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

Abstract

Purpose: Radixact Synchrony corrects for target motion during treatment by adjusting the jaw and MLC positions in real time. As the jaws move off axis, Synchrony attempts to adjust for a loss in output due to the un-flattened 6 MV beam by increasing the jaw aperture width. The purpose of this work was to assess the impact of the variable-width aperture on delivered dose using measurements and simulations.

Methods: Longitudinal beam profile measurements were acquired using an Edge diode with static gantry. Jaw-offset peak, width, and integral factors were calculated for profiles with the jaws in the extreme positions using both variable-width (Synchrony) and fixed-width apertures. Treatment plans with target motion and compensation were compared to planned doses to study the impact of the variable aperture on volumetric dose.

Results: The jaw offset peak factor (JOPF) for the Synchrony jaw settings were 0.964 and 0.983 for the 1.0- and 2.5-cm jaw settings, respectively. These values decreased to 0.925 and 0.982 for the fixed-width settings, indicating that the peak value of the profile would decrease by 7.5% compared to centered if the aperture width was held constant. The IMRT dose distributions reveal similar results, where gamma pass rates are above tolerance for the Synchrony jaw settings but fall significantly for the fixed-width 1-cm jaws.

Conclusions: The variable-width behavior of Synchrony jaws provides a larger output correction for the 1-cm jaw setting. Without the variable-aperture correction, plans with the 1-cm jaw setting would underdose the target if the jaws spend a significant amount of time in the extreme positions. This work investigated the change in delivered dose with jaws in the extreme positions, therefore overall changes in dose due to offset jaws are expected to be less for composite treatment deliveries.

Keywords: radixact; synchrony; tomotherapy.

Fig. 1

Schematic of collimation and jaw compensation on Radixact. Beam’s eye view (a) and side view (b) with centered jaws and central three leaves open. Beam’s eye view (c) and side view (d) with jaws compensating in the positive IEC‐Y direction. Note that there are 64 total MLC leaves on Radixact and drawings are not to scale.

Fig. 2

Photograph of the measurement setup with Edge diode aligned perpendicular to the direction of couch travel. A cutout was made in the 5 mm thick bolus material to reduce the air gap around the detector. The 10 cm of solid water on top of the gel and detector was removed for the photograph. The detector volume was centered in IEC‐X.

Fig. 3

Profiles at 10‐cm depth and 85‐cm source‐to‐surface distance measured with an Edge diode. The y‐axis displays the signal relative to the maximum signal for the profile measured with the centered aperture. The colored horizontal ticks indicate the level of the FWHM for each curve. Displayed at the off‐axis locations are profiles acquired with varying jaw aperture widths (see Table 1).

Fig. 4

Simulated dose profiles in the IEC‐Y direction in the frame of reference of a cylindrical target from a helical IMRT delivery. Profiles are centered in X and Z. The vertical lines indicate the limits of the target in IEC‐Y (5‐cm diameter, 5‐cm length cylinder). The prescription dose was 50 Gy. Negative and positive refer to the target shifted to the positive or negative extreme location throughout treatment and compensated with jaw motion. The resolution is 0.5 mm in the IEC‐Y direction. Gamma profiles (1%, 1 mm) are shown for the Synchrony plans compared to planned.

Fig. 5

Jaw‐offset peak factors (JOPFs) as a function of target shift in IEC‐Y for the variable‐aperture jaw behavior during Synchrony treatments. Profiles were calculated with the TPS at a depth of 1.5‐cm and 85‐cm SSD. The values are an average of positive and negative JOPFs. Values at the extreme positions agree with the measured values in Table 2.