Document Type: Original Research
PhD, DRP, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India
PhD, Maharishi Dayanand University, Rohtak, Haryana, India
PhD, Chaudhary Ranbir Singh University, Jind, Haryana, India
PhD, DRP, BLK Super Speciality Hospital, New Delhi India
Background: This study aims to investigate radiation beam geometry of Cyberknife beam and change in dosimetric characteristics of six megavoltage (6MV) flattening filter free (FFF) beam after passing through high density cadmium free compensator alloy.
Material and Methods: In this experimental study, changes in FFF beam dosimetric characteristics after passing through compensator alloy was measured. Transmitted intensity of FFF beam was measured in air by an ion chamber at a source to detector distance (SDD) of 800mm. Extended SDD measurement also has been performed at a distance of 1270mm to analyze scattering due to compensator. Linear attenuation coefficient (µeff) was measured for cadmium free compensator alloy using simple exponential attenuation model. Percentage depth doses (PDDs) have been measured by a radiation field analyzer with compensator material to observe the beam hardening and change in surface doses and depth doses.
Results: Linear attenuation coefficient of compensator alloy was measured 0.042 (Standard Deviation ±0.00099) mm-1 and it was found that there is no change with increase in collimator size. Even after increasing distance source from detector, µeff has no change. PDDs were found to increase with thickness of compensator. PDD from a 60mm collimator size increased by 5% and 6% at a depth of 100mm and 200mm, respectively in water. PDD also increased with collimator size less significantly. Surface dose was found to decrease with increase in compensator thickness.
Conclusion: Cyberknife beam has been found to be narrow beam geometry. FFF beam contains lesser scattered photons. Presence of high density compensator filters out the soft x-ray photon causes significant dosimetric changes.
- Cashmore J. The characterization of unflattened photon beams from a 6 MV linear accelerator. Phys Med Biol. 2008;53:1933-46. doi: 10.1088/0031-9155/53/7/009. PubMed PMID: 18364548.
- Lee PC. Monte Carlo simulations of the differential beam hardening effect of a flattening filter on a therapeutic x-ray beam. Med Phys. 1997;24:1485-9. doi: 10.1118/1.598037. PubMed PMID: 9304577.
- Vassiliev ON, Titt U, Ponisch F, Kry SF, Mohan R, Gillin MT. Dosimetric properties of photon beams from a flattening filter free clinical accelerator. Phys Med Biol. 2006;51:1907-17. doi: 10.1088/0031-9155/51/7/019. PubMed PMID: 16552113.
- Attix FH. Introduction to radiological physics and radiation dosimetry. Weinheim: Wiley-VCH; 1th ed. 1986. p. 222.
- Dieterich S, Cavedon C, Chuang CF, Cohen AB, Garrett JA, Lee CL, et al. Report of AAPM TG 135: quality assurance for robotic radiosurgery. Med Phys. 2011;38:2914-36. doi: 10.1118/1.3579139. PubMed PMID: 21815366.
- Dehlaghi V, Taghipour M, Haghparast A, Roshani GH, Rezaei A, Shayesteh SP, et al. Prediction of the thickness of the compensator filter in radiation therapy using computational intelligence. Med Dosim. 2015;40:53-7. doi: 10.1016/j.meddos.2014.09.003. PubMed PMID: 25498836.
- El-Khatib EE, Podgorsak EB, Pla C. Broad beam and narrow beam attenuation in Lipowitz’s metal. Med Phys. 1987;14:135-9. doi: 10.1118/1.596100. PubMed PMID: 3104738.
- Tyagi A, Nangia S, Chufal K, Mishra M, Ghosh D, Supe S, et al. Quality assurance and dosimetric analysis of intensity modulation radiotherapy using compensators for head and neck cancers. Polish Journal of Medical Physics And Engineering. 2009;15:193-208. doi:10.2478/v10013-009-0019-3.
- Kaushik S, Punia R, Tyagi A, Malik A. Effect of scattering and differential attenuation of beam profile in the presence of high-density intensity modifying compensator. J Can Res Ther. 2019;15(8):110-4. doi: 10.4103/jcrt.JCRT_661_17.
- Khan FM. Physics of Radiation Therapy. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2003. p. 60-1.
- Arora VR, Weeks KJ. Characterization of gypsum attenuators for radiotherapy dose modification. Med Phys. 1994;21:77-81. doi: 10.1118/1.597364. PubMed PMID: 8164592.
- Ponisch F, Titt U, Vassiliev ON, Kry SF, Mohan R. Properties of unflattened photon beams shaped by a multileaf collimator. Med Phys. 2006;33:1738-46. doi: 10.1118/1.2201149. PubMed PMID: 16872081.
- Vassiliev ON, Titt U, Kry SF, Ponisch F, Gillin MT, Mohan R. Monte Carlo study of photon fields from a flattening filter-free clinical accelerator. Med Phys. 2006;33:820-7. doi: 10.1118/1.2174720. PubMed PMID: 16696457.
- Huang PH, Chin LM, Bjarngard BE. Scattered photons produced by beam-modifying filters. Med Phys. 1986;13:57-63. doi: 10.1118/1.595923. PubMed PMID: 3951410.
- Podgorsak EB. Review of radiation oncology physics: a handbook for teachers and students. International Atomic Energy Agency Educational reports series. Vienna, Austria. May 2003. Page.181, 278.
- Kaushik S, Punia R, Tyagi A, Singh MP. Dosimetric study of cadmium free alloy used in compensator based intensity modulated radiotherapy. Radiat Phys Chem. 2017;139:184-9. doi.org/10.1016/j.radphyschem.2017.05.022.