On the determination of the effective transmission factor for stainless steel ovoid shielding segments and estimation of their shielding efficacy for the clinical situation.

Commercially available ovoid tubes for gynecological applications used in conjunction with the microSelectron-HDR (Nucletron International B.V., Waardegelder 1, 3905 TH Veenendaal, The Netherlands) for 192Ir sources, allow for shielding. Publications concerning the transmission properties of these 4.5-mm thick stainless steel (AISI number 303/304) shielding segments are scarce and not compatible for implementation in treatment planning. Therefore the effect of shielding on dose distribution is unknown. The effective transmission factor has been measured and implemented in the planning computations. Screening efficacy was evaluated on 20 actual treatment plans, analyzing dose reduction to critical tissue and comparing dose distribution in planes relevant for this particular application. Due to high transmission (effective transmission factor = 0.85), stainless steel screening segments only provide low, local dose reductions of maximum 15%. A new approach with regard to optimization and source configuration is needed to reduce dose to vulnerable tissue, exploiting the screening segments to a maximum extent. Better shielding, especially at the midline (plane bisecting the ovoids) could be expected by using shielding segments with other geometrical characteristics.