Evaluation of the Effects of Inhomogeneities on Dose Profiles Using Polymer Gel Dosimeter and Monte Carlo Simulation in Gamma Knife

Document Type: Original Paper

Authors

1 Biochemistry and Biophysics Dept., Faculty of Medicine, Mazandaran University of Medical Sciences, Sari, Iran

2 Medical Physics Dept., Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran

3 Novin Medical Radiation Institute, Tehran, Iran

Abstract

Introduction
Polymer gel dosimeters offer a practical solution to 3D dose verification for conventional radiotherapy as well as intensity-modulated and stereotactic radiotherapy. In this study, EGSnrc calculated and PAGAT polymer gel dosimeter measured dose profiles from single shot irradiation with 18 mm collimator of Gamma Knife in homogeneous and inhomogeneous phantoms were compared with each other.
Materials and Methods
The head phantom was a custom-built 16 cm diameter plexiglas sphere. Inside the phantom, there were two cubic cutouts for inserting the gel vials and inhomogeneities. Following irradiation with the Gamma Knife unit, the polymer gel dosimeters were scanned with a 1.5 T MRI scanner. For the purpose of simulation the simplified channel of 60Co source of Gamma Knife BEAMnrc and for extracting the 3D dose distribution in the phantom, DOSXYZnrc codes were used.
Results
Within high isodose levels (>80%), there are dose differences higher than 7%, especially between air inserted and PTFE inserted phantoms, which were obtained using both simulation and experiment. This means that these values exceed the acceptance criterion of conformal radiotherapy and stereotactic radiosurgery (i.e., within some isodose levels, less than 93% of prescription dose are delivered to the target).
Conclusion
The discrepancies observed between the results obtained from heterogeneous and homogeneous phantoms suggest that Leksell Gamma Knife planning system (LGP) predictions which assume the target as a homogeneous material must be corrected in order to take care of the air- and bone-tissue inhomogeneities.

Keywords

Main Subjects


  1. Leksell L. The stereotaxic method and radiosurgery of the brain. Acta Chir Scand. 1951;102:316-19.
  2. Mack A, Scheib S, Major J, Gianolini S, Pazmandi G, Feist H, Czempiel H, Kreiner H. Precision dosimetry for narrow photon beams used in radiosurgery-determination of Gamma Knife output factors. Med Phys. 2002;29(9):2080-9.
  3. Yu C, Shepard D. Treatment planning for stereotactic radiosurgery with photon beams. Technol Cancer Res Treat. 2003;2:93–104.
  4. Moskvin V, Timmerman R, DesRosiers C, Randall M, DesRosiers P, Dittmer P, Papiez L. Monte Carlo simulation of the Leksell gamma Knife (R): II. Effects of heterogeneous versus homogeneous media for stereotactic radiosurgery. Phys Med Biol. 2004;49(21):4879-4895.
  5. Rogers D, Faddegon B, Ding G, Ma C, Wei J, Mackie T. BEAM: A Monte Carlo code to simulate radiotherapy treatment units. Med Phys. 1995;22:503 – 524.
  6. Nelson W, Hirayama H, Rogers D. The EGS4 Code System. Stanford Linear Accelerator Center, Stanford, California; 1985.Report SLAC–265.
  7. Kawrakow I, Mainegra-Hing E, Rogers D, Tessier F, Walters B. The EGSnrc Code System: Monte Carlo Simulation of Electron and Photon Transport. Ottawa : National Research Council of Canada; 2011.Report PIRS-701.
  8. Fong PM, Keil DC, Does MD, Gore JC. Polymer gels for magnetic resonance imaging of radiation dose distributions at normal room atmosphere. Phys Med Biol. 2001;46(12):3105-13.
  9. Venning AJ, Nitschke KN, Keall PJ, Baldock C. Radiological properties of normoxic polymer gel dosimeters. Med Phys. 2005;32(4):1047-53.
  10. Al-Dweri FM, Rojas EL, Lallena AM. Effects of bone- and air-tissue inhomogeneities on the dose distributions of the Leksell Gamma Knife calculated with PENELOPE. Phys Med Biol. 2005;50(23):5665-78.
  11. Cheung JY, Yu KN. Dose distribution close to metal implants in gamma knife radiosurgery: A Monte Carlo study [letter]. Med Phys. 2005;32(5):1448-9.
  12. Lewis RD, Ryde SJ, Seaby AW, Hancock DA, Evans CJ. Use of Monte Carlo computation in benchmarking radiotherapy treatment planning system algorithms. Phys Med Biol. 2000;45(7):1755-64.
  13. Pourfallah TA, Allahverdi M, Alam NR, Ay MR, Zahmatkesh MH, Ibbott GS. Performance evaluation of MRI-based PAGAT polymer gel dosimeter in an inhomogeneous phantom using EGSnrc code on a Co-60 machine. Appl Radiat Isot. 2009;67(1):186-91.
  14. Allahverdi Pourfallah T, Allahverdi M, Riahi Alam N, Ay MR, Zahmatkesh MH. Verifying the accuracy of dose distribution in gamma knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and MC simulation. Iran J Radiat Res. 2009;7:49-56.
  15. Pourfallah TA, Allahverdi M, Alam NR, Ay MR, Zahmatkesh MH. Differential dose volume histograms of Gamma Knife in the presence of inhomogeneities using MRI–polymer gel dosimetry and MC simulation. Med Phys. 2009;36(7):3002-12.
  16. De Deene Y, Vergote K, Claeys C, De Wagter C. The fundamental radiation properties of normoxic polymer gel dosimeters: a comparison between a methacrylic acid based gel and acrylamide based gels. Phys Med Biol. 2006;51(3):653-73.
  17. Venning AJ, Hill B, Brindha S, Healy BJ, Baldock C. Investigation of the PAGAT polymer gel dosimeter using magnetic resonance imaging. Phys Med Biol. 2005;50(16):3875-88.
  18. Rogers DWO, Walters B, Kawrakow I. BEAMnrc Users Manual. Ottawa : National Research Council of Canada; 2007 May. 268 p. Report PIRS-0509(A)revL.
  19. Treurniet JR, Walters BR, Kawrakow I, Rogers DWO. BEAMnrc, DOSXYZnrc and BEAMDP GUI User’s Manual. Ottawa: National Research Council of Canada; 2005 Jul. 17 p. Report PIRS-0623(rev C).
  20. Al-Dweri FM, Lallena AM, Vilches M. A simplified model of the source channel of the Leksell GammaKnife (R) tested with PENELOPE. Phys Med Biol. 2004;49(12):2687-703.
  21. Walters B, Kawrakow I, Rogers DWO. DOSXYZnrc Users Manual. Ottawa : National Research Council of Canada; 2009 Jul. Report PIRS-794revB.
  22. Perez CA, Brady LW, Halperin EC, Schmidt-Ullrich RK. Principles and Practice of Radiation Oncology.4nd ed.  Philadelpia: LIPPINCOTT WILLIAMS & WILKINS; 2004.
  23. Isbakan F, Ulgen Y, Bilge H, Ozen Z, Agus O, Buyuksarac B. Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry. Med Phys. 2007;34(5):1623-30.
  24. De Deene, Y. On the accuracy and precision of gel dosimetry. J Phys Conf Ser. 2006;56:72–85.