Appendix A

Beam Commissioning Input

The beam commissioning procedure requires the user to input some information about the system:

• Machine specifications: vendor, source-axis-distance (SAD), number, size and position of leaves and whether there is a backup jaw or not.
• Depth dose curves: measured at the central axis for at least four field sizes. One of the fields should be the reference size, 10 x 10 cm², one should be smaller and the largest field size should be no smaller than 20 x 20 cm². A recommended set of field sizes could be 3 × 3 cm², 5 × 5 cm² , 10 × 10 cm², 15 × 15 cm², 20 × 20 cm², 30 × 30 cm², 40 × 40 cm² (1).
• Inline and crossline profiles for the same field sizes as the depth dose curves and at a few different depths, for example 1.5 cm, 5 cm, 10 cm and 20 cm.
• Absolute calibration (dose per monitor units) for the 10 × 10 cm² fields at the same source surface distance (SSD) as the dose curves. The depth of the calibration point is a required input to the program; it should preferably be situated below the region where the electron contamination is important, i.e. at 10 cm.
• Output factors for all measured field sizes with the same SSD and depths as the calibration point.
• The approximate shape of the ionization detector convolution kernel for the detector that was used to measure the dose curves: stated as height, width and off-set. The height mainly influences the dose curve in the build-up region, the width mainly affects the profile penumbra and the shift should only be applied if you know that the curves are shifted with respect to their proper dose measurement positions. It is recommended using a detector height of at least 1 mm to achieve proper electron contamination smear.
• The collimator settings for the square fields during depth dose curve and profile measurements: jaw, MLC or both.

Possible input from machine specifications that could be adjusted further manually:

• Transmission of the MLC and the jaws.
• Flattening filter position.

Possible extra measurements that could be important for more detailed MLC calibration:

• Any DICOM TPS plan, for example a square or a strip field, could be used as input to get a COMPASS computed dose which could then be compared with any reference dose – film, ionization chamber, diodes or other measurement devices. The COMPASS beam model parameters can then be tuned until a good fit is obtained.

(1) IBA Dosimetry binary (.rbf) files, ASCII (.asc) files or .csf files.