotherapy.

Fig. 1

Photograph of a Radixact at UW‐Madison with the cover removed, showing the kV tube and flat panel kV detector. The MV source‐to‐axis distance (SAD) is 85 cm, the kV SAD is 57.5 cm, and the kV source to imager distance is 113.5 cm. The MV source is hidden in the picture by the couch. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 2

Geometry of the MCNP simulations showing the kV tube shell (a), the tungsten anode (b), the 1 mm aluminum and 0.5 mm copper filters (c), the static tungsten collimator (d), and the 1.6 mm polycarbonate bore material (e). The anode–cathode axis is parallel to the direction of table travel. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 3

Measured (A12) and simulated (MCNP) point‐dose data in water for open‐field radiographs (20 × 20 cm²). Error bars indicate standard error (k = 1). The standard error of all simulated points was less than 0.5%. The heel effect can be observed in the inline direction via the asymmetric profile. The gray lines indicate the location of the reference point, at 2 cm depth and 55 cm from the source along the central axis. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 4

Example computed tomography (CT) axial slices through isocenter and dose distributions in mGy resulting from 100 radiographs on the Radixact for two patient cases. Four imaging angles were used for the cases in this example. The target was placed at the geometric isocenter. The red line on the CT slice displays the AP location of the coronal dose distribution in this figure. Organ‐specific dose statistics are provided for each patient case in Table III. [Color figure can be viewed at wileyonlinelibrary.com]