Journal of Medical Physics
 Home | Search | Ahead of print | Current Issue | Archives | Instructions | Subscription | Login  The official journal of Association of Medical Physicists of India      
 Users online: 343  Home  EMail this page Print this page Decrease font size Default font size Increase font size 


 
ORIGINAL ARTICLE
Year : 2007  |  Volume : 32  |  Issue : 3  |  Page : 103-107
 

Dose linearity and uniformity of Siemens ONCOR impression plus linear accelerator designed for step-and-shoot intensity-modulated radiation therapy


Department of Radiation Oncology, Ruby Hall Clinic, Pune, India

Date of Submission10-Mar-2007
Date of Acceptance19-Jun-2007

Correspondence Address:
Janhavi R Bhangle
Department of Radiation Oncology, Ruby Hall Clinic, 40, Sassoon Road, Pune - 411 001
India
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-6203.35722

Rights and Permissions

 

   Abstract 

For step-and-shoot type delivery of intensity-modulated radiation therapy (IMRT), beam stability characteristics during the first few monitor units need to be investigated to ensure the planned dose delivery. This paper presents the study done for Siemens ONCOR impression plus linear accelerator before commissioning it for IMRT treatment. The beam stability for 6 and 15 MV in terms of dose monitor linearity, monitor unit stability and beam uniformity is investigated in this work. Monitor unit linearity is studied using FC65G chamber for the range 1-100 MU. The dose per MU is found to be linear for small monitor units down to 1 MU for both 6 and 15 MV beams. The monitor unit linearity is also studied with portal imaging device for the range 1-20 MU for 6 MV beam. The pixel values are within ±1σ confidence level up to 2 MU; for 1 MU, the values are within ±2σ confidence level. The flatness and symmetry analysis is done for both energies in the range of 1-10 MU with Kodak diagnostic films. The flatness and symmetry are found to be within ±3% up to 2 MU for 6 MV and up to 3 MU for 15 MV.


Keywords: Flatness, linearity, low MU, step-and-shoot intensity-modulated radiation therapy, symmetry


How to cite this article:
Bhangle JR, Sathiya Narayanan V K, Deshpande SA. Dose linearity and uniformity of Siemens ONCOR impression plus linear accelerator designed for step-and-shoot intensity-modulated radiation therapy. J Med Phys 2007;32:103-7

How to cite this URL:
Bhangle JR, Sathiya Narayanan V K, Deshpande SA. Dose linearity and uniformity of Siemens ONCOR impression plus linear accelerator designed for step-and-shoot intensity-modulated radiation therapy. J Med Phys [serial online] 2007 [cited 2020 Jan 20];32:103-7. Available from: http://www.jmp.org.in/text.asp?2007/32/3/103/35722


Intensity-modulated radiation therapy (IMRT) is becoming the commonly chosen treatment modality because of its dose conformity to the concave tumor volumes, sparing surrounding normal organs. Siemens ONCOR Impression Plus linear accelerator with photon beam energies of 6 and 15 MV equipped with double-focused multileaf collimator (MLC) has been installed in our clinic. These accelerators are designed to deliver intensity-modulated fields using step-and-shoot technique. In this type of IMRT delivery, multiple segments are the result of the optimization process to conform the dose to the target volume. These segments are delivered in a stack arrangement. Each beam segment is delivered separately with the radiation beam turned off (paused) [1] when the MLC moves to take a shape of the next segment. The composite of dose increment delivered to each segment delivers the planned intensity modulation. The accuracy of intensity modulation depends on the capabilities and limitations of the linear accelerator equipped with the multi-leaf collimator to deliver low monitor unit (MU) segments. For complex tumor-normal tissue configuration, IMRT plan can involve large numbers of field segments, leading to many very small fields and low doses per segment.

The operation of the linear accelerator delivering low monitor unit segments needs to be investigated for every linear accelerator prior to commissioning the linear accelerator for IMRT. [2] Many authors have studied the performances of different linear accelerators in low-MU region using different methods. [1],[3],[4],[5],[6] Dose linearity, uniformity and monitor unit stability in low-MU region for Siemens ONCOR Impression Plus linear accelerator for both 6 and 15 MV photon energies were investigated in this work.


   Materials and Methods Top


Dose monitor linearity and monitor unit stability at small monitor units for 6 and 15 MV were examined using a FC65G ionization chamber connected to DOSE1 electrometer. The measurements were done at 100 cm SSD with the chamber at d max in the SP34 slab phantom. [1],[4] The collimator settings were 10 cm × 10 cm. The measurements were done for 6 MV photon beam with dose rate of 300 MU/min and for 15 MV with dose rate of 500 MU/min. For low MU linearity, ionization readings were recorded for 100, 50, 20, 15, 10, 5, 3, 2 and 1 MU. These readings were normalized to 2 MU and to 100 MU reading, and the corresponding percentage variation for all other MUs was calculated. [3] The monitor unit linearity for 6 MV was also studied using OPTIVUE AG9 flat panel with 1024 x 1024 pixels. For the field size of 10 cm x10 cm, portal images were obtained for 20, 15, 10, 5, 3, 2 and 1 MU with the flat panel at 145 cm. The statistical analysis of the data is presented.

