STUDY ON THE REFERENCE DOSE LEVEL IN RADIOTHERAPY TREATMENT PLANNING

Purpose: The reference dose level of the dose distribution in the tumo r volume is studied. Methods and Materials: The study is performed usi ng a formula based on the Linear Quadratic (LQ) model. The calculated reference dose level to which the prescribed dose must be referred, fo r the eradication of a homogeneous tumor, is investigated by varying t he dose distribution, that is, the dose volume histogram shape, its ra nge, the prescribed total dose, the fraction size and the linear quadr atic model parameters, or and P. Results: For all the simulated dose v olume histograms the calculated reference dose level is lower than the mean dose level, depending on the range of dose variation and the con sidered tumor sensitivity. When the dose nonuniformity is not too grea t the reference dose level is very near to the mean dose level; when t he inhomogeneity of dose distribution is high the reference level is c learly lower than the mean level but not necessarily equal to the mini mum level in the tumor. For the dose volume histograms derived from th e actual dose distributions obtained from a two tangential beams techn ique, a four beams technique and a moving beam technique, the referenc e levels are calculated and compared with the ICRU 29 reference point dose level. In two cases the reference levels are lower than the level at the ICRU 29 reference point. In the case of the four beams techniq ue, the two levels are equal. Conclusion: These theoretical results sh ow the possibility of administering the prescribed dose to a dose leve l higher than the minimum in the tumor, with the same value of Tumor C ontrol Probability (TCP) as the one corresponding to a uniform tumor i rradiation. The application of the proposed study can offer a general support to the choice of the reference dose level, based on the actual dose distribution in the tumor volume.