Telecobalt Machine Beam Intensity Modulation with Aluminium Compensating Filter Using Missing Tissue Approach

Document Type: Original Paper


1 National Centre for Radiotherapy & Nuclear Medicine, Korle Bu Teaching Hospital, Accra, Ghana; Physics Department, University of Cape Coast, Ghana; and School of Biomedical and Allied Health Sciences, University of Ghana, Accra, Ghana.

2 Department of Physics, School of Physical Sciences, University of Cape Coast, Cape Coast, Ghana.

3 Department of Applied Physics, University for Development Studies, Navrongo Campus, Navrongo, Ghana.

4 National Centre for Radiotherapy and Nuclear Medicine, Korle Bu Teaching Hospital, Accra, Ghana.


Introduction: The present study aimed to generate intensity-modulated beams with Aluminium compensating filters for a conventional telecobalt machine based on the outputs of a treatment planning system (TPS) performing forward planning and cannot simulate directly the compensating filter.
Materials and Methods: In order to achieve the beam intensity modulation during treatment planning with the TPS, we used a bolus placed on the surface of a tissue-equivalent phantom. The treatment plans replicated on the telecobalt machine with the bolus were represented with compensating filters placed at a certain distance from the phantom surface. An equation was proposed for the conversion of the bolus thickness to the compensating filter thickness such that any point within the phantom would receive the planned dose. Correction factors were introduced into the proposed equation to account for the influences of field size, treatment depth, and applied bolus thickness. The proposed equation was obtained based on the analyses of empirical data measured in a full scatter water phantom with and without the compensating filter. 
Results: According to the results, the dosimetric verification of the proposed approach outputs in a solid water phantom with calibrated Gafchromic EBT2 films were comparable to that of the TPS with deviation of ±4.73% (mean: 2.98±1.05%).
Conclusion: As the findings of the present study indicated, the discrepancy between the measured doses and TPS-estimated doses was within the tolerance of ±5%, which is recommended for dose delivery in external beam radiotherapy. Therefore, the proposed approach is recommended for clinical application. 


