title: The dilemma of parotid gland and pharyngeal constrictor muscles preservation-Is daily online image guidance required? A dosimetric analysis.
abstract: With margin reduction common in head and neck radiotherapy, it is critical that the dosimetric effects of setup deviations are quantified. With past studies focusing on the quantification of positional and volumetric changes of organs at risk (OARs), this study aimed to measure the dose delivered to these the parotid gland (PG) and pharyngeal constrictor muscles (PCMs) using cone beam computed tomography (CBCT). Furthermore, this investigation sought to establish a potential time trend of change in dose delivered to target volumes secondary to ascertaining the need for daily image guidance (IG) to reduce the dose burden to these important OARs. Intensity modulated radiotherapy (IMRT) plans for 5 locally advanced head and neck patients׳ plans were created and mapped to weekly CBCTs. Each plan was recalculated without heterogeneity correction allowing for dosimetric comparison. Dosimetric endpoints recorded to assess the effect of positional variation were as per ICRU 83 and included D95and D98for the target volumes, mean dose (MD) and V30Gyfor the PGs, and V50Gyand MD for the PCMs. Results were deemed statistically significant if p < 0.05. No significant time trends were established for these OARs. A significant decrease in V50Gywas observed for all PCMs (p < 0.001) on all CBCTs relative to the original plan. Regarding target volumes, a highly significant decrease in MD (MD = 20Gy, CI: -20.310 to -19.820) in D98of the high-dose planning target volume (PTV [70Gy]; PTVD98%= 70Gy) for case 3 was found (p ≤ 0.001). A nonpredictable, yet significant dosimetric effect was found. A clinically acceptable balance must be achieved between OAR dosimetry and target coverage as can be achieved by frequent IG.
authors.affiliation: Applied Radiation Therapy Trinity (ARTT), Discipline of Radiation Therapy, Trinity College, Dublin, Ireland.
authors.forename: Olivia
authors.initials: O
authors.lastname: Duffy
authors.affiliation: Applied Radiation Therapy Trinity (ARTT), Discipline of Radiation Therapy, Trinity College, Dublin, Ireland.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Forde
authors.affiliation: Applied Radiation Therapy Trinity (ARTT), Discipline of Radiation Therapy, Trinity College, Dublin, Ireland. Electronic address: leechm@tcd.ie.
authors.forename: Michelle
authors.initials: M
authors.lastname: Leech
mesh_terms: D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006801:Humans; D059351:Organ Sparing Treatments; D058958:Organs at Risk / Q000000981:diagnostic imaging*; D010306:Parotid Gland / Q000000981:diagnostic imaging*; D010609:Pharyngeal Muscles / Q000000981:diagnostic imaging*; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control*; D011880:Radiotherapy Planning, Computer-Assisted
keywords: Daily image guidance; Head and neck cancer; Parotid glands; Pharyngeal constrictor muscles