Figure 1.

Creation of PTV_opt structure. This figure illustrates the steps necessary to creating the PTV_opt structure that dose is prescribed to. The PTV is generated using a 5-mm volumetric expansion of the GTV (Step 2). Gradient margins are created in Step 3, which are volumetric expansions of critical OARs. Finally, in Steps 4 and 5, the PTV_opt structure is created, which is the PTV minus the gradient margins. The PTV_opt structure is commonly used to prescribe dose to in standard adaptive radiation therapy practice and allows for de-escalation of dose at the interface of targets and critical OARs. Abbreviations: GTV = gross tumor volume; OAR = organ at risk; PTV = planning target volume.