The purpose of this study was to evaluate the effect of a lead block for alveolar
bone protection in image-guided high-dose-rate interstitial brachytherapy for tongue
cancer.We treated 6 patients and delivered 5,400 cGy in 9 fractions using a lead block.
Effects of lead block (median thickness, 4 mm) on dose attenuation by distance were
visually examined using TG-43 formalism-based dose distribution curves to determine
whether or not the area with the highest dose is located in the alveolar bone, where
there is a high-risk of infection. Dose re-calculations were performed using TG-186
formalism with advanced collapsed cone engine (ACE) for inhomogeneity correction set
to cortical bone density for the whole mandible and alveolar bone, water density for
clinical target volume (CTV), air density for outside body and lead density, and silastic
density for lead block and its' silicon replica, respectively.The highest dose was
detected outside the alveolar bone in five of the six cases. For dose-volume histogram
analysis, median minimum doses delivered per fraction to the 0.1 cm3 of alveolar bone
(D0.1cm3TG-43, ACE-silicon, and ACE-lead) were 344.3 (range, 262.9-427.4) cGy, 336.6
(253.3-425.0) cGy, and 169.7 (114.9-233.3) cGy, respectively. D0.1cm3ACE-lead was
significantly lower than other parameters. No significant difference was observed
between CTV-related parameters.The results suggested that using a lead block for alveolar
bone protection with a thickness of about 4 mm, can shift the highest dose area to
non-alveolar regions. In addition, it reduced D0.1cm3 of alveolar bone to about half,
without affecting tumor dose.
