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Minimizing the Heel Effect in Orthovoltage Calibrations

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K McCollough

K McCollough*, C Blackwell , M Grams , Mayo Clinic, Rochester, MN


SU-I-GPD-T-508 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: With an increase in Skin cancers the popularity of orthovoltage machines has been on the increase in Radiation oncology and Dermatology departments. Most Physicists in Radiation Oncology are familiar with well documented procedures for linear accelerator steering to achieve optimum symmetry and flatness of the beam while keeping the percent depth dose matched to published data. X-ray tubes do not have this ability. Knowledge of the beam profile is vital in getting a proper calibration of the unit.

Methods: Gafchormic film was used to measure the x-ray beam profile. Film was placed at 1cm depth and exposed to 200cGY. Circular applicators ranging in size from 3cm to 10cm were evaluated. Film was analyzed using a flatbed scanner. Profiles were acquired in both the In-plane (Anode – Cathode) and Cross-plane direction. In phantom Ionization chamber measurements were made with the long axis of the chamber oriented in In-plane and Cross-plane directions.

Results: The heel affect is more prominent at lower energies 60kVp vs 250kVp and has a much larger impact on dosimetry for larger field sizes 10cm vs 3cm. Positional mis-placement (not centered) of a calibration chamber is more significant in small cones and low energies. Film profiles suggest ±2% variation across the central 2 cm of the treatment field, which could translate to a ±2% variation in absolute calibration.

Conclusion: As with any calibration procedure it is important to have an accurate and robust method. Orthovoltage units without projected crosshairs or light fields can make accurate central axis chamber alignment challenging. Chamber alignment errors in the in-plane (Anode – Cathode) axis is less critical than cross-plane due to large volume. Positioning of the long axis of your chamber perpendicular to the Anode-Cathode plane will result in larger variation in absolute calibration due to the asymmetry of the beam profile.

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