Measuring Interfraction and Intrafraction Motion with Cone Beam Computed Tomography (CBCT) and an Optical Localization System (OLS) for Lower Extremity Soft Tissue Sarcoma Patients Treated with Preoperative Intensity Modulated Radiation Therapy (IMRT)

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

To evaluate residual positional and out of plane rotational errors for lower extremity soft tissue sarcoma patients using Image-Guided IMRT.

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Materials/Methods 

29 patients receiving IMRT were scanned with a pre-treatment CBCT, and manual registration to the initial CT planning scan was performed using commercial software (Elekta Synergy XVI). Patients were immobilized with a custom device or polystyrene vacuum cradle. Any deviation >3mm from the planned isocenter was corrected by couch translation. The last scan acquired prior to treatment represented the patient in the actual treatment position. An automated matching algorithm was used to re-register these scans to the planning CT to estimate residual setup errors retrospectively. Mean residual set up uncertainties and out of plane rotations were calculated in the left-right (LR), supero-inferior (SI), and antero-posterior (AP) dimensions. Intrafraction motion was assessed using the OLS system combined with pre and post fraction CBCT scans performed for 14 of the 29 patients once per week (5 times total). The OLS system consists of an infrared emitting camera mounted on the treatment room ceiling, 5 single non-invasive reflective markers placed on the patient, and a PC-based software program designed to interpret the reflected signal and to record the marker position relative to the isocenter co-ordinates of the linear accelerator at a speed of 30 Hertz. The maximum relative displacement was calculated for both the OLS system and the pre and post fraction CBCT scans. Systematic error (SE) and random error (RE) was calculated for the residual positioning uncertainty and combined with the intrafraction motion for planning target volume (PTV) margin calculation.

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Results 

The mean standard deviation (SD) of the residual RE was 1.9 mm LR, 2.1 mm SI, and 1.8 mm AP, and the SE SD was 0.8 mm, 1.3 mm, and 0.9 mm in each dimension, respectively. The overall out of plane rotation was 1.1 degrees LR, 2.1 degrees SI, and 0.8 degrees AP. The overall maximum displacement of the OLS markers was 0.9 mm LR, 1.3 mm SI, and 1.3 mm AP (±0.7 mm, ±1.0 mm, and ±1.1 mm SD, respectively). The max difference in positional displacement for pre and post fraction CBCT scans was 1.0 mm LR, 0.2 mm SI, and −0.1 mm AP (±1.9 mm, ±1.6 mm, and ±1.4 cm SD, respectively). Using the published van Herk “margin recipe” formula, these data support a 4mm LR, 5mm SI, and 4mm AP PTV margin.

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Conclusions 

The largest RE, SE, and rotation was seen in the SI direction, which may have implications for the quantification of non-uniform PTV margins. Due to their small magnitude, intrafraction motion and out of plane rotations were not a significant concern in margin design.