Evaluation of Six-Dimensional Cranial Target Positioning Accuracy in Two Different Immobilization Methods Using Exactrac System

Document Type : Original Paper


1 Department of Oncology, Apollo Cancer Hospital, Hyderabad, Telangana, India

2 Centre for Biomaterials, Cellular and Molecular Theranostics, Vellore Institute of Technology, Vellore, Tamil Nadu, India

3 Department of Physics, School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu, India


Introduction: The aim of this study was to determine the accuracy of two different immobilization methods in patient positioning in cranial radiotherapy. The six-dimensional (6D) target localization accuracy of using a dedicated stereotactic mask was compared with that of a conventional head mask by the ExacTrac system.
Material and Methods: A total of 56 patients with cranial lesions were included in this study (26 patients with a dedicated stereotactic mask and 30 subjects with a conventional head mask). The ExacTrac image-guided positioning system was utilized to obtain daily translational and rotational patient positioning displacement from the intended position. The 6D setup data was analyzed to obtain population mean, systematic and random errors, and three-dimensional (3D) vector shifts in all the patients.
Results: The population mean values of setup errors were comparable with both immobilization systems; however, the spread as indicated by population systematic and population random errors was more in the use of a conventional head mask. The mean values of the 3D vector shifts were 2.09±1.00 and 4.51±3.38 mm with the use of a dedicated stereotactic mask and conventional head mask, respectively. The frequency distribution of maximum rotational deviation and statistical analysis demonstrated a significant difference in immobilization accuracy between stereotactic immobilization and 3-clamp immobilization (p <0.05).
Conclusion: The results revealed that there was a significant reduction in target positioning errors with a dedicated stereotactic mask, compared to that with a conventional cranial mask. Furthermore, a dedicated stereotactic mask is required to keep rotational deviations within system correctable limits.


Main Subjects



    1. Stroom JC, de Boer HC, Huizenga, Visser AG. Inclusion of geometrical uncertainties in radiotherapy treatment planning by means of coverage probability. Int J. Radiat. Oncol. Biol. Phys. 1999;43: 905-19.
    2. Van Herk M, Remijer P, Rasch C, Lebesque JV. The probability of correct target dosage: dose-population histogram for deriving treatment margins in radiotherapy. Int J. Radiat. Oncol. Biol. Phys. 2000; 47: 1121-35.
    3. Lightstone AW, Benedict SH, Bova FJ, Solberg TD, Stern RL. Intracranial stereotactic positioning systems: Report of the American Association of Physicists in Medicine radiation therapy committee Task Group No. 68. 2005.
    4. Meeks SL, Bova FJ, Friedman WA, Buatti JM, Moore RD. IrLED-based patient localization for linac radiosurgery. Int J. Radiat. Oncol. Biol. Phys. 1998; 41:433-9.
    5. Burton KE, Thomas SJ, Whitney D, Routsis DS, Benson RJ, Burnet NG. Accuracy of a relocatable stereotactic radiotherapy head frame evaluated by use of a depth helmet. Clin Oncol.  2002; 14:31-9.
    6. Masi L, Casamassima F, Polli C, Menichelli C, Bonucci I, Cavedon C. Cone beam CT image guidance for intracranial stereotactic treatments: Comparison with a frame guided setup. Int J. Radiat. Oncol. Biol. Phys. 2008; 71:926-33.
    7. Tryggestad E, Christian M, Ford E, Kut C, Le V, Sanguineti G, et al. Inter-and intrafraction patient positioning uncertainty for intracranial radiotherapy: A study of four frameless thermoplastic mask based immobilization strategies using daily cone-beam CT. Int J. Radiat. Oncol. Biol. Phys. 2011; 80:281-90.
    8. Infusino E, Trodella L, Ramella S, D'Angelillo RM, Greco C, Lurato A, et al. Estimation of patient setup uncertainty using BrainLAB ExacTrac X-ray 6D system in image-guided radiotherapy. J Appl Clin Med Phys. 2015; 16:99-107.
    9. Se An Oh, Ji Woon Yea, Min Kyu Kang, Jae Won Park, Sung Kyu Kim. Analysis of setup uncertainty and margin of the daily ExacTrac 6D image guide system for patients with brain tumors. PLos ONE. 2016; 11.
    10. Dhabaan A, Schreibmann E, Siddiqi A, Elder E, Fox T, Oqunleye T, et al. Six degrees of freedom CBCT-based positioning for intracranial targets treated with frameless stereotactic radiosurgery. J Appl Clin Med Phys. 2012; 13:215-25.
    11. Guckenberger M, Meyer J, Vordermark D, Baier K, Wilbert J, Flentie M. Magnitude and clinical relevance of translational and rotational patient setup errors: A cone-beam CT study.  Int J. Radiat. Oncol. Biol. Phys. 2006; 65:934-42.
    12. Winey B, Bussiere M. Geometric and dosimetric uncertainties in intracranial stereotactic treatments for multiple non-isocentric lesions. J Appl Clin Med Phys. 2014; 15: 122-32.
    13. Roper J, Chanyavanich V, Betzel G, Switchenko J, Dhabaan A. Single-isocenter multiple target SRS: Risk of compromised coverage. Int J. Radiat. Oncol. Biol. Phys. 2015; 93:540-6.
    14. Briscoe M, Voraney JP, Ploquin N. Establishing a threshold for rotational patient setup errors in linear accelerator based stereotactic radiosurgery. Biomed Phys Eng Exp. 2016; 2:045018.
    15. Van Herk M. Errors and margins in radiotherapy. Seminars in radioation oncology. 2004; 14: 52-64.
    16. Stanhope C, Chang Z, Wang Z, Yin FF, Kim G, Salama JK, et al. Physics considerations for single-isocenter volumetric modulated arc radiosurgery for treatment of multiple intracranial targets. Practical Radiation Oncology. 2016; 6:207-13.
    17. Lutz W, Winston KR, Maleki N. A system for sterotactic radiosurgery with a linear accelerator. Int J. Radiat. Oncol. Biol. Phys. 1988; 14: 373-81.
    18. Gevaert T, Verellen D, Tournel K, Linthout N, Bral S, Engels B, et al. Setup accuracy of the Novalis ExacTrac 6DOF system for frameless radiosurgery. Int J. Radiat. Oncol. Biol. Phys. 2012; 82: 1627-35.
    19. Schell Mc, Bova FJ, Larson DA, Leavitt DD, Lutz WR, Podgarsak EB, et  al. AAPM Report No.54:Stereotactic radiosurgery. Report of Task Group 42 Radiation Therapy Committee. American Institute of Physics. 1995.
    20. Takakura T, Mizowaki T, Nakata M, Yano S, Fujimoto T, Miyabe Y, et al. The geometric accuracy of frameless stereotactic radiosurgery using a 6D robotic couch system. Phys Med Biol. 2010; 55:1-10.
    21. Keeling V, Hossain S, Jin H, Algan O, Ahmad S, Ali I. Quantitative evaluation of patient setup uncertainty of stereotactic radiotherapy with the frameless 6D ExacTrac system using statistical modeling. J Appl Clin Med Phys. 2016; 17:111-27.


Volume 17, Issue 5
September and October 2020
Pages 308-315
  • Receive Date: 18 October 2018
  • Revise Date: 10 October 2019
  • Accept Date: 11 October 2019