Measurements and comparisons for data of small beams of linear accelerators

State Key Laboratory of Oncology in South China, Guangzhou, Guangdong, P. R. China.
Ai zheng = Aizheng = Chinese journal of cancer (Impact Factor: 2.16). 04/2009; 28(3):328-32.
Source: PubMed


Accurate data acquisition is very important to establish a reliable dose calculation model of the treatment planning system for small radiation fields in intensity modulated radiation therapy (IMRT) and stereotactic radiotherapy (SRT). This study was to analyze and compare small-field measurements using different methods and ionization chambers.
Three types of farmer chambers were used, with active volumes of 0.65 cc, and 0.13 cc, 0.01 cc, respectively. The beam data, including the total scatter factor (Scp), collimator scatter factor (Sc), tissue-maximum ratio (TMR), were acquired in a 30 cm x 30 cm x 30 cm3 water phantom under two linear accelerators. Measurements were performed at accelerating potentials of 4, 6, and 8 MV with the beam size ranging from 1 cm x 1 cm to 10 cm x 10 cm. The measurements were analyzed and compared.
For the beam size of >or=3 cm x 3 cm, the differences in Scp and Sc measurements of the 0.65 cc, 0.13 cc and 0.01 cc ion chambers were within 0.8%, while the differences were much greater for the beam size of less than 3 cm x 3 cm (the maximum difference reached 64%). Using 4, 6 and 8 MV X-rays, Sc measured by the 0.13 cc chamber with an elongated source-to-surface distance (SSD) (>150 cm) were 25.4%, 6.9%, 24.6%, and 1.4%, 1.4%, 2.2% greater than those measured by a standard SSD (100 cm) for 1 cm x 1 cm and 2 cm x 2 cm beams respectively; although there was no significant difference in Sc measurements for the beams of >or=2 cm x 2 cm using the elongated SSD of the 0.13 cc and the 0.01 cc ion chambers, Sc measured by the 0.13 cc ion chamber were 0.2%, 8.5%, 3.4% less than those measured by the 0.01 cc ion chamber for the 1 cm x 1 cm beam. For the 1 cm x 1 cm beam, the TMR of the depth deeper than 15 cm measured with the 0.01 cc ion chamber was about 4% different compared with that measured with the 0.13 cc ion chamber; for radiation fields of >or=2 cm x 2 cm, the differences of TMR between the 0.01 cc and 0.13 cc chambers were within 1%. For the radiation fields of >or=3 cm x 3 cm, the measured TMR values had a good consistency with the calculated values obtained from the percentage depth doses (PDDs) at the depth of 0 to 15 cm; but the two values were obviously different at the depths of deeper than 15 cm (>2%).
For the measurement of small fields, the choice of a suitable detector is important due to the lack of lateral electron equilibrium. Misuse of the detector may affect the accuracy of the measurements for small radiation fields. When the lateral electron equilibrium is not established, the size of the detector used to measure the absorbed dose on the central axis should be considerably smaller than the field size.

Download full-text


Available from: Chen Lixin, May 24, 2015
44 Reads
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The advent of modern technologies and mega voltage beam energies in radiation therapy has increased the demand of higher degree of accuracy and efficiency. We intended to increase the efficiency of the radiation therapy by applying mathematical and computational tools not only to verify the measured Percentage Depth Dose (PDD) data but also to aid in rapid machine commissioning. Lagrange’s interpolation method was used to calculate the absorbed doses at depths with step size of 1 cm. The measured PDD data at selected depths with 5 to 8 cm separation have been used as the points of interpolations. The interpolated results precisely agree with the measured data, and can be used as the input data for treatment planning. This exploration should be extended for the interpolation of dose for different field sizes as well so that the commissioning time can be further decreased and the reliability of the measured data is assured.