Dependence of tolerance dose on irradiated volume in radiotherapy, and determination of volume corrected time, dose and fractionation factors.

Clinical observations demonstrate that the tolerance dose depends on the volume of normal tissues irradiated. It has been found to decrease if volume size irradiated increases. A relationship between tolerance dose and volume irradiated has been derived. The time, dose and fractionation (TDF) factors given by Orton and Ellis (1973) do not include the effect of volume size of irradiation. That relationship has been used to derive equations of partial tolerance (PT) and TDF factor incorporating the treatment volume size. The incorporation of this factor becomes more important when treating deep-seated tumours and when large volumes of normal tissue are involved. The value of the TDF factor changes by 30% when volume irradiated increases from 500 to 1400 cc for 200 cGy per fraction in three fractions per week schedule.