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EDITORIAL
Year : 2008  |  Volume : 33  |  Issue : 1  |  Page : 1-2
 

Quality of high-energy X-ray radiotherapy beams: Issues of adequacy of routine experimental verification


Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, CT & CRS, Anushaktinagar, Mumbai - 400 094, India

Correspondence Address:
S D Sharma
Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, CT & CRS, Anushaktinagar, Mumbai - 400 094
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-6203.39416

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How to cite this article:
Sharma S D. Quality of high-energy X-ray radiotherapy beams: Issues of adequacy of routine experimental verification. J Med Phys 2008;33:1-2

How to cite this URL:
Sharma S D. Quality of high-energy X-ray radiotherapy beams: Issues of adequacy of routine experimental verification. J Med Phys [serial online] 2008 [cited 2017 May 29];33:1-2. Available from: http://www.jmp.org.in/text.asp?2008/33/1/1/39416


Medical electron linear accelerator is an important equipment used in radiotherapy departments worldwide. Measurement of beam quality of high-energy X-rays generated by a medical electron linear accelerator (LINAC) is required for three different purposes, namely, (i) to verify the stated specification of the vendor, (ii) to determine the appropriate beam-quality-correction factor for the ionization chamber and (iii) to determine the shielding thickness of the primary and secondary barriers of the accelerator housing. Recently, a few instances of significant mismatch between quoted nominal X-ray-beam energy (>6 MV) and measured X-ray-beam energy have been noticed by radiotherapy centers during acceptance and commissioning of newly installed medical LINACs; therein, it is important to note that one of the beam-quality indicators (TPR 20,10 ) indicated a particular value of nominal X-ray energy while the measured dosimetry data (percentage depth dose) indicated a significantly different nominal energy. Such observations create confusion during the clinical use of the accelerator, and the user becomes unsure about whether to use the dosimetry data measured locally or to use the standard data set available in the literature corresponding to the measured quality of the X-ray beam. The significant variation in quoted and measured values of X-ray-beam energy also raises doubt over the adequacy of shielding thickness of the accelerator housing.

All the major manufacturers of medical electron linear accelerators (e.g., Elekta, Siemens, Varian) declare IEC 60977 [1] compliance of their accelerators. Accordingly, the energy of the photon beam is specified in terms of nominal accelerator potential/ beam energy (MV or MeV), as well as in terms of a parameter which indicates penetrative quality of the beam. Commonly quoted penetrative quality indicator of the X-ray beam by the manufacturer is percentage depth dose at 10-cm depth (D 10 ) for 10 × 10 cm 2 field size at the phantom surface and 100-cm source surface distance (SSD) [e.g., Varian Medical Systems, USA, quotes D 10 of their 10-MV X-ray beam as 74.0 ± 1.0%]. In addition, the manufacturer also quotes the depth of dose maximum (d m ) as an alternative quality indicator of the beam. These two X-ray penetrative quality indicators suffer from the effect of electron contamination and may change their magnitude due to variation in components and accessories in the head of the accelerator. [1],[2],[3],[4] AAPM has also adopted D 10 as a quality-specifying parameter for X-rays under the recommendation that measurement should be done using 0.1-cm lead filter to limit the contribution of contamination electrons. [4] Other recommended X-ray beam quality indicators are depth of 80% dose (d 80 ) for 10 × 10 cm 2 field at an SSD of 100 cm along the central axis of the beam [5] and the ratio of tissue phantom ratio (TPR) at the depths of 20 and 10 cm (TPR 20,10 ). [2],[3],[6] The method of d 80 for energy specification also has a practical shortcoming related to electron contamination. The parameter TPR 20,10 is a measure of the effective attenuation coefficient describing approximately the exponential decrease of X-ray depth - dose curve beyond d m and is independent of the electron contamination in the incident beam. [2],[3]

