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.

Clinical Implementation and Initial Experience of Real-Time Motion Tracking With Jaws and Multileaf Collimator During Helical Tomotherapy Delivery

“…The maximum number of radiograph images per unit dose among all patients was 38.9/Gy, which is equivalent to approximately 0.3% of the planned dose. 10,17 The maximum values of patient overall average of Potential Diff, Rigid Body, and Target Offset were 1.4 AE 3.3, 3.9 AE 1.7, and 4.8 AE 2.7 mm, respectively. The variations of their daily averages are shown in Figure 5B for the first patient.…”

“…For the cases treated, the maximum average number of radiograph images (including those with MV beam off) per unit Gy RT dose was 45.3/Gy (typically below 10/ Gy), which leads to an imaging dose of approximately 0.5% of the prescription dose. 10,17…”

Effects of variable‐width jaw motion on beam characteristics for Radixact Synchrony®

“…A description of the Synchrony system has been provided by others in previous publications. [1][2][3][4][5][6] The Radixact uses an un-flattened 6-MV beam for helical tomotherapy treatment delivery. The un-flattened beam profile is not an issue during standard intensity-modulated radiation therapy (IMRT) treatments because the change in fluence off-axis is accounted for in the treatment planning system.…”

“…The jaws compensate for motion in the IEC‐Y (superior/inferior) direction, and the multi‐leaf collimator (MLC) compensates for motion in the transverse plane (IEC‐X [left/right] and IEC‐Z [anterior/posterior] directions). A description of the Synchrony system has been provided by others in previous publications 1–6 …”

Technical note: On the impact of the kV imaging configuration on doses from planar images during motion‐synchronized treatments on Radixact®

“…Dose to water at the reference depth of 2 cm was calculated using equation 4 in the TG‐61 report 6 . The measurement geometry of a previous publication on kV dose from Radixact was repeated to isolate differences in point doses due to changing the kV configuration alone 5 . The same tube potentials were explored: 100, 120, and 140 kVp.…”