Home About us Editorial board Ahead of print Current issue Search Archives Submit article Instructions Subscribe Contacts Login 


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2012  |  Volume : 8  |  Issue : 1  |  Page : 86-90

Dosimetric validation of new semiconductor diode dosimetry system for intensity modulated radiotherapy


1 Department of Medical Physics, Tata Memorial Hospital, Parel, Mumbai, India
2 Department of Electronics, Brijlal Biyani Science College, Amravati, Maharashtra, India

Date of Web Publication19-Apr-2012

Correspondence Address:
Rajesh Kinhikar
Department of Medical Physics, Tata Memorial Hospital, Parel, Mumbai - 400 012
India
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.95180

Rights and Permissions
 > Abstract 

Introduction: The new diode Isorad was validated for intensity modulated radiotherapy (IMRT) and the observations during the validation are reported.
Materials and Methods: The validation includes intrinsic precision, post-irradiation stability, dose linearity, dose-rate effect, angular response, source to surface (SSD) dependence, field size dependence, and dose calibration.
Results: The intrinsic precision of the diode was more than 1% (1 σ). The linearity found in the whole range of dose analyzed was 1.93% (R 2 = 1). The minimum and maximum variation in the measured and calculated dose were found to be 0.78% (with 25 MU at ioscentre) and 4.8% (with 1000 MU at isocentre), respectively. The maximal variation in angular response with respect to arbitrary angle 0° found was 1.31%. The diode exhibited a 51.7% and 35% decrease in the response in the 35 cm and 20 cm SSD range, respectively. The minimum and the maximum variation in the measured dose from the diode and calculated dose were 0.82% (5 cm × 5 cm) and 3.75% (30 cm × 30 cm), respectively. At couch 270°, the response of the diode was found to vary maximum by 1.4% with ΁ 60 gantry angle. Mean variation between measured dose with diode and planned dose by TPS was found to be 1.3% (SD 0.75) for IMRT patient-specific quality assurance.
Conclusion: For the evaluation of IMRT, use of cylindrical diode is strongly recommended.

Keywords: Semiconductor diode, intensity modulated radiotherapy, quality assurance


How to cite this article:
Kinhikar R, Chaudhari S, Kadam S, Dhote D, Deshpande D. Dosimetric validation of new semiconductor diode dosimetry system for intensity modulated radiotherapy. J Can Res Ther 2012;8:86-90

How to cite this URL:
Kinhikar R, Chaudhari S, Kadam S, Dhote D, Deshpande D. Dosimetric validation of new semiconductor diode dosimetry system for intensity modulated radiotherapy. J Can Res Ther [serial online] 2012 [cited 2019 Oct 21];8:86-90. Available from: http://www.cancerjournal.net/text.asp?2012/8/1/86/95180


 > Introduction Top


Semiconductor diodes have been identified as a useful and practical device for in vivo dosimetry in radiotherapy (RT). [1],[2],[3],[4],[5],[6],[7] Various diodes are commercially available to implement in vivo dosimetry. Important factor to consider is the type of detector used for its geometry. Rapid developments in RT in terms of image-guided radiotherapy, intensity-modulated radiotherapy (IMRT) has imposed a new dosimetric challenge due to the inhomogeneous dose distribution in the planning target volume (PTV) and sharp dose gradient outside PTV. In addition, volumetric modulated arc therapy (VMAT) is now being increasingly preferred due to its advantage of minimal treatment time. To validate the VMAT dosimetry, the semiconductor diode with cylindrical shape may be useful. The Isorad? (Sun Nuclear, ML) diode is commercially available which fulfils these conditions. The validation of this diode for IMRT (static and VMAT) has not been reported yet.

This diode was validated for IMRT delivered with LINAC. The purpose of this article was to report the observations during the validation of this diode for IMRT.


 > Materials and Methods Top


The treatment machine used for all measurements is a dual-energy linear accelerator (Novalis Tx, Varian, Palo Alto, CA). The X-ray beam used from this machine has a nominal energy of 6 and 15 MV. All the IMRT plans and measurements were done with 6 MV. The 20- 10 cm depth tissue phantom ratio at 100-cm target-to-axis distance (TAD) for linac is 0.67. The reference dose rate for this beam has been established for a 10 × 10 cm 2 field size, at a target-to-surface distance (TSD) of 95 cm and 5 cm depth in water.