Main Subjects


  1. Schlegel W, Bortfeld T, Grosu A. New technologies in radiation oncology. Springer-Verlag Berlin Heidelberg, Germany. 2006.
  2. Podgorsak E. B. Radiation oncology Physics: A handbook for teachers and students. International Atomic Energy Agency (IAEA). 2005.
  3. Brewster L, Mohan R, Mageras G, Burman, C, Leibel S, Fuks Z. Three dimensional conformal treatment planning with multileaf collimators. Int. J. Rad. Onc. Biol. Phys. 1995 Dec 1; 33(5): 1081–9. DOI: 10.1016/0360-3016(95)02061-6.
  4. McNair HA, Adams EJ, Clark CH, Miles EA, Nutting CM. Implementation of IMRT in the radiotherapy department. Br J Radiol. 2003 Dec;76(912): 850-6. DOI: 10.1259/bjr/19737738.
  5. Vaarkamp J, Adams EJ, Warrington AP, Dearnaley DP. A comparison of forward and inverse planned conformal, multi segment and intensity modulated radiotherapy for the treatment of prostate and pelvic nodes. Radiother. Oncol. 2004 Oct; 73(1):65-72. DOI: 10.1016/j.radonc.2004.07.015.
  6. Shepard DM, Earl M A, Li X A, Naqvi S, Yu C. Direct aperture optimization: A turnkey solution for step-and-shoot IMRT. Med. Phys. 2002 Jun; 29 (6): 1007-18. DOI: 10.1118/1.1477415.
  7. Chang S. Compensating filter-intensity-modulated Radiotherapy – A Traditional Tool for Modern Application. Radiotherapy and Imaging. 2006: 82- 6.
  8. Vassy D L, Turmel J, Josey J C. Solid Modulation: Problem-Solving IMRT. American College of Radiology. 2008 Nov; 5(11): 1150 - 3. DOI: 10.1016/j.jacr.2008.08.001.
  9. Rawlinson JA, Islam MK, Galbraith DM. Dose to radiation therapists from activation at high-energy accelerators used for conventional and intensity-modulated radiation therapy. Med Phys. 2002 Apr; 29(4): 598–608. DOI: 10.1118/1.1463063.
  10. George R, Keall PJ, Kini VR, et al. Quantifying the effect of intrafraction motion during breast IMRT planning and dose delivery. Med Phys 2003; 30: 552–62. DOI: 10.1118/1.1543151.
  11. Buckey CR, Stathakis S, Papanikolaou N. The inter- and intrafraction reproducibilities of three common IMRT delivery techniques. Med. Phys. 2010; 37( 9): 4854 - 60. DOI: 10.1118/1.3476413.
  12. Chang SX, Cullip TJ, Deschesne KM. Intensity modulation delivery techniques: “Step & shoot” MLC auto sequence versus the use of a modulator. Med Phys. 2000 May; 27(5): 948–59. DOI: 10.1118/1.598989.
  13.  Sharma SD. Challenges of small photon field dosimetry are still challenging. Journal of medical physics/Association of Medical Physicists of India. 2014 Jul;39(3):131-2. DOI: 10.4103/0971-6203.138998. .
  14. Stanton R, Stinson D. Applied physics for radiation oncology. Revised edition. Medical Physics Publishing. 2009.
  15. Khan FM. The Physics of Radiation Therapy. Fourth Edition. Lippincott Williams and Wilkins. 2010.
  16. O-Hoon C, Jung-Eun L, Hong-Seok N, Doo-Kwon B. Modeling for Missing Tissue Compensator Fabrication Using RFID Tag in U-Health . International Symposium on Biological and Medical Data Analysis. Biological and Medical Analysis. 2006: 463-71.
  17. Paliwal BR, Rommelfanger S, Das RK. Attenuation characteristics of a new compensating filter material: Thermo-Shield for high energy electron and photon beams. Med. Phys. 1998 Apr; 25( 4): 484 - 7. DOI: 10.1118/1.598223.
  18. Report of Task Group No. 65 of the Radiation Therapy Committee of the American Association of Physicists in Medicine: AAPM Report 85. Tissue inhomogeneity corrections for megavoltage photon beams. American Association of Physicists in Medicine. 2004 Jul: 3- 107.
  19. Kutcher G J, Coia L, Gillin M, Hanson W F. et al. Comprehensive QA for radiation oncology: Report of AAPM radiation therapy committee task group 40. Med. Phys. 1994 Apr; 21(4): 581- 618. DOI: 10.1118/1.597316.
  20. International Electrotechnical Commission (IEC): Medical electrical equipment – Dosimeters with ionization chambers as used in radiotherapy. IEC 60731. IEC. 1997.
  21. International Atomic Energy Agency (IAEA). Technical report series 398. Absorbed dose determination in external beam radiotherapy. IAEA. 2000.
