QUANTITATIVE DATA TO SHOW EFFECTS OF GEOMETRIC ERRORS AND DOSE GRADIENTS ON DOSE DIFFERENCE FOR IMRT DOSE QUALITY ASSURANCE MEASUREMENTS

  • Park, So-Yeon (Department of Radiation Applied Life Science, Seoul National University Graduate School) ;
  • Park, Jong-Min (Department of Radiation Applied Life Science, Seoul National University Graduate School) ;
  • Ye, Sung-Joon (Department of Radiation Applied Life Science, Seoul National University Graduate School)
  • Received : 2011.09.02
  • Accepted : 2011.10.11
  • Published : 2011.12.30

Abstract

To quantitatively evaluate how setup errors in conjunction with dose gradients contribute to the error in IMRT dose quality assurance (DQA) measurements. The control group consisted of 5 DQA plans of which all individual field dose differences were less than ${\pm}5%$. On the contrary, the examination group was composed of 16 DQA plans where any individual field dose difference was larger than ${\pm}10%$ even though their total dose differences were less than ${\pm}5%$. The difference in 3D dose gradients between the two groups was estimated in a cube of $6{\times}6{\times}6\;mm^3$ centered at the verification point. Under the assumption that setup errors existed during the DQA measurements of the examination group, a three dimensional offset point inside the cube was sought out, where the individual field dose difference was minimized. The average dose gradients of the control group along the x, y, and z axes were 0.21, 0.20, and 0.15 $cGy{\cdot}mm^{-1}$, respectively, while those of the examination group were 0.64, 0.48, and 0.28 $cGy{\cdot}mm^{-1}$, respectively. All 16 plans of the examination group had their own 3D offset points in the cube. The individual field dose differences recalculated at the offset points were mostly diminished and thus the average values of total and individual field dose differences were reduced from 3.1% to 2.2% and 15.4% to 2.2%, respectively. The offset distribution turned out to be random in the 3D coordinate. This study provided the quantitative data that support the large individual field dose difference mainly stems from possible geometric errors (e.g., random setup errors) under the influence of steep dose gradients of IMRT field.

Acknowledgement

Supported by : National Research Foundation of Korea (NRF)