An n-type Isorad (Yellow) semiconductor diode as shown in [Figure 1] was used in all measurements (Sun Nuclear Corporation, Melbourne, FL). This diode has a cylindrical geometry and a buildup cap made of molybdenum with 1.6 g/cm 2 equivalent thickness. The detector diameter is 9.7 mm while the active diameter is 1.4 mm. The active detection area is 1.5 mm 1.5 mm with active volume of 0.02 mm 3 . Diode outputs were measured in an IVD model 1135 system (Sun Nuclear Corporation) consisting of two small electrometers (pods) interfaced to a PC computer.
Figure 1: An n-type Isorad (yellow) semiconductor diode

Click here to view


A solid water phantom was used for most diode tests and calibrations. The total phantom dimensions are 30 × 30 × 30 cm 3 and it is composed of several slabs with variable thickness, ranging between 0.1 and 7.0 cm.

A number of precalibration tests were performed to establish some important properties of the diodes before use for clinical purpose. Those were intrinsic precision, post-irradiation stability, dose linearity, dose-rate effect, angular response, SSD dependence, field size dependence, and dose calibration.

The intrinsic precision was evaluated with the diodes placed at the surface of the solid water phantom. TAD (center of diode) was 100 cm. Field size of 10 cm × 10 cm was selected, and 50 monitor units were delivered at the isocentre. Ten measurements of the diode response to the same irradiation conditions were carried out, and the standard deviation was calculated. [Figure 2] shows the setup of diode measurements with a Novalis Tx linear accelerator.
Figure 2: The setup of diode measurements with a Novalis Tx linear accelerator

Click here to view


For post-irradiation stability, the amount of variation in diode response was recorded 5 minutes after irradiation ceased.

Dose linearity was assessed by recording the diode response to incremental amounts of monitor units or dose. The diode was irradiated for various MUs (25, 50, 100, 150, 200, 300, 400, 600, 800, and 1000) at TAD 100 cm for same setup. The measured dose from the diode was compared with the calculated dose.

Under same experimental setup, diode was irradiated at various dose rates from the linear accelerator and the response was recorded.

It is known that, theoretically, angular dependence is not a factor to be taken into account in measurements carried out with this kind of diodes due to their cylindrical symmetry; however, to verify this assumption, diode was irradiated with various gantry angle (incremental of 40°). For all the irradiations, diode was placed at the edge of the treatment couch top for field size of 10 cm × 10 cm with TAD 100 cm (100 MU). The angular response was recorded for 9 angles around the diode and normalized to the response obtained for gantry angle 0°.

The diode was kept at TAD 100 cm at 5 cm depth in solid water slabs. The TSD was changed and the diode reading was recorded. The measurements were carried out for 10 cm × 10 cm field size.

Under same experimental setup, diode was irradiated for various field sizes (5, 10, 15, 20, 25, and 30 cm × cm) for 100 MU at TAD 100 cm. The measured dose with diode was compared with the calculated dose.

The diode calibration was done for 10 cm × 10 cm field size at TAD 100 cm. The dose rate was selected as 300 MU/minute and diode was irradiated for 100 and 200 MU. The diode was placed at the surface of the solid water phantom slab at TAD 100 cm. The diode reading was recorded as a measured reading and was compared with the calculated.

The diode was placed with TAD 100 cm at the edge of the treatment couch with couch angle as 270°. The field size was 10 cm × 10 cm and the diode was irradiated for 100 MU for various gantry angles (30° increments). The measured dose from the diode was recorded and the response of the diode with respect the non-coplanar irradiations was estimated. Similar measurements were repeated for couch angle of 90° as well.