  22. National Institute of Standards and Technology (NIST),USA. Physical measurement laboratory. Available from:
  23. Day, M.J., A note on the calculation of dose in x-ray fields. British Journal of Radiology. 1950; 23(270): 368-9.
  24.  Day MJ, Aird EG. The equivalent field method for dose determinations in rectangular fields. Br J Radiol Suppl. 1983;17:105-14.
  25.  Bjärngard BE, Siddon RL. A note on equivalent circles, squares, and rectangles. Medical physics. 1982 Mar 1;9(2):258-60. DOI: 10.1118/1.595161.
  26.  Clarkson, J.R. A note on depth doses in fields of irregular shape. British Journal of Radiology. 1941 Aug; 14(164): 265- 8. DOI: 10.1259/0007-1285-14-164-265.
  27.  Sathiyan S, Ravikumar M, Keshava SL. Relative Output Factors and Absolute Equivalent Square Fields at Depths for High Energy X-Ray and Gamma Ray Beams. Austral Asian Journal of Cancer. 2006; 5(4): 225-35.
  28.  Sterling T, Perry H, and Weinkam J. Automation of radiation treatment planning. VI. A general field equation to calculate percent depth dose in the irradiated volume of a cobalt 60 beam. Br. J. Radiol. 1967 Jun; 40(474): 463-74. DOI: 10.1259/0007-1285-40-474-463.
  29.  Vadash P, Bjärngard B. An equivalent-square formula for head scatter factors. Med Phys. 1993 ; 20(3):733-4. DOI: 10.1118/1.597024.
  30.  Monti AF, Ostinelli A, Frigerio M, Gelosa S. An equivalent square method for irregular photon fields. Med Dosim. 1995 Winter; 20(4): 275-7.
  31.  Sanz, DE. Accuracy limits of the equivalent field method for irregular photon fields. Phys Med Biol. 2002 Sep 7; 47(17): 3073-85.
  32.  Thomas SJ, Eaton DJ, Tudor GS, Twyman NI. Equivalent squares for small field dosimetry. Br J Radiol. 2008 Nov; 81(971): 897-901. DOI: 10.1259/bjr/27713136.
  33. Araki F, Ikeda R, Moribe N, Shirakawa Y, Hatemura M, Shi-monobou T, et al. Dose calculation for asymmetric photon fields with independent jaws and multileaf collimators. Med Phys. 2000 Feb; 27(2): 340-5. DOI: 10.1118/1.598836.
  34.  Kwa W, Kornelsen RO, Harrison RW, el-Khatib E. Dosimetry for asymmetric x-ray fields. Med Phys. 1994 Oct; 21(10):1599-604. DOI: 10.1118/1.597260.
  35. Day MJ. The equivalent field method for axial dose determinations in rectangular fields. Br J Radiol. 1972;11: 95-10.
  36. Tagoe SNA, Nani EK, Yarney J, et al. Semi-empirical equivalent field method for dose determination in midline block fields for cobalt-60 beam. Journal of Applied Science and Technology. 2012; 17: 70 - 7. 
  37. Chengeni N, Tahmasebi Birgani MJ. Equivalent field calculation for irregular symmetric and asymmetric photon fields. International Scholarly and Scientic research and innovation. 2013; 7(9): 1430 - 5.
  38. International Atomic Energy Agency (IAEA). IAEA-TECDOC-896. Radiation dose in radiotherapy from prescription to delivery. IAEA, Vienna. 1996.
  39. Martišíková M, Ackermann B, Jäkel O. Analysis of uncertainties in Gafchromic® EBT film dosimetry of photon beams. Phys. Med. Biol. 2008 Nov 18;53(24):7013. DOI: 10.1088/0031-9155/53/24/001.
  40. Devic S, Seuntjens J, Hegyi G, , Podgorsak EB, et al. Dosimetric properties of improved GafChromic films for seven different digitizers. Med. Phys. 2004 Sep 1; 31 (9): 2392 - 401. DOI: 10.1118/1.1776691.
  41. Levitt SH, Purdy JA, Perez CH, Vijayakuma S. Technical basis of radiation therapy: Practical clinical application. 4th Revised Edition. Springer; 2008.
  42. Haghparast A, Hashemi B, and Eivazi M T. Influence of compensator thickness, field size, and off-axis distance on the effective attenuation coefficient of a cerrobend compensator for intensity-modulated radiation therapy. Medical Dosimetry. 2013 Spring; 38(1): 25-9. DOI: 10.1016/j.meddos.2012.06.001.
  43. Iwasaki A, KuIwasaki A, Kubota M, Fujimori A, Suzaki K, Abe Y, et al. Formulation of spectra-based attenuation coefficients in water as a function of depth and off-axis distance for 4, 10 and 15MV X-ray beams. Radiation Physics and Chemistry. 2005 Apr; 72(6): 657-61. DOI: 10.1016/j.radphyschem.2004.05.051.