In clinical photon beam therapy planning, the PTV must be normally enclosed by what?

In clinical photon beam therapy planning, the Planning Target Volume (PTV) is typically required to be enclosed by the 95% isodose line. This guideline is crucial as it ensures that a sufficient dose is delivered to the tumor while minimizing exposure to surrounding healthy tissues. The 95% isodose line indicates that 95% of the prescribed dose is delivered adequately across the PTV, which is essential for effective treatment and improving patient outcomes.

Enclosing the PTV with the 95% isodose line helps to provide a balance between coverage of the tumor and limiting the dose to critical structures, thus reducing the risk of adverse effects associated with radiation therapy. This standard reflects best practices in dosimetry, aligning with radiobiological principles and clinical evidence supporting favorable response rates for tumor control.

The other percentage options, like 90%, 98%, and 85%, do not meet the established guidelines in most clinical settings. While they may offer various degrees of coverage, they do not ensure the same level of confidence and safety in treatment delivery as adhering to the 95% isodose constraint. Hence, using the 95% isodose line represents a benchmark for optimal treatment planning in photon beam therapy.