Using 4D dose accumulation to calculate organ-at-risk dose deviations from motion-synchronized liver and lung tomotherapy treatments

Abstract

Tracking systems such as Radixact Synchrony change the planned delivery of radiation during treatment to follow the target. This is typically achieved without considering the location changes of organs at risk (OARs). The goal of this work was to develop a novel 4D dose accumulation framework to quantify OAR dose deviations due to the motion and tracked treatment. The framework obtains deformation information and the target motion pattern from a four-dimensional computed tomography dataset. The helical tomotherapy treatment plan is split into 10 plans and motion correction is applied separately to the jaw pattern and multi-leaf collimator (MLC) sinogram for each phase based on the location of the target in each phase. Deformable image registration (DIR) is calculated from each phase to the references phase using a commercial algorithm, and doses are accumulated according to the DIR. The effect of motion synchronization on OAR dose was analyzed for five lung and five liver subjects by comparing planned versus synchrony-accumulated dose. The motion was compensated by an average of 1.6 cm of jaw sway and by an average of 5.7% of leaf openings modified, indicating that most of the motion compensation was from jaw sway and not MLC changes. OAR dose deviations as large as 19 Gy were observed, and for all 10 cases, dose deviations greater than 7 Gy were observed. Target dose remained relatively constant (D95% within 3 Gy), confirming that motion-synchronization achieved the goal of maintaining target dose. Dose deviations provided by the framework can be leveraged during the treatment planning process by identifying cases where OAR doses may change significantly from their planned values with respect to the critical constraints. The framework is specific to synchronized helical tomotherapy treatments, but the OAR dose deviations apply to any real-time tracking technique that does not consider location changes of OARs.

Keywords: Radixact; Synchrony; tomotherapy.

FIGURE 1

(a) Location of the target centroid in each phase relative to the reference phase derived from the 4DCT images for Lung 3. (b) The continuous respiratory amplitude pattern. (c) The discrete respiratory pattern mapped to each of the 10 phases. (d) The discrete mathematical target motion pattern. 4DCT, four‐dimensional computed tomography

FIGURE 2

Example of assigning sinogram projections from the original treatment plan to each phase of the 4DCT for Lung 3. The central 30 MLC leaves and the first 250 projections of the treatment are shown. 4DCT, four‐dimensional computed tomography; MLC, multi‐leaf collimator

FIGURE 3

Example of generation of motion‐synchronized treatment plans for Lung 3. (a) Differential sinograms expressed as the absolute difference between the original MLC sinogram and the motion‐synchronized sinogram. The central 30 MLC leaves and the first 250 projections of the treatment are shown. (b) Jaw field edges after compensation projected to isocenter for each phase as a function of projection in the treatment. The dotted gray lines indicate the centered jaw positions. MLC, multi‐leaf collimator

FIGURE 4

Dose differences (Synchrony accumulation minus plan) for each lung subject for the full prescription. For example, a 5‐Gy dose difference is a 10% difference relative to the prescription of 50 Gy

FIGURE 5

Dose differences (Synchrony accumulation minus plan) for each liver subject for the full prescription. For example, a 5‐Gy dose difference is a 10% difference relative to a prescription of 50 Gy