Monitor unit stability was studied with the same setup as described above. For the collimator setting of 10 cm x 10 cm, the ionization was recorded for 10 MU. It was compared with the cumulative ionization of 5 MU measured two times; 2 MU measured five times, 1 MU measured ten times. [4] Similarly, ionization recorded for 15 MU was compared with the cumulative ionization of 5 MU measured three times; 3 MU measured five times, 1 MU measured fifteen times.

Film dosimetry was used to study dose uniformity for low monitor units. [5] Beam flatness and symmetry for 6 and 15 MV were evaluated with Kodak diagnostic film. For SFD 100 cm with collimator setting of 10 cm x 10 cm, exposures of 10, 5, 3, 2 and 1 MU were given. Vidar VXR16 Dosimetry Pro scanner, along with Omni-Pro IMRT software, was used to analyze these films for flatness and symmetry. [7]


   Results and Discussion Top


For low-MU linearity, ionization readings were recorded in the range of 1-100 MU. [Figure - 1] shows the percentage variation when these readings were normalized to 2 MU reading and to 100 MU reading. The percentage variation in the relative ionizations is within 2% for low monitor units such as 1 MU.

Similar study done for 15 MV is shown in [Figure - 2]. For 15 MV beam, the percentage variation in the relative ionization is within 2% up to 2 MU. For 1 MU the variation is high.

[Figure - 3],[Figure - 4] show linearity of charge measured as a function of monitor unit for 6 MV and 15 MV beam respectively. For both the energies, the charge is found to be linear down to 1 MU.

[Figure - 5] represents the change in the pixel values obtained from the portal imaging device as a function of monitor units in the range of 1-20 MU. It is observed that the pixel values are not exactly linear with monitor units. This can be because of the response of the flat panel. The flat panel gets saturated above 15 MU. The deviation from the mean curve is calculated for every data set. It is observed that the pixel values for 2-20 MU are within ±1σ confidence level. Only for 1 MU, the pixel values are within ±2σ confidence level.

[Figure - 6]A and B show monitor unit stability for 6 MV. The variation in the cumulative ionization recorded for 5 MU, measured two times, is −1.0%. The variation in the cumulative ionization recorded for 2 MU, measured five times; and that for 1 MU, measured ten times, from the ionization recorded for 10 MU is within 1.0%. Similarly, for 15 MU the variation in the cumulative ionization recorded for 5 MU, measured three times, is −1.45%; while that measured for 3 MU five times and 1 MU fifteen times is 0.31 and 1.01% respectively.

For 15 MV [Figure - 7]A, the variation in the cumulative ionization recorded for 5 MU, measured two times, from the ionization recorded for 10 MU is 1.06%; for 2 MU, measured five times, the variation is 0.45%. But for the cumulative ionization recorded for 1 MU ten times, the variation is found to be 16%.

For 15 MU, as shown in [Figure - 7]B, the variation for 5 MU, measured three times; and 3 MU, measured five times, from the ionization recorded for 15 MU is 0.24 and −0.21% respectively. The variation in the cumulative ionization for 1 MU, measured fifteen times, from that of 15 MU is found to be 13.29%. The time required to deliver 1 MU of 15 MV at the dose rate of 500 MU/min is ~ 120 msec, and that required for 1 MU of 6 MV at the dose rate of 300 MU/min is ~ 200 msec. It is interesting to note that the ramp-up time specified by the manufacturer is 250 msec. [8] This could be one of the reasons for the high variation in the cumulative ionization of 1 MU, measured ten and fifteen times, for 6 and 15 MV.

Flatness symmetry study was done with Kodak diagnostic films. Flatness was calculated using 'variation over mean' (80%) method. The symmetry value was obtained with 'point difference quotient' method. [5] For 6 MV beam [Figure - 8], flatness in both planes is within 3%. The crossplane symmetry for 1 MU of 6 MV is 3.41%; and for 2 MU, 3.16%. For 3, 5 and 10 MU, the crossplane symmetry is within 2% for 6 MV; while the inplane symmetry values are within 2% for 1-10 MU [Figure - 9].

For 15 MV beam, inplane and crossplane flatness for 1 MU is 4.11 and 4.88% respectively. For 2, 3 5 and 10 MU, the crossplane flatness values are within 2%. The inplane flatness is within 3% for 2, 5 and 10 MU; while it is 3.97% for 3 MU [Figure - 10].

The symmetry analysis for 15 MV [Figure - 11] shows that for 1 MU of 15 MV beam, the crossplane and inplane symmetry are 6.24 and 9.59% respectively. For 2, 3, 5 and 10 MU, the crossplane symmetry values are within 3%. The inplane symmetry for 2 MU is 4%; for 3 MU, 7%; for 5 MU, 3.3%; and for 10 MU, it is 2.84%.


   Conclusion Top


For 6 MV beam, the stability of the beam at low MU in terms of cGy/MU is within ±2%, assuming all other factors are proportional while converting the charge to cGy. The stability in terms of flatness and symmetry is also acceptable and is within ±3% up to 2 MU. The stability of 15 MV beam in terms of flatness and symmetry is within ±3% above 3 MU.

