A new method for ideal dose distribution of two adjacent fields for external beam radiation therapy

Background: Adjacent treatment fields are commonly used in external beam radiation therapy, such as mantle and inverted-Y fields for the treatment of Hodgkin’s disease and craniospinal fields used in the treatment of medulloblastoma and head and neck tumors when the lateral neck fields are placed adjacent to the anterior supraclavicular field. In each of these situations, there is a possibility of introducing very large dosage errors across the junction. Consequently, this region is at risk for severe complications if it is overdosed, or tumor recurrence if it is underdosed. Methods: For prevention of adjacent field overlapping a new method is introduced. In this method the patient’s couch of the treatment machine is rotated for 90o clockwise and counterclockwise. Then the gantry is rotated for α and β that are measured by geometrical methods in opposite direction for each field. The adjacent fields have a common edge and then the overlap region in treatment volume is eliminated. Results: By phantom dosimetry, the maximum dose in the junctional volume of the two adjacent treatment fields is measured to be 102%. This technique provides an inhomogeneity of about 2%. Conclusion:In some cases, the measurements have shown that the dose inhomogeneity is as large as 45%. Compared with the dynamic intensity-modulated radiotherapy (IMRT), this technique also provides a superior dose homogeneity such that inhomogeneity becomes about 2%.