Integrity of Parotid Gland Delineation for H&N IMRT: What are we sparing?

Reviewer: Voika BarAd, MD
Abramson Cancer Center of the University of Pennsylvania
Last Modified: October 19, 2005

Presenter: T.S. Hong
Presenter's Affiliation: Department of Human Oncology, University of Wisconsin Medical School, Madison, WI, Department of Medical Physics, University of Wisconsin Medical School, Madison, WI, Department of Radiology, Universi
Type of Session: Scientific

Background

Parotid–sparing head and neck (H&N) IMRT affords the potential to preserve salivary function and to diminish the adverse impact of chronic xerostomia on patient quality of life.
Although the literature provides several guidelines for H&N IMRT tumor target definition, guidelines for parotid gland delineation remain ill-defined.
Despite the intuitive simplicity of parotid contouring, successful execution of parotid-sparing IMRT requires accurate and consistent delineation of the parotid gland (both the superficial and deep lobes).
In this study, we examine the process of parotid gland contouring and assess the integrity of contouring on resultant IMRT dose reporting.

Materials and Methods

Comprehensive treatment plans for 50 H&N cancer patients who underwent parotid-sparing IMRT were analyzed; detailed analysis was performed with particular focus on parotid gland contouring.
The physician-defined parotid gland contours from original treatment plans were compared with contours that were retrospectively defined using specific neuroradiologist-defined criteria.
A comformality index (CI) was calculated , where perfect comformality =1, and values >1 denote decreasing overlap between contours from original treatment plans compared with contours retrospectively defined using specific neuroradiologist-defined criteria.
Mean dose was calculated for the total and superficial lobe of the parotid gland for physician-contoured and neuroradiologist-contoured parotid glands.

Results

Preliminary analysis demonstrates a mean comformality index of 1.26 when comparing H&N IMRT of original treatment plans with contours retrospectively defined using specific neuroradiologist-defined criteria.
The most common contour variations were a result of failure to appreciate an accessory parotid gland, inaccurate definition of the deep parotid lobe, or suboptimal CT window settings, particularly from images associated with significant dental artifact.
Mean parotid gland dose from IMRT plans varied significantly based on inclusion or exclusion of the deep lobe in dose calculations; these distinctions introduced a 2-4 Gy variation in the mean parotid gland lobe.

Author's Conclusions

Accurate delineation of parotid glands requires careful attention to anatomic detail and can significantly impact calculated mean dose to "spared" parotid glands in H&N planning.
Several simple educational tips can improve the integrity of parotid gland contouring by radiation oncologists.

Clinical/Scientific Implications

Detailed parotid gland contouring is seldom reported in the literature.
Contouring parotid structure may influence the mean dose to the superficial lobe and total parotid gland.
Attention to properly defining parotid gland contours represents a critical part of xerostomia prevention during IMRT H&N treatment, and can influence the accuracy and interpretation of the parotid doses reported in the IMRT literature.