Anatomy-based definition of posterior neck in head and neck VMAT plan optimization as potential new organ at risk

BACKGROUND/OBJECTIVES: The advantages of intensity modulated radiotherapy (IMRT) in target coverage and organ at risk (OAR) sparing come with a low- and intermediate-dose bath effect. To control this unwanted dose dumping, planners use helping structures in the plan optimization. We hypothesized that using an anatomic definition of the posterior neck would result in reduced doses to the posterior neck muscles without compromising target coverage or increasing doses to organs at risk. METHODS: We randomly selected twelve head and neck cancer patients treated with volume modulated arc therapy (VMAT). For each case, we generated 3 plans with different optimization methods: first, with a dummy volume behind the spinal vertebrae; second, a volume with the posterior neck and upper shoulder muscles according to anatomic criteria; and third, a plan without any helping structure. RESULTS: There was no statistically significant difference between the 3 plans in terms of coverage, conformity index or homogeneity index. All the OAR dose constraints were respected, and there was no significant increase of the dose to these organs when the anatomic definition for the posterior neck was used. However, the posterior neck muscles and upper shoulder muscles received significantly lower mean doses, 31.9 Gy vs 36.8 Gy vs 37.6 Gy, p = 0.0004. CONCLUSIONS: Using an anatomically defined posterior neck avoidance volume in the plan optimization significantly reduces doses to the posterior neck and upper shoulder muscles, potentially with less severe neck fibrosis, without compromising target volume coverage or increasing doses to other organs at risk.