Generation of iso-TDF maps: considerations for radiation therapy planning

The NSD (nominal standard dose) concept has been useful in therapy for comparing various radiation treatment schedules. More recently, TDF (time, dose, and fractionation) factors were introduced. By incorporating treatment schedules (time and fractionation) with isodose summations, TDFs can be determined throughout the patient contour, including tissue in and outside the tumor volume. For this purpose an isodose summation program was modified to generate iso-TDF maps based on the treatment schedule. For schedules in which all fields are not treated every day, recovery is not inhibited throughout the total treatment volume. It was necessary to reevaluate the concept of "recovery time" for tissues outside the tumor volume and to introduce a "threshold to inhibit recovery" model. Clinical examples assuming this recovery model are presented.