What must the anatomical axis be when treating an extremity?

The anatomical axis must be perpendicular to the beam when treating an extremity because this orientation optimizes the delivery of radiation to the targeted tissue while minimizing exposure to surrounding healthy tissues. Positioning the anatomical axis perpendicularly allows for the most effective concentration of the radiation dose at the tumor site.

When the beam is perpendicular, the radiation is directed straight through the area of interest, ensuring that the dose distribution is accurately aligned with the treatment target, thus enhancing therapeutic effect. This alignment is crucial in targeting the malignancy and sparing critical structures, particularly in complex anatomy associated with extremities, where improper angling could lead to underdosing the tumor or overdosing adjacent normal tissue.

Other orientations, such as parallel or oblique, do not maximize the therapeutic dose at the tumor while simultaneously minimizing collateral damage to surrounding healthy tissues, making them less effective for treatment planning in this context. A 45-degree angle, in particular, introduces complexities that complicate dose distribution and may not effectively target the specific area of interest, leading to suboptimal treatment outcomes.