Why are diagnostic CTs not ideal for treatment planning?

Diagnostic CTs are not ideal for treatment planning primarily because they do not provide electron density data. This type of imaging is generally optimized for high-resolution anatomical detail rather than the specific tissue characterization needed for radiation treatment planning. In treatment planning, it's crucial to know the electron density of different tissues because it affects how radiation interacts with those tissues. Accurate electron density information is essential for calculating radiation dose distributions and ensuring that the treatment effectively targets the tumor while sparing surrounding healthy tissue.

In contrast, while the other options may seem relevant, they do not capture the essential reason for the limitation of diagnostic CT usage in treatment planning. The resolution is typically sufficient for most anatomical delineation tasks, the aperture size is not generally an issue in the context of diagnostic imaging, and imaging time is typically acceptable for diagnostic procedures. Thus, the lack of electron density data is the most significant limitation in the context of preparing for effective radiation therapy.