How is the surface area of an interstitial implant defined in the Manchester system?

In the Manchester system, the surface area of an interstitial implant is defined by the active lengths of the peripheral needles. This approach is essential because the peripheral needles are responsible for delivering the majority of the radiation dose to the tumor and surrounding tissues. The calculation takes into account the lengths of the active segments of these needles, which emit radiation and contribute to the treatment effectiveness.

Understanding the significance of the active lengths of the peripheral needles is crucial, as it directly impacts the dose distribution within the targeted area. This aspect of the Manchester system allows dosimetrists to accurately calculate the surface area and thus plan treatment effectively, ensuring that the tumor receives an adequate dose while minimizing exposure to surrounding healthy tissue.

Other considerations, such as the total number of needles, the length of the central needle, and the spacing between needles, do not directly determine the surface area as defined in this system. These factors may play roles in overall dosimetry and treatment planning but are not the primary basis for defining the surface area of the implant.