However, it is suggested that segments with monitor units less than 5 MU should be avoided. The inverse planning system used at our center takes care of prevention of small MU segments such as 5 MU from the treatment plan. So for the range of monitor units used for patient treatment, the Siemens ONCOR Impression Plus linear accelerator has been delivering stable beams for step-and-shoot IMRT treatments.

 
   References Top

1.Cheng CW, Das IJ. Comparison of beam characteristics in intensity modulated radiation therapy (IMRT) and those under normal treatment condition. Med Phys 2002;29:226-30.  Back to cited text no. 1  [PUBMED]  
2.Williams PC. IMRT: Delivery techniques and quality assurance. Br J Radiol 2003;76:766-76.  Back to cited text no. 2  [PUBMED]  [FULLTEXT]
3.Sharpe MB, Miller BM, Yan D, Wong JW. Monitor unit settings for intensity modulated beams delivered using a step-and-shoot approach. Med Phys 2000;27:2719-25  Back to cited text no. 3  [PUBMED]  
4.Aspradakis MM, Walker CP, Steele A, Lambert GD. Chapter 12- Radiotherapy Physics "IPEM 8 th Annual Scientific Meeting, Durham," 73-74, September 2002.  Back to cited text no. 4    
5.Saw CB, Li S, Ayyangar KM, Yoe-Sein M, Pillai S, Enke CA, et al . Dose linearity and uniformity of a linear accelerator designed for implementation of multileaf collimation system-based intensity modulated radiation therapy. Med Phys 2003;30:2253-6.  Back to cited text no. 5  [PUBMED]  
6.Reena P, Dayananda S, Pai R, Jamema SV, Gupta T, Deepak D, et al . Performance characterization of Siemens Primus linear accelerator under small monitor unit and small segments for the implementation of step-and shoot IMRT. J Med Phys 2006;31:269-74.  Back to cited text no. 6    
7.Method to calculate flatness and symmetry values - OmniPro IMRT software.  Back to cited text no. 7    
8.Mitra R, Bayouth J, Das I. Startup characteristics and Dosimetry of small monitor unit segments in Step-and-Shoot IMRT delivery for Two Types of Digital Linear Accelerators" Presented at the 43 rd annual AAPM meeting, 2001.  Back to cited text no. 8    


    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11]


This article has been cited by
1 Characteristics of flattening filter free beams at low monitor unit settings
Yuichi Akino,Seiichi Ota,Shinichi Inoue,Hirokazu Mizuno,Iori Sumida,Yasuo Yoshioka,Fumiaki Isohashi,Kazuhiko Ogawa
Medical Physics. 2013; 40(11): 112101
[Pubmed] | [DOI]
2 The quality control of intensity modulated radiation therapy (IMRT) for ONCOR siemens linear accelerators using film dosimetry
Jabbari, K. and Amouheidari, A. and Babazadeh, S.
Iranian Journal of Medical Physics. 2012; 9(2 A): 111-125
[Pubmed]
3 Dose linearity and monitor unit stability of a G4 type cyberknife robotic stereotactic radiosurgery system
Sudahar, H. and Kurup, P.G.G. and Murali, V. and Velmurugan, J.
Journal of Medical Physics. 2012; 37(1): 4-7
[Pubmed]
4 Beam characteristics of megavoltage beams at low monitor unit settings
G. Kemikler, H. Acun
Physica Medica. 2011; 27(4): 203
[VIEW] | [DOI]
5 Beam characterization for step-and-shoot IMRT
Batel, V.P., Figueira, A.R., Carvalho, A.L., Reis, T.
IFMBE Proceedings. 2009; 25(1): 1038-1041
[Pubmed]
6 Clinac performance: Watching the first seconds
Figueira, A.R., Batel, V.P., Carvalho, A.L., Reis, T.
IFMBE Proceedings. 2009; 25(1): 1060-1062
[Pubmed]
7 Evaluation of dosimetric performance of "Siemens ONCOR" linear accelerator with small monitor unit values and small segment sizes and consistency with treatment planning system | ["Siemens ONCOR" tedavi cihazinin küçük monitor unit deǧerlerinde ve küçük segment boyutlarinda dozimetrik performansinin deǧerlendirilmesi ve planlama sistemi ile uyumu]
Ceylan, C., Baş, H., Kiliç, A., Küçük, N., Güden, M., Engin, K.
Turk Onkoloji Dergisi. 2009; 24(2): 73-79
[Pubmed]



 

Top
Print this article  Email this article
Previous article Next article

    

 
   Search
 
   Next article
   Previous article 
   Table of Contents
  
    Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
    Article in PDF (283 KB)
    Citation Manager
    Access Statistics
    Reader Comments
    Email Alert *
    Add to My List *
* Registration required (free)  


    Abstract
    Materials and Me...
    Results and Disc...
    Conclusion
    References
    Article Figures

 Article Access Statistics
    Viewed4800    
    Printed157    
    Emailed2    
    PDF Downloaded367    
    Comments [Add]    
    Cited by others 7    

Recommend this journal