Simulation And Delineating The Radiation Target Target Volumes

Compared to the level of precision and accuracy that can be achieved in radiation delivery, target volume delineation remains one of the most important areas of uncertainty and error. The physician must first outline on the planning CT scan a gross tumor volume (GTV), corresponding to any gross tumor that can be felt on physical examination, or visualized with imaging. Following this, a margin of variable size must be added to this volume to define the clinical target volume (CTV). CTV should include those tissues suspected to contain microscopic tumor cells that are not visualized with current imaging techniques. It is estimated depending on histological subtype and anatomical location. Advances in imaging microscopic disease in situ would significantly improve our ability to define the CTV. Finally, a margin that accounts for setup uncertainty and organ motion must be added to the CTV to generate a planning target volume (PTV) (43). By convention, radiation treatment portals are designed with the goal to entirely cover the PTV, deliver a uniform dose distribution to it, and spare the normal tissues that lie in close proximity to the PTV.

Definitions for target volumes will continue to evolve as imaging and delivery techniques improve. The "rule" for dose uniformity within the PTV has been largely a matter of tradition and convention. More recently, there have been suggestions that nonuniformity within the PTV, specifically regions of increased dose, may actually increase the local control. The ability of IMRT to deliver nonuniform dose patterns by design, introduces the concept of "dose sculpting" (44). New terms, such as biological target volume (BTV), have been introduced to describe biological properties of tumors. The BTV could be imaged with novel techniques, reflecting the radiosensitivity of a given tumor subvolume (44).

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