The contribution of interstitial implants to treatment planning.

Delivery of radiation to a deeply located tumor site by external radiation is only accomplished by traversal of normal tissues. Even with the physical advantages of high energy radiation or heavy particles, this is still unwanted radiation. Even a tumor-seeking isotope, such as radio-iodine for carcinoma of the thyroid, must still have an initial total body distribution. While interstitial sources, removable or permanent, present a personnel exposure problem, the delivery and deposition of ionizing radiation in this way has unequalled simplicity, versatility, and availability. Although the method is as old as radiotherapy itself, three dimensional computer dosimetry has given it extended applicability, and extends an invitation to innovations in achieving ideal dose distribution.