Purpose: AAPM Task Group 43 recently revised the dosimetry recommendations for 125I seeds. We reviewed these guidelines and studied the effects of the recommendations on the prescription absorbed dose for patients who have undergone eye plaque therapy in our clinic.
Methods and materials: 95 consecutive patients were chosen for this study. Absorbed doses at various points of clinical interest were computed based on conventional dose calculation algorithm (3, 4, 7) and TG-43 recommendations. For three representative plaques chosen, the seeds are approximated by isotropic point and line sources, respectively, and absorbed doses were calculated at all points on the central axis of the plaque.
Results: For apical heights shorter than 5 mm, treatment plans using model 6711 seeds delivered 10-13% lower absorbed doses than that calculated previously. For lesions with apical heights 5 mm or larger, the absorbed dose was 6-12% lower than prescribed. Calculations for model 6702 seeds indicated that TG-43 recommendations would produce 0-6% lower absorbed doses. Point doses calculated along the central axis of the plaque and isodose distributions at various levels showed that point source approximation of the seeds was clinically acceptable.
Conclusions: TG-43 recommendations, if implemented, would result in lower absorbed doses unless the dose prescription is modified. The clinicians need to be aware of the dosimetric implications of these recommendations. The seeds may be approximated by isotropic point sources.