Dose distribution in the irradiation with adjacent fields. Medical application

In this paper we analyzed the absorbed dose distribution for the irradiation with two adjacent fields in the case of medulloblastoma treatment and we optimized the irradiation technique by using three adjacent fields. As a purpose of this study we considered the secondary effects of the irradiation with two adjacent fields at the sensitive organs such as heart and lung. The objectives of this paper are: to achieve dose uniformity in the field junction region; to avoid the underdosage or overdosage in the junction area; the hot spot created due to the overlap of the beams in the depth has to be clinically acceptable; the dosage received by a sensitive structure such as the spinal cord must not exceed its tolerance dose. We should specify that this treatment technique was developed considering the following risk factors: the possibility of introducing very large dosage errors across the junction; the tumor recurrence if it is underdosed; severe complications if it is overdosed. The analysis of the irradiation technique was made from two points of view, geometric and dosimetric. The analysis from geometric and dosimetric points of view entails the conclusion that at the cord and lung level exists the risk of an important overdosage. This inconvenience was solved by moving one of the fields at the other side of the risk organs (if this procedure is possible) and by optimizing the irradiation technique by using three adjacent fields. (authors)