Measurement of LINAC 90° head leakage radiation TVL values

Zuofeng Li, Sasa Mutic, Daniel Low

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


One of the key components in modern LINAC room shielding design is the amount of 90° head leakage radiation levels. With the general clinical acceptance of intensity-modulated radiation therapy (IMRT) technique, accurate knowledge of this quantity has become even more important. Measurement of 90° head leakage radiation of medical linear accelerators can be technically challenging due to the low dose rate causing poor signal-to-noise ratios in most detectors. 90° leakage tenth-value layer (TVL) values in concrete have not been reported for the Elekta linear accelerators. This report describes our measurements of 90° leakage TVL values for 6, 10, and 18 MV x-ray beams for an Elekta Precise Treatment System. A large-volume (1000 cm3) unpressurized ionization chamber and a high sensitivity electrometer, together with a separate chamber bias power supply, were used in these measurements in order to maximize the signal-to-noise ratio. A lead enclosure, of minimum thickness 10 cm, was constructed inside the treatment room to house the ion chamber to reduce the influence of room-scattered radiation. A square aperture of 10 x 10 cm2 area was left in the shield and aimed towards the accelerator head. Measurements were performed with the chamber placed at approximately 2 m from the accelerator isocenter. Concrete slabs with individual dimensions of approximately 40 x 40 cm2 cross-sectional area and 5 cm thickness were placed between the accelerator head and the ion chamber for these measurements. The measurements were performed with total concrete thickness of up to 80 cm, so that values up to the third TVL were measured. These measurements showed that the first concrete TVL values are 22, 23, and 28 cm (8.6, 9.1, and 10.5 in.) for 6, 10, and 18 MV beams, while the average of the first 3 TVL's were 25, 26, and 29 cm (9.9, 10.2, and 11.5 in.). Measured values agreed to within 10% of previously reported values for Varian linear accelerators for equivalent radiation beam qualities.

Original languageEnglish
Pages (from-to)3541-3545
Number of pages5
JournalMedical physics
Issue number9
StatePublished - 2006


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