Room: Davidson Ballroom A
Purpose: The unidirectional planar Pd-103 LDR source (Civa-sheet) is often used for intraoperative implantation at the interface between the tumor site and healthy tissue. Its dosimetric properties have been studied in the ideal case of application on a flat surface. Our goal is to quantify the dosimetric impact of implanting this highly directional source on curved surfaces that are often encountered in patients.
Methods: From the post-operative CT scans of three recent patients we have reconstructed the shape of each implanted Civa-sheet. In order to obtain realistic estimation of the distribution of curvatures, the mean radius of curvature at the location of each Civa-dot was calculated. A Monte Carlo simulation of a single Civa-dot was designed, based upon published geometry and material specifications. Knowledge of the dose distribution in the vicinity of each source allowed us to compute the dose at any point around Civa-sheets of different curvatures and orientations. Mean dose, as well as dosimetric inhomogeneity were studied and compared to the expected values from a flat implant.
Results: Local radius of curvature between 2cm and 8cm was found to be dominant in all three patient implants. On the unshielded side of an inward-facing curved implant of radius 2cm, the calculated dose at 0.5 cm depth exceeded the prescribed dose by ~25%, while on the shielded side the dose increased by a factor of two, thus compromising the shielding efficiency of the original design. When applied as an outward-facing implant, the dose on the unshielded side decreased by ~20%. On the shielded side, the curved geometry resulted in 25% improved tissue sparing.
Conclusion: Although accurate pre-planning based on surface curvature may be impractical, the dosimetric differences reported in this study will help avoid implantation geometries that lead to either underdosing the target, or excess dose to healthy tissue.