Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_3_3144 - Interfraction Variability in the Position of Soft-Tissue Structures of the Head and Neck

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Interfraction Variability in the Position of Soft-Tissue Structures of the Head and Neck
K. Tatebe1, M. Malin1, J. M. Melotek2, and D. J. Haraf1; 1University of Chicago, Chicago, IL, 2The University of Chicago Medicine, Chicago, IL

Purpose/Objective(s): To assess the interfraction motion of soft-tissue sites in the head and neck to help inform planning margins in radiation therapy.

Materials/Methods: Points on soft tissue structures of the head and neck were retrospectively identified on the simulation scan and weekly cone beam computed tomography (CBCT) scans for 60 of 62 patients enrolled on a previously published phase II randomized trial for de-escalation of therapy in p16 positive oropharyngeal patients. All patients were immobilized with custom aquaplast masks. The points identified were: the tip of the oral tongue, the tip of the uvula, and the midline, superior-most point of the epiglottis. These were chosen due to high visibility on CBCT due to the interface between soft tissue and air, and for their ease of repeatable and consistent identification. Analysis was limited to points on normal structures to avoid confounding effects from tumor regression. Displacements of these structures on CBCT were then compared to the simulation scan after registration to bony anatomy. Means and standard deviations of misalignments along the coordinate axes were quantified for greater than 220 registrations per site. Total magnitude of 3D motion, and appropriate planning target volume (PTV) margins per Van Herck methodology, were computed.

Results: The average magnitude of 3D displacement, and displacements averaged along each Cartesian axis, are shown, along with standard deviations (SD), in Table 1. The mean across all registrations showed no statistically significant systematic errors in the left/right (X), anterior/posterior (Y), or superior/inferior (Z) directions. Most mean displacements along Z were larger than in the X or Y directions. Motions of the uvula and epiglottis were most strongly correlated, with a correlation coefficient of 0.28. Correlations between the tongue and epiglottis, and the tongue and uvula were 0.14 and 0.02, respectively. A PTV margin for a minimum dose to the CTV of 95% of prescription for 90% of patients was determined to be 13.0 mm, 9.7 mm, and 9.9 mm for the oral tongue, uvula, and epiglottis, respectively.

Conclusion: Interfraction motion of soft tissue structures can be significantly greater than that of setup variability in bony anatomy. The larger systematic displacements in the Z-direction are likely due to the slice thickness being large compared to the resolution within an axial slice. PTV margins based solely on interfraction error after alignment to bony structures may lead to underdosing of soft tissue targets and, potentially, marginal failures. Care should be taken to ensure that motions of soft tissue structures do not exceed those anticipated by the planning margins. Frequent CBCT with alignment to soft tissue anatomy may facilitate reduction of PTV margins and hence toxicity. Table 1
Displacement (SD) in mm 3D X Y Z
Tongue 6.9 (4.2) -0.26 (2.0) -0.14 (4.0) +0.88 (3.9)
Uvula 5.5 (3.1) +0.20 (1.8) -1.15 (2.9) +0.94 (2.9)
Epiglottis 4.6 (2.7) -0.19 (1.5) +0.42 (1.7) +0.66 (2.9)

Author Disclosure: K. Tatebe: Employee; Advocate Good Samaritan Hospital. Project Lead; Northwestern Trauma and Surgical Initiative. M. Malin: None. J.M. Melotek: None. D.J. Haraf: None.

Ken Tatebe, MD, PhD

University of Chicago Medicine

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