Principal Component Analysis to Design Planning Target Volume in Oropharyngeal Cancers

Abstract

Standard translational shifts of the Clinical Target volume (CTV) to generate the Planning Target Volume (PTV) do not account for rotations. Head and neck positional misalignments derive in large part from rotations due to cervical spine arching and twisting in Cone Beam Computed Tomography (CBCT). Translational expansions do not track rotations, yielding coverage envelopes that unnecessarily overlap with adjacent structures. This work examines whether principal component analysis of the motion along all 6 degrees of freedom may be used to produce a more favorable PTV. Seventy-five CBCTs of ten oropharyngeal cases were included. The records of couch shifts needed to align individual bony structures (C1-5, mandible and mastoid) between the planning image and CBCTs were recorded. A Principal Component Analysis of the shifts was used to generate an ellipsoid inflation of each CTV vertex along 6 degrees of freedom. The result was compared to a 3D ellipsoid based translational expansion, and to a described ellipsoid based vertex expansion along 6 degrees of freedom, with axes oriented in parallel to the treatment reference frame. Themean (x, y) shifts in mm needed to align individually bodies C1 - C5 were respectively (-0.4, 0.5), (+.5, -0.2), (+-0.2, -0.2), (-0.2, +0.4), and (-0.5, +0.7), the monophasic pattern showing acquired curvature along both axes during treatment and demanding a PTV for coverage. A PTV was constructed using a described 6D ellipsoidal based boundary point expansion aligned along the reference frame axis or using a new theory to align against the principal components of the motion. A cyclical one-out method was used to validate the PTV models. Selected confidence intervals yielded complete coverage in >80% weeks in 80% cases. Validation testing disclosed similar complete coverage in 83-86% weekly CBCTs in the test cases with either method. The PCA 6D PTV could yield less normal structure overlap. A one out method was used to test overlap avoidance from PTVs constructed from a population of weekly CBCTs drawn from seven cases with one excluded. PTVs were drawn around target and constrictors on an extraneous case and imaged on a CT slice. Both a rolling 'ball' expansion of the vertices that applies a 3D translational ellipsoid and a PTV constructed using a 6D ellipsoid aligned against the standard reference frame overlapped with all or nearly all the constrictors in all but one trial (1/7). The 6D ellipsoid aligned against the principal motion components spared >70% of a constrictor in all trials (7/7). PTVs remain needed to ensure target coverage in head and neck radiotherapy even with daily CT accuracy because of acquired spinal curvatures resulting in rotational displacements. A described 6D ellipsoid oriented to the reference frame can yield good coverage, but with unneeded constrictor coverage. A PCA analysis yields a PTV with equally good coverage but able to spare 70% of a constrictor.