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

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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.


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