The diode was validated for patient-specific IMRT quality assurance for two patients on Novalis Tx linear accelerator for 6 MV X-rays. The diode was placed at 5 cm depth with TAD 100 cm in a solid water slab. The diode was irradiated for the IMRT fluence from the patient treatment plan (with the gantry and the couch fixed at 0°). The reading obtained from the diode (measured dose) was compared with the planned dose from the Eclipse treatment planning system.


 > Results Top


The intrinsic precision of the diode was more than 1% (1 σ). In the case of post-irradiation stability, the diode did not show significant variation in reading. The maximal variation in reading observed was 0.15% in 5 minutes. [Figure 3] shows response of the diode with respect to the monitor units (25-1000 MU). The linearity found in the whole range of dose analyzed was 1.93% (R2 = 1). The minimum and maximum variation in the measured and calculated dose were found to be 0.78% (with 25 MU at ioscentre) and 4.8% (with 1000 MU at isocentre), respectively.
Figure 3: Response of the diode with respect to the monitor units (25– 1000 MU)

Click here to view


[Figure 4] shows the response of the diode with respect to various gantry angles. The maximal variation in angular response with respect to arbitrary angle 0° found was 1.31%. The minimum variation was 0.8%.
Figure 4: Response of the diode with respect to various gantry angles

Click here to view


[Figure 5] shows the SSD dependence of the diode. The diode exhibited a 51.7% decrease in the response in the 35 cm SSD range, while it showed a decrease in the response by 35% in 20 cm SSD range.
Figure 5: SSD dependence of the diode

Click here to view


[Figure 6] shows the response of the diode with respect to the field size. The minimum and the maximum variation in the measured dose from the diode and calculated dose were 0.82% (5 cm × 5 cm) and 3.75% (30 cm × 30 cm), respectively.
Figure 6: Response of the diode with respect to the fi eld size

Click here to view


The variation between measured dose by the diode and calculated dose for 100 and 200 MU was found to be 0.49% and 1.07%, respectively. The calculated dose was from the ion chamber measurements. Thus, the reading to dose calibration was satisfactory. [Table 1] shows the comparison of the measurements and percentage variation.
Table 1: The comparison of the measurements and percentage variation between measured dose by the diode and calculated dose for 100 and 200 MU. The calculated dose was from the ion chamber measurements. Thus, the reading to dose calibration was satisfactory

Click here to view


[Table 2] shows the response of the diode in the non-coplanar irradiation geometry. At couch 270°, the response of the diode was found to vary maximum by 1.4% with ± 60° gantry angle. This proved that the diode does not show significant variation in the response in the oblique irradiation with couch at 270°. Similar was the case for 90° as well. However, beyond 60° gantry angle, the variation in the response was around 13.5%. Nevertheless, this irradiation geometry is not clinically practised.
Table 2: The response of the diode in the non-coplanar irradiation geometry. At couch 270 degrees, the response of the diode was found to vary maximum by 1.4% with +/- 60 degrees gantry angle. Couch position was fi xed at 270 degrees for all measurements in this setup

Click here to view


Mean variation between measured dose with diode and planned dose by TPS was found to be 1.3% (SD 0.75). [Table 3] shows the validation of the diode for IMRT patient-specific QA for two patients. The diode response was satisfactory for the IMRT as well.
Table 3: The validation of the diode for IMRT patient specifi c QA for two patients

Click here to view



 > Discussion Top


The diodes exhibit a good intrinsic precision, stability, and linearity in the range of dose used. The buildup cap seems to be enough to guarantee charged particle equilibrium in the quality of our beam, and cylindrical symmetry is also good enough for not having to consider major correction factors. The diode response was found stable after irradiation time of 5 minutes. The diode showed a linear relation with the delivered monitor units. The diode response was also compared with the delivered dose for respective monitor units. The diode showed a good agreement with the calculated dose. The authors [3],[4],[5],[6] studied various parameters for the diode which is different from this diode. However, our results are in good agreement with these studies.

The diode showed almost an isotropic response for various gantry angles. This has been an advantage similar to the ion chamber since this diode is cylindrical in shape.

The diode exhibited a significant perturbation in the response with SSD in the range of 20-35 cm from the isocentre. The decrease in the response was correlated to the calculated readings by inverse square law method and was comparable.

