Real-time high spatial resolution dose verification in stereotactic motion adaptive arc radiotherapy

Abstract

Purpose: Radiation treatments delivered with real-time multileaf collimator (MLC) tracking currently lack fast pretreatment or real-time quality assurance. The purpose of this study is to test a 2D silicon detector, MagicPlate-512 (MP512), in a complex clinical environment involving real-time reconfiguration of the MLC leaves during target tracking.

Methods: MP512 was placed in the center of a solid water phantom and mounted on a motion platform used to simulate three different patient motions. Electromagnetic target tracking was implemented using the Calypso system (Varian Medical Systems, Palo Alto, CA, USA) and an MLC tracking software. A two-arc VMAT plan was delivered and 2D dose distributions were reconstructed by MP512, EBT3 film, and the Eclipse treatment planning system (TPS). Dose maps were compared using gamma analysis with 2%/2 mm and 3%/3 mm acceptance criteria. Dose profiles were generated in sup-inf and lateral directions to show the agreement of MP512 to EBT3 and to highlight the efficacy of the MLC tracking system in mitigating the effect of the simulated patient motion.

Results: Using a 3%/3 mm acceptance criterion for 2D gamma analysis, MP512 to EBT3 film agreement was 99% and MP512 to TPS agreement was 100%. For a 2%/2 mm criterion, the agreement was 95% and 98%, respectively. Full width at half maximum and 80%/20% penumbral width of the MP512 and EBT3 dose profiles agreed within 1 mm and 0.5 mm, respectively. Patient motion increased the measured dose profile penumbral width by nearly 2 mm (with respect to the no-motion case); however, the MLC tracking strategy was able to mitigate 80% of this effect.

Conclusions: MP512 is capable of high spatial resolution 2D dose reconstruction during adaptive MLC tracking, including arc deliveries. It shows potential as an effective tool for 2D small field dosimetry and pretreatment quality assurance for MLC tracking modalities. These results provide confidence that detector-based pretreatment dosimetry is clinically feasible despite fast real-time MLC reconfigurations.

Keywords: 2D silicon array; MLC tracking; QA of adaptive radiotherapy; small field dosimetry.

Figure 1

Rendering of MP512 detector mounted on printed circuit board (PCB) and encapsulated in a PMMA phantom.

Figure 2

Three‐dimensional lung motion traces corresponding to (a) Patient 1, (b) Patient 2 and (c) Patient 3. Data are from 4DCT sampled every 25 ms. X component corresponds to motion in left‐right direction, Y component corresponds to motion in sup‐inf direction, and Z corresponds to motion in ant‐pos direction.

Figure 3

Cross‐section of scattering conditions of the solid water phantom. Units are mm. Detector surface is at an approximate water equivalent depth of 1.5 cm and the solid PMMA phantom is encapsulated by aluminum to minimize induced noise from RF field of Calypso. Coordinate system is marked with the Y direction coming out of the page.

Figure 4

GTV (blue) and PTV (red) margins defined on a CT dataset of the detector phantom represented in a transverse plane view. Directions are indicated with Y (sup‐inf) coming out of page.

Figure 5

2D dose map extracted from Eclipse TPS overlaid on CT slice of MP512. Boxes highlight the sensitive area of the detector and represent the dose map region used for gamma analysis comparison (coronal plane views).

Figure 6

Linear dose response exhibited by MP512 for cumulative dose delivery. The slope of the linear fit is used to convert measured detector charge (nC) to absorbed dose (cGy). Error bars indicate measured baseline fluctuation of 1%.

Figure 7

2D integral dose maps reconstructed from (a) EBT3, (b) MP512 and (c) TPS for the no‐motion case.

Figure 8

MP512 vs EBT3 Dose Profiles for various motion modalities with Patient 1 motion applied. Left Column: Profiles in the sup‐inf direction. Right Column: Profiles in the left‐right direction.

Figure 9

Dose profiles measured with MP512 when the Patient 1 motion was applied for each motion modality, with dose error. Left corresponds to Sup‐Inf direction and right corresponds to Left‐Right direction. In the error plots, NM corresponds to no motion, M to motion, MT to motion tracking (i.e., passive algorithm) and MTP to motion tracking with prediction.

Figure 10

Dose profiles measured with MP512 when the Patient 2 motion was applied for each motion modality, with dose error. Left corresponds to vertical direction and right corresponds to horizontal direction. In the error plots, NM corresponds to no motion, M to motion, MT to motion tracking (i.e., passive algorithm) and MTP to motion tracking with prediction.

Figure 11

Dose profiles measured with MP512 when the Patient 3 motion was applied for each motion modality, with dose error. Left corresponds to vertical direction and right corresponds to horizontal direction. In the error plots, NM corresponds to no motion, M to motion, MT to motion tracking (i.e., passive algorithm) and MTP to motion tracking with prediction.