![constrictor muscles constrictor muscles](https://i.imgur.com/l535DWU.png)
The rotational component is of great importance especially when the target has a non-spherical shape or the rotation is off axis, as a small rotation can cause deviation of the dose distribution. Target displacement can be decomposed into translations and rotations. Historically, a uniform margin around the clinical target volume (CTV) has been used to define the PTV. 2 This expanded safety volume is known as planning target volume (PTV). 1 In order not to miss the target, safety margins are applied which account for anatomic motion, delineation errors, and setup errors. These techniques offer dose distributions conformal to the tumor with superior sparing of the organs at risk (OARs). Intensity-modulated radiation therapy (IMRT) and rotational intensity-modulated techniques, including volumetric-modulated arc therapy (VMAT) have been used in patients with head and neck cancer. This new rotational PTV causes significant reduction of the overlap volume between PCMs and PTVs in order to spare the PCMs compared to isotropic expanded PTV. The smallest isotropic expansion that covers the new rotational PTV was between 3 and 5mm with the average tumor center shift of 0.49 cm. Average percent change for PTV volume and overlap with the superior, middle, and inferior PCMs are as followed: −19%, −37%, −59.4%, and −45.2. The new rotational PTV causes reduction in the superior PCM overlap in the base of tongue (BOT) lesions compared to tonsillar lesion, 57.8% vs 25.8%, P = 0.01, as well as middle PCM overlap, 73% vs 49%, P = 0.04. The overlap between the pharyngeal constrictor muscles (PCMs) and both PTVs was then evaluated. This created a new set of CTVs that were combined to form the new rotational PTV. In order to perform an off-axis rotation, a hypothetical point was placed through the center of the cervical spinal canal and the image was then rotated around the longitudinal axis ±5 degrees.
![constrictor muscles constrictor muscles](https://i.ytimg.com/vi/K-YtCIXr3Cc/maxresdefault.jpg)
We retrospectively evaluated 20 volumetric-modulated arc therapy (VMAT) plans. The aim of this study was to design a new margin formula that would cover the space occupied by an oropharyngeal clinical target volume (CTV) with ±5-degree rotation around the spine in order to reduce the pharyngeal constrictors overlap with PTV compared to an isotropic expanded PTV. In oropharyngeal cancers, an isotropic expanded PTV has been used. Planning target volume (PTV) has been used to account for variations in tissue, patient and beam position.