Adaptive radiation therapy for postprostatectomy patients using real-time electromagnetic target motion tracking during external beam radiation therapy

Abstract

Purpose: Using real-time electromagnetic (EM) transponder tracking data recorded by the Calypso 4D Localization System, we report inter- and intrafractional target motion of the prostate bed, describe a strategy to evaluate treatment adequacy in postprostatectomy patients receiving intensity modulated radiation therapy (IMRT), and propose an adaptive workflow.

Methods and materials: Tracking data recorded by Calypso EM transponders was analyzed for postprostatectomy patients that underwent step-and-shoot IMRT. Rigid target motion parameters during beam delivery were calculated from recorded transponder positions in 16 patients with rigid transponder geometry. The delivered doses to the clinical target volume (CTV) were estimated from the planned dose matrix and the target motion for the first 3, 5, 10, and all fractions. Treatment adequacy was determined by comparing the delivered minimum dose (Dmin) with the planned Dmin to the CTV. Treatments were considered adequate if the delivered CTV Dmin is at least 95% of the planned CTV Dmin.

Results: Translational target motion was minimal for all 16 patients (mean: 0.02 cm; range: -0.12 cm to 0.07 cm). Rotational motion was patient-specific, and maximum pitch, yaw, and roll were 12.2, 4.1, and 10.5°, respectively. We observed inadequate treatments in 5 patients. In these treatments, we observed greater target rotations along with large distances between the CTV centroid and transponder centroid. The treatment adequacy from the initial 10 fractions successfully predicted the overall adequacy in 4 of 5 inadequate treatments and 10 of 11 adequate treatments.

Conclusion: Target rotational motion could cause underdosage to partial volume of the postprostatectomy targets. Our adaptive treatment strategy is applicable to post-prostatectomy patients receiving IMRT to evaluate and improve radiation therapy delivery.

Figure 1

The schematic workflow of the Delivered Dose Investigation Tool (DiDIT) implemented in the commercial Pinnacle³ treatment planning system. The processes inside the dashed rectangle are performed in the Pinnacle³ workstation.

Figure 2

Histogram of the inter-transponder distance variation compared with planned distances for all treatment session of all 16 patients. The overall variation was 0.05 ± 0.15 cm.

Figure 3

(a) Mean translational displacement and standard deviation in left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions. (b) Mean magnitude of rotation and the standard deviation in pitch, yaw, and roll directions. (c) Eccentricity of all 16 patients. (d) The delivered to planned D_min ratio of after all treatment sessions. Rectangles indicate patients with inadequate treatment to the CTV in terms of D_min.

Figure 4

The planned (a) and estimated delivered (b) dose to the target for patient 11 overlaying on the CT images. The white arrow in (b) indicates a cold spot. The DVH (c) of the planned (solid) and delivered (dashed) dose to CTV shows that only a small percentage of the CTV volume is under-dosed.