Comprehensive assessment of the effect of eye plaque tilt on tumor dosimetry

Abstract

Purpose: Tilting of the posterior plaque margin during eye plaque brachytherapy can lead to tumor underdosing and increased risk of local recurrence. We performed a quantitative analysis of the dosimetric effects of plaque tilt as a function of tumor position, basal dimension, height and plaque type using 3D treatment planning software.

Materials and methods: Posterior and anterior tumors with largest basal dimensions of 6, 12 and 18 mm and heights of 4, 7 and 10 mm were modeled. Both Eye Physics and COMS plaques were simulated and uniformly loaded. Plans were normalized to 85 Gy at the tumor apex. Posterior plaque tilts of 1, 2, 3 and 4 mm were simulated.

Results: Volumetric coverage is more sensitive to tilt than the area coverage. Wide, flat tumors are more susceptible to tilt. Apical dose changed significantly as a function of tumor height and diameter. No other parameter exhibited significant differences. Posterior tumors are slightly more susceptible to tilt due to the use of notched plaques. Plaque type does not significantly alter the effect of plaque tilt.

Conclusions: Wide, flat tumors are the most susceptible to plaque tilt. Tumor location or plaque type does not have a significant effect on dosimetry changes from plaque tilt. Robust clinical procedures such as the use of mattress sutures, pre- and post-implant ultrasound and post-implant dosimetry can all mitigate the risk associated with plaque tilt.

Keywords: Brachytherapy; Dosimetry; Eye plaque; Tilt.