Podgorsak M B, DeWerd L A, Paliwal B R, Ho A K, Sibata C H
Department of Radiation Medicine, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
Med Dosim. 1995 Fall;20(3):177-81. doi: 10.1016/0958-3947(95)00023-p.
The accuracy of the point source approximation used in dose calculations for an implant comprised of multiple high dose rate (HDR) Ir-192 source dwell positions is investigated. First, a single dwell position implant is modeled. The exposure rate about the source is calculated using both the point source approximation and the more rigorous line source formalism. A comparison of these calculated exposure rates is made. It is found that for each HDR Ir-192 source dwell position, the point source approximation results in a dose overestimation of 1% at a distance of 1 cm on the source transverse axis, while dose underestimations of more than 2% can be found at a distance of 1 cm on the source longitudinal axis. Even larger errors occur closer to the source. The results of this academic study are then extended to two clinical cases--an endobronchial treatment and a tandem and ovoids setup, both involving multiple source dwell positions. Since clinical HDR Ir-192 implants are comprised of many individual source dwell positions, there will be inaccuracy in the calculated overall dose distribution leading to dose delivery errors. For example, the dose delivered to a prescription point located 0.5 cm from an endobronchial applicator will be 3% lower than prescribed. Similar errors are produced in gynecologic implants. To decrease below 0.5% the dose delivery error resulting from the point source approximation, prescription points should be at a distance of at least 1 cm from any applicator. Since the dosimetry error is a direct result of the choice of model used to describe the source, the use of anisotropy factors accounting for the variation of photon fluence around the HDR Ir-192 source will not completely correct the calculation.
研究了用于由多个高剂量率（HDR）铱 - 192源驻留位置组成的植入物剂量计算的点源近似的准确性。首先，对单个驻留位置的植入物进行建模。使用点源近似和更严格的线源形式主义来计算源周围的曝光率。对这些计算出的曝光率进行比较。结果发现，对于每个HDR铱 - 192源驻留位置，点源近似在源横轴上1 cm处导致剂量高估1%，而在源纵轴上1 cm处可发现剂量低估超过2%。在更靠近源的位置会出现更大的误差。然后将该学术研究的结果扩展到两个临床病例——支气管内治疗和串联及卵形装置，两者都涉及多个源驻留位置。由于临床HDR铱 - 192植入物由许多单独的源驻留位置组成，计算出的总体剂量分布会存在不准确，从而导致剂量传递误差。例如，传递到距离支气管内施源器0.5 cm处的处方点的剂量将比规定剂量低3%。在妇科植入物中也会产生类似的误差。为了将点源近似导致的剂量传递误差降低到0.
