SU‐GG‐T‐449: Peripheral Dose Investigation of a Corpus Collosotomy by Gamma Knife, Model C

Purpose: Corpus collosotomy by Gamma Knife Radiotherapy is a treatment for medically refractory epilepsy. Due to the long treatment duration (3–6 hours), the peripheral dose received by patients becomes a concern. In this study, peripheral doses for a posterior corpus collosotomy by Gamma Knife, Model C were investigated. Method and Materials: A Rando‐Phantom was used to measure the peripheral doses. Three types of dosimetry systems were used, which included ionization chambers, TLDs (TLD‐100 3×3×1mm chips), and OneDose™ MOSFET dosimeters. The dosimeters were placed on the surface of the Rando‐Phantom under 0.5 cm of build up at distances of 5.0, 17.5, 30.0, 42.5, 54.0, 70.0 and 84cm from the center of the treatment volume. Seven cylindrical ionization chambers with buildup caps were placed at the same distances, one chamber at each position. The leakage of the ionization chambers was measured before irradiation and recorded during treatment sessions. The phantom was irradiated using a clinical treatment plan for a posterior corpus collosotomy. The prescription dose was 130Gy (max dose), and the delivery dose was reduced to one third of the prescription dose for measurement purposes. The treatment time was 99.38 minutes. Results: The measured doses by TLD were 17.9, 9.9, 5.8, 2.9, 2.4, 1.5, and 1.1cGy for the seven positions respectively. The measured doses by ionization chambers were 16.9, 12.0, 5.2, 2.9, 2.8, 2.2, and 2.0cGy. The results obtained with the OneDose™ were not consistent due to the decay of the signal over the period of measurement. The doses measured by TLD and ionization chamber for the same position were averaged. The total doses to the measurement positions were 52.2, 33.0, 16.5, 8.7, 7.8, 5.7, and 4.8cGy, respectively, for the full prescription dose of 130Gy. Conclusion: The peripheral doses were significant, especially in the head and neck region.