Results of the TrueBeam External Dosimetry Audit, Dec 2012

Before going clinical, we checked the dosimetry of all five photon and six electron beams by performing an external dosimetry audit (MTK, PTW-Freiburg).

Two TLDs were irradiated per photon energy (6X, 6FFF, 10X, 10FFF, 15X), one per electron energy (6E, 9E, 12E, 15E, 18E, 22E). In total, 16 TLDs were irradiated.

Dose to water (Dw) was measured in a small water phantom for horizontal beams (PTW T41023), according to the German DIN 6800-2 protocol. All available chamber/dosemeter combinations were used.

Due to the high dose-per-pulse values of unflattened beams, we used Farmer chambers only for FFF. For this chamber type, there are published values for gamma and delta, if the saturation correction method of Bruggmoser is used.

Ten readings were taken per energy, and averaged. Immediately after the dose measurement for one energy, the respective TLD was irradiated. Then the next energy was chosen.

Here is the calculation sheet:

(click to enlarge)

Results

In the last three rows of the calculation sheet, the dose values stated by us are compared to the true dose values determined by PTW. All dose deviations are within ±1.5%. The largest positive deviation is +1.2% (6E), the largest negative deviation -1.3% (6FFF).

Photon results and certificates (2 TLDs per energy):

• 06X: -0.2% and -0.1%
• 10X: +0.6% and +0.3%
• 15X: -0.2% and +0.3%
• 6FFF: -1.3% and -1.1%
• 10FFF: +0.2% and -0.1%

Electron results¹ and certificates (1 TLD per energy):

• 06E: +1.2%
• 09E: +1.1%
• 12E: +0.8%
• 15E: +0.5%
• 18E: +0.3%
• 22E: -0.2%

¹Note: a small error has been corrected which affects the saturation correction of Electron beams: the pulse frequency of DR 300 MU/min is 60 Hz, not 54 Hz.