Applications of two-step intensity modulated arc therapy

Purpose: Organs at risk sometimes are surrounded by the target volume. At a first glance it seems to be impossible, to spare the organs at risk, i.e. the spinal cord, without underdosage of parts of the concave target volume. A fast method called "two-step intensity modulated arc therapy" (two-step IMAT) will be shown that avoids underdosage in the target volume near such organs at risk. Materials and Methods: Simple rotational techniques reduce the dose to the surrounded organ, however, the blocking of the organ at risk spoils the homogeneity of dose in the target volume in the vicinity of the organ. A further narrow rotation field, tangential to the concave part of the target volume, increases the dose there and homogenizes the dose distribution without deteriorating the dose in the organ at risk significantly. Results: Some cases show that the two-step IMAT and its modifications guarantee a sufficient dose homogeneity in the target volume surrounding an organ at risk. Besides basic theoretical considerations exemplary solutions for head and neck tumors, tumors adjacent to the spinal cord and mamma carcinoma of patients with extremely arched chest are demonstrated. Conclusion: Double rotation techniques can provide sufficient dose homogeneity for concave target volumes with excellent sparing of the surrounded organ at risk. They are not time consuming and can he used until IMRT will be applied routinely with adequate time Load and effort.