[Table - 1] lists the values of D 10 , d 80 and TPR 20,10 at different values of nominal energy (MV) of X-ray beams generated by medical LINACs. It can be observed from this table that 1.0% change in D 10 value (>6 MV) results in about 1.0 MV corresponding change in nominal beam energy. This means the nominal X-ray energy specified by the vendor has a tolerance of about 1.0 MV. This in turn indicates that the exact matching of the X-ray-beam energy quoted by the vendor with the X-ray-beam energy measured by the user would definitely be a random coincidence and can be an issue of dispute. D 10 and d 80 do not show saturation with increasing nominal beam energy and hence can be regarded as sensitive beam-quality indicators. TPR 20,10 shows saturation at higher beam energy (>12 MV), which is a major drawback of this parameter. As far as the use of TPR 20,10 is concerned for determining MV of an X-ray beam, it would be an inferior choice. But for determining beam-quality conversion factor for ionization chambers used for dose rate calibrations of output of X-rays from medical LINACs, this parameter is found to be relatively more suitable. [2] It is also observed from the data in [Table - 1] that among the three recommended X-ray-beam quality indicators, d 80 has comparatively higher sensitivity.

[Table - 2] lists the values of tenth value thickness [both first TVL (TVL1) and equilibrium TVL (TVLe)] for ordinary concrete from NCRP Report 151. [7] These two TVLs are used to determine the absolute thickness of a radiation barrier [wall thickness of a barrier = TVL1 + (n-1) TVLe]. To determine the shielding thickness of primary and secondary barriers of the accelerator housing, it is important to determine the beam energy in terms of MV. However, determining the accelerating potential directly is not practical at the hospitals. Quality of the X-ray beam is measured in terms of D 10 and d 80 or TPR 20,10 , and correlation curves generated using data from [Table - 1] are used to calculate corresponding MV values. However, accuracy of MV value so determined depends on the accuracy and sensitivity of beam-quality indicator selected. Change in values of TVL with changing nominal beam energy, particularly for high-energy X-rays (>15 MV), is relatively small and hence an error of 1.0 MV or so in determining the nominal beam energy would not affect the shielding thickness drastically.

Dosimetry and quality assurance protocols recently in use recommend a number of parameters for qualifying beam quality for heterogeneous X-rays generated by medical LINACs. The choice of parameters for qualifying beam quality is governed by the objective of the given task. The parameter d 80 measured under the electron-contamination-free condition as recommended by AAPM [4] for D 10 will be a better choice for verifying manufacturer-specified nominal X-ray-beam energy and determining shielding thickness, as it shows an increasing nature with increasing nominal beam energy. However, for determining the beam-quality conversion factor for beam dosimetry purposes, protocol-specific beam-quality indicator should be measured and used. Dosimetry data generated following a standard technique at the hospital site should be used for patient dosimetry, irrespective of the difference in stated and measured energy of the X-ray beam.

 
   References Top

1.International Electrotechnical Commission (IEC), Medical electrical equipment: Medical electron accelerators in the range 1 MeV to 50 MeV - Guidelines for functional performance characteristics. IEC 60977, 1989.  Back to cited text no. 1    
2.International Atomic Energy Agency (IAEA), Absorbed dose determination in external beam radiotherapy: An international code of practice for dosimetry based on absorbed dose to water. Technical Report Series No. 398, 2000.  Back to cited text no. 2    
3.International Commission on Radiation Units and Measurements (ICRU), Dosimetry of high energy photon beams based on standards of absorbed dose to water, ICRU Report 64, 2001.  Back to cited text no. 3    
4.Almond PR, Biggs PJ, Coursey BM, Hanson WF, Huq MS, Nath R, et al. AAPM's TG-51 protocol for clinical reference dosimetry of high energy photon and electron beams. Med Phys 1999;26:1847-70.   Back to cited text no. 4  [PUBMED]  
5.British Institute of Radiology, Central axis depth dose data for use in radiotherapy. Br J Radiol 1996;Suppl 25.  Back to cited text no. 5    
6.Radiological Physics and Advisory Division (RPandAD), Bhabha Atomic Research Centre (BARC), Acceptance/quality assurance tests for medical linear accelerator. RPandAD/ACC/QA/04, 2004.  Back to cited text no. 6    
7.National Council on Radiation Protection and Measurements (NCRP), Structural shielding design and evaluation for x- and gamma ray radiotherapy facilities. Report No. 151, 2005.  Back to cited text no. 7    



 
 
    Tables

  [Table - 1], [Table - 2]


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