SU-E-J-195: Quantification of Rotations and Deformations in Head and Heck Radiotherapy

Authors


Abstract

Purpose:

To quantify the magnitude of patient rotations and deformations over the course of head and neck radiotherapy.

Methods:

45 CBCT images acquired weekly were selected retrospectively from 5 head and neck cancer patients. CBCT images were registered to the corresponding planning CT images using translation, rigid-body, and cubic B-spline deformable transformations. Elastix (elastix.isi.uu.nl), an open-source public domain registration algorithm, was used for all registrations with mutual information as similarity metric and gradient descent as optimization. Four-level multiresolution approach was employed with 2 cm B-spline grid spacing at the finest resolution. Registration qualities were visually verified. Rotational occurrences were quantified from the rigid-body registrations. The magnitude of deformations in planning target volumes (PTV) and organs were quantified from the deformation vector fields with rigid registrations as baseline.

Results:

The measured rotations (mean±std) were 1.0±1.0 deg, – 0.3±1.0 deg, and 0.1±0.8 deg in the sagittal, coronal, and axial plains respectively. The estimated magnitudes of deformations (mean±std) were 3.4±2.1 mm for PTVs, 2.9±1.2 mm for parotid glands, 2.8±1.2 mm for mandible, and 2.2±1.1 mm for spinal cord. The overall deformation in the body contour was 4.3±4.3 mm. The organ spatial deviation due to rotation was small and approximately 8.4% of that from deformations. Results varied among patients.

Conclusion:

Rotations occurred about 1.0 deg (1SD) in the H' N area. Daily geometric deviation of target volumes was considerable and mainly from tissue deformation than rotations. Proper safety margin is necessary for adequate target coverage. Online plan adaptation may mitigate the daily differences between the plan and actual patient poses.

Partially supported by a grant from Varian Medical Systems, Palo Alto, CA.

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