A Practical Way to Assess the Effectiveness of the Existing Vault Shielding for IMRT
J.B. Wojcicka1; J.A. Gresick-Schugsta2; and J.R. Nace2 (1York Cancer Center, WellSpan Health; 2York Hospital, WellSpan Health)
Shielding in the existing vault of a radiation therapy accelerator was re-evaluated due to the recent clinical implementation of the new treatment technique called Intensity Modulated Radiation Therapy (IMRT). It is estimated that the required number of MU needed for IMRT treatment increases by a factor of 2 to 5 over conventional techniques. This increase in MU per fraction has a direct impact on the shielding of the vaults used to deliver IMRT. Two methods were used. First, is based on direct radiation measurements, a Modulation Scaling Factor (MSFmod) comparing the average monitor units for IMRT to conventional radiotherapy (3DCRT) and a gantry use factor. In the second method, area and personnel film badges were analyzed for a two year period, because of a change from one year to the next in the clinical protocol offered to prostate patients. Specifically, in the first year, prostate patients were treated with conventional 3DCRT and 15 MV photons, whereas in the second year, the prostate patient population was offered IMRT, which uses 6 MV photons. Radiation surveys identified 2 previously unidentified areas of elevated radiation levels, but not in excess of the regulatory limits for radiation exposure. Radiation exposure results of the area and personnel monitoring from the first year to the second decreased, due to reduced production of neutrons from the decrease in use of 15 MV photons for patient treatments. The radiation survey demonstrated that the existing shielding was effective for the increased IMRT workload. Standard personnel and area dosimeters, available from commercial dosimetry vendors, can also be used effectively to demonstrate and document continued compliance with evolving treatment modalities.