References

  1. Ezzell GA, Galvin JM, Low D, et al. Guidance document on delivery, treatment planning, and clinical implementation of IMRT: report of the IMRT Subcommittee of the AAPM Radiation Therapy Committee. Med Phys. 2003 Mar;30(8):2089-2115. https://doi.org/10.1118/1.1591194
  2. Basran PS and Woo MK. An analysis of tolerance levels in IMRT quality assurance procedures. Med Phys. 2008 Apr;35(6):2300-2307. https://doi.org/10.1118/1.2919075
  3. Alber M, Broggi S, Wagter CD, Eichwurzel I, Engstrom P, Fiorino C, Georg D, Hartmann G, Knoos T, Leal A, Marijnissen H, Mijnheer B, Paiusco M, Sanchez.Doblado F, Schmidt R, Tomsej M, Welleweerd H: Guidelines for the verification of IMRT. Brussels, Belgium: ESTRO, 2008.
  4. Woo MK and Nico A. Impact of multileaf collimator leaf positioning accuracy on intensity modulation radiation therapy quality assurance ion chamber measurements. Med Phys. 2005 May;32(5):1440-1445. https://doi.org/10.1118/1.1901843
  5. Breen SL, Moseley DJ, Zhang B, Sharpe MB. Statistical process control for IMRT dosimetric verification. Med Phys. 2008 Oct;35(10):4417-4425. https://doi.org/10.1118/1.2975144
  6. Dong L, Antolak J, Salehpour M, et al. Patient‐specific point dose measurement for IMRT monitor unit verification. Int J Radiat Oncol Biol Phys. 2003 Feb;56(3):867−877. https://doi.org/10.1016/S0360-3016(03)00197-4
  7. Bouchard H and Seuntjens J. Ionization chamberbased reference dosimetry of intensity modulated radiation beams. Med Phys. 2004 Sep;31(9):2454- 2465. https://doi.org/10.1118/1.1781333
  8. Tsai JS, Wazer DE, Ling MN, et al. Dosimetric verification of the dynamic intensity.modulated radiation therapy of 92 patients. Int J Radiat Oncol Biol Phys. 1998 Dec;40(5):1213-1230. https://doi.org/10.1016/S0360-3016(98)00009-1
  9. Stasi M, Baiotto B, Barboni G, Scielzo G. The behavior of several micro.ionization chambers in small intensity modulated radiotherapy fields. Med Phys. 2004 Oct;31(10):2792-2795. https://doi.org/10.1118/1.1788911
  10. Martens C, De Wagter C, De Neve W. The value of the PinPoint ion.chamber for characterization of small field segments used in intensity.modulated radiotherapy. Phys Med Biol. 2000;45:2519-2530. https://doi.org/10.1088/0031-9155/45/9/306
  11. Leybovich LB, Sethi A, Dogan N. Comparison of ionization chambers of various volumes for IMRT absolute dose verification. Med Phys. 2003 Feb;30(2):119.123. https://doi.org/10.1118/1.1536161
  12. Capote R, Sanchez.Doblado F, Leal A, Lagares JI, Arrans R, Hartmann GH. An EGSnrc Monte Carlo study of the microionization chamber for reference dosimetry of narrow irregular IMRT beamlets. Med Phys. 2004 Sep;31(9):2416-2422. https://doi.org/10.1118/1.1767691
  13. Ezzell GA, Burmeister JW, Dogan N, et al. IMRT commissioning: multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119. Med Phys. 2009 Nov;36(11):5359. 5373. https://doi.org/10.1118/1.3238104
  14. Clark CH, Hansen VN, Chantler H, et al. Dosimetry audit for a multi.centre IMRT head and neck trial. Radiother Oncol. 2009;93:102-108. https://doi.org/10.1016/j.radonc.2009.04.025
  15. Clark CH, Miles EA, Guerrero Urbano MT, et al. Pre.trial quality assurance processes for an intensitymodulated radiation therapy (IMRT) trial: PARSPORT, a UK multicentre Phase III trial comparing conventional radiotherapy and parotid.sparing IMRT for locally advanced head and neck cancer. Br J Radiol. 2009 Jul;82:585-594. https://doi.org/10.1259/bjr/31966505
  16. Pawlicki T, Yoo S, Court LE, et al. Moving from IMRT QA measurements toward independent computer calculations using control charts. Radiother Oncol. 2008;89:330-337. https://doi.org/10.1016/j.radonc.2008.07.002
  17. Palta JR, Liu C, Li JG. Quality assurance of intensity. modulated radiation therapy. Int J Radiat Oncol Biol Phys. 2008;71:S108-112. https://doi.org/10.1016/j.ijrobp.2007.05.092
  18. Losasso T. IMRT delivery performance with a Varian multileaf collimator. Int J Radiat Oncol Biol Phys. 2008;71:S85-88. https://doi.org/10.1016/j.ijrobp.2007.06.082
  19. Hellman S, Ling CC, Leibel SA, et al. A practical guide to intensity.modulated radiation therapy. A practical guide to intensity.modulated radiation therapy. Madison (WI): Medical Physics Publishing; 2003:158-159.
  20. Kim H, Park Y, Park J, et al. Assessment of Setup Errors and a New PTV Margin for Prostate Cancer Patients with an Endorectal Balloon. Int J Radiat Oncol Biol Phys. 2009;75:S637.
  21. Ibbott GS, Followill DS, Molineu HA, Lowenstein JR, Alvarez PE, Roll JE. Challenges in credentialing institutions and participants in advanced technology multi.institutional clinical trials. Int J Radiat Oncol Biol Phys. 2008;71:S71-75. https://doi.org/10.1016/j.ijrobp.2007.08.083
  22. Giorgia N, Antonella F, Eugenio V, Alessandro C, Filippo A, Luca C. What is an acceptably smoothed fluence? Dosimetric and delivery considerations for dynamic sliding window IMRT. Radiat Oncol. 2007 Nov;2:42-254. https://doi.org/10.1186/1748-717X-2-42
  23. Yan G, Liu C, Simon TA, Peng LC, Fox C, Li JG. On the sensitivity of patient.specific IMRT QA to MLC positioning errors. J Appl Clin Med Phys. 2009;10(1):120-128.
  24. Breen SL, Moseley DJ, Zhang B, Sharpe MB. Statistical process control for IMRT dosimetric verification. Med Phys. 2008 Oct;35(10):4417-4425. https://doi.org/10.1118/1.2975144