|IMRT Planning for Head and Neck Cancers:
Sparing of the Larynx in Helical Tomotherapy Treatment Plans
R.J. Staton, S.L. Meeks. Volume 69, Issue 3, Page S455 (1 November 2007)
There has been concern posed in the literature about the possibility of overdose to the larynx when the entire neck is included in the IMRT field during H&N radiation planning. A potential solution is to match the IMRT field border to a single AP field and feather the field border. However, this technique is subject to variations in dose near the match line. Helical tomotherapy provides improved target homogeneity and OAR avoidance for head and neck cancers compared to linac-based IMRT. The purpose of this study is to evaluate the ability of helical tomotherapy to provide adequate sparing of the larynx when the entire neck is included in the IMRT field.
The study cohort consisted of five patients being treated with helical tomotherapy for oropharyngeal cancer (tonsil or base of tongue). GTVs and CTVs were contoured by the attending radiation oncologist and then expanded to PTVs: 0.30.4 cm margin for CTV volumes above the larynx and 0.50.6 cm below the larynx. The treatment plans were designed with the following PTV objectives: 70 Gy (35 fractions) to the primary, 6668 Gy to gross nodes, 60 Gy to the high risk neck, and 5457 Gy to low risk neck. All five patients were planned using the TomoTherapy clinical treatment planning system (TomoTherapy Inc., Madison, WI) with a jaw setting of 2.56 cm, pitch of 0.287, and initial modulation factor of 2.5. The optimization parameters were set to deliver the prescription dose to 95% of the PTVs.
All five treatment plans were able to achieve homogenous PTV coverage while sparing the larynx and other critical structures (see Fig. 1). The treatment plans did not demonstrate underdosing to the nodal PTVs in attempting to spare the larynx. The mean larynx dose of the cohort of patients was 22.6 Gy (±3.0 Gy). The mean dose to the parotid adjacent to the low risk neck was 24.4 Gy (±2.7 Gy).
Helical tomotherapy treatment plans for oropharyngeal cancers achieved mean larynx doses of 22.6 Gy while obtaining acceptable PTV coverage while sparing normal critical structures. Reports in the literature suggest that the lower limit of mean larynx dose achieved by linac-based whole-field head and neck IMRT is 3550 Gy. Recent data suggest that a mean dose to the larynx above 43.5 Gy (at 2 Gy per fraction) may increase the incidence of Grade 2+ laryngeal edema. The mean larynx doses achieved using helical tomotherapy are comparable to those reported for split-field linac-based treatment plans1,3, i.e. approximately 1719 Gy.