The minimum and the maximum variation in the measured dose were 0.82% (5 cm × 5 cm) and 3.75% (30 cm × 30 cm), respectively. The response of the diode for respective field size was compared with the ion chamber measured output which is clinically used.

Before using diode for clinical applications, it was necessary to compare its response with the reference detector. The observations in the errors during in vivo measurements for individual patients have been reported. [7],[8],[9],[10] The variation between measured dose by the diode and calculated dose for 100 and 200 MU was found to be 0.49% and 1.07%, respectively. The calculated dose was from the cylindrical ion chamber measurements. In few clinical situations (CNS and head and Neck Tumours), non-coplanar beams need to be planned. The diode response was verified in this geometry as well. [Table 2] shows the response of the diode in the non-coplanar irradiation geometry. At couch 270°, the response of the diode was found to vary maximum by 1.4% with ± 60° gantry angle. This proved that the diode does not show significant variation in the response in the oblique irradiation with couch at 270°. Similar was the case for 90° as well. Usually, non-coplanar beams upto ± 30° are used. However, beyond 60° gantry angle, the variation in the response was around 13.5%. But this irradiation geometry is not advised which involves the stem and cable irradiations.

The diode was validated for IMRT patient-specific dosimetry for two patients. The measurements with the diode were in good agreement with the TPS. Thus, the diode was validated for IMRT patient-specific quality assurance.


 > Conclusion Top


This work described the validation of a cylindrical diode for IMRT patient-specific quality assurance. The behavior of this diode was found adequate for the assessment of the dose delivered in IMRT treatments. The results of this study have demonstrated that this diode is suitable for in vivo dosimetry for IMRT. For the evaluation of IMRT, use of cylindrical diode is strongly recommended.

 
 > References Top

1.Alecu R, Feldmeier JJ, Alecu M. Dose perturbations due to in vivo dosimetry with diodes. Radiother Oncol 1997;42:289-91.  Back to cited text no. 1
    
2.Rodríguez ML, Abrego E, Pineda A. Implementation of in vivo dosimetry with Isorad semiconductor diodes in radiotherapy treatments of the pelvis. Med Dosim 2008;33:14-21.  Back to cited text no. 2
    
3.Saini AS, Zhu TC. Energy dependence of commercially available diode detectors for in-vivo dosimetry. Med Phys 2007;34:1704-11.  Back to cited text no. 3
    
4.Saini AS, Zhu TC. Dose rate and SDD dependence of commercially available diode detectors. Med Phys 2004;31:914-24.  Back to cited text no. 4
    
5.Saini AS, Zhu TC. Temperature dependence of commercially available diode detectors. Med Phys 2002;29:622-30.  Back to cited text no. 5
    
6.Zhu XR. Entrance dose measurements for in-vivo diode dosimetry: Comparison of correction factors for two types of commercial silicon diode detectors. J Appl Clin Med Phys 2000;1:100-7.  Back to cited text no. 6
    
7.Noel A, Aletti P, Bey P, Malissard L. Detection of errors in individual patients in radiotherapy by systematic in vivo dosimetry. Radiother Oncol 1995;34:144-51.  Back to cited text no. 7
    
8.Kutcher GJ, Coia L, Gillin M, Hanson WF, Leibel S, Morton RJ, et al. Comprehensive QA for radiation oncology: Report of AAPM Radiation Therapy Committee Task Group 40. Med Phys 1994;21:581-618.  Back to cited text no. 8
    
9.Essers M, Mijnheer BJ. In vivo dosimetry during external photon beam radiotherapy. Int J Radiat Oncol Biol Phys 1999;43:245-59.  Back to cited text no. 9
    
10.Meiler RJ, Podgorsak MB. Characterization of the response of commercial diode detectors used for in vivo dosimetry. Med Dosim 1997;22:31-7.  Back to cited text no. 10
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

  >Abstract>Introduction>Materials and Me...>Results>Discussion>Conclusion>Article Figures>Article Tables
  In this article
>References

 Article Access Statistics
    Viewed2415    
    Printed118    
    Emailed2    
    PDF Downloaded194    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]