Further Tweaks to Peak Skin Dose Calculation
Abstract
Purpose
This study investigates correction factors commonly used in calculating peak skin dose (PSD) from IR procedures. Specifically, it examines 1- effect of patient-induced compression of the table/pad on transmission factors and, 2- the contribution of scatter from beams outside the skin patch of interest as a function of C-arm angulation.
Methods
For compression effects, transmission factors of table and pad were first measured with no object on table. A 32-cm CTDI phantom was then positioned just above an ionization chamber and supported by Styrofoam blocks. Measurements were performed at various kV/filter combinations and beam angulations. The phantom was then placed directly on the table pad and measurements repeated. Additional measurements were acquired with added weight to simulate very large patients. Lateral images were used to assess compression and adjust table height to maintain the chamber at isocenter. To evaluate off-center and out-of-field dose contributions, an anthropomorphic chest phantom was used with the chamber positioned beneath it. Fluoroscopy and acquisition runs were performed at angles ranging from -60° to +60° and using multiple fields of view (FOVs). For each run, the ratio of chamber-measured air-kerma to system-displayed air kerma (C/D) was calculated. Additionally, chamber location was recorded for each view angle.
Results
Compression of the table and pad increased transmission factors (including forward-scatter) by 8% to 15%, depending on beam quality and angle of incidence. For off-center and out-of-field contributions, the normalized C/D ratio decreased to approximately 0.7 at the field edge and ranged from 0.1 to 0.4 outside the field, depending on FOV and angulation. Manual PSD calculations for several patients demonstrated that neglecting these effects could result in up to ±20% inaccuracy.
Conclusion
These findings indicate that updates to current correction factors used in PSD calculations are warranted to improve accuracy, particularly for large patients and complex geometries.