Journal of Cancer Research and Therapeutics

: 2021  |  Volume : 17  |  Issue : 4  |  Page : 870--874

Output factor measurements with multiple detectors in CyberKnife® Robotic Radiosurgery System

Muthukumaran Manavalan1, Manigandan Durai2, Ganesh Narayanasamy3, Sotirios Stathakis4, Henry Finlay Godson1, Vikraman Subramani5,  
1 Research & Development Centre, Bharathiyar University, Coimbatore, Tamil Nadu, India
2 Department of Radiation Oncology, The Medicity, Gurugram, Haryana, India
3 Department of Radiation Oncology, University of Arkansas for Medical Sciences, Little Rock, AR, USA
4 University of Texas Health Science Center San Antonio, TX, USA
5 Brunei National Cancer Centre, Jerudong, Brunei

Correspondence Address:
Muthukumaran Manavalan
Research & Development Centre, Bharathiar University, Coimbatore, Tamil Nadu 641 046


Aim: The aim of this study was to measure and compare the output factor (OF) of a CyberKnife Robotic Radiosurgery System with eight different small field detectors and validate with Technical Report Series (TRS) report 483. Background: Accurate dosimetry of CyberKnife system is limited due to the challenges in small field dosimetry. OF is a vital dosimetric parameter used in the photon beam modeling and any error would affect the dose calculation accuracy. Materials and Methods: In this study, the OF was measured with eight different small-field detectors for the 12 IRIS collimators at 800 mm SAD setup at 15 mm depth. The detectors used were PTW 31016 PinPoint 3D, IBA PFD shielded diode, IBA EFD unshielded diode, IBA SFD unshielded diode (stereotactic), PTW 60008 shielded diode, PTW 60012 unshielded diode, PTW 60018 unshielded diode (stereotactic), and PTW 60019 CVD diamond detector. OF was obtained after correcting for field output correction factors from IAEA TRS No. 483. Results: The field OFs in CyberKnife are derived from the measured data by applying the correction factors from Table 23 in TRS 483 for the eight small field detectors. These field OFs matched within 2% of peer-reviewed published values. The range and standard deviation showed a decreasing trend with collimator diameter. Conclusion: The field OF obtained after applying the appropriate correction factor from TRS 483 matched well with the peer-reviewed published OFs. The inter-detector variation showed a decreasing trend with increasing collimator field size. This study gives physicists confidence in measuring field OFs while using small field detectors mentioned in this work.

How to cite this article:
Manavalan M, Durai M, Narayanasamy G, Stathakis S, Godson HF, Subramani V. Output factor measurements with multiple detectors in CyberKnife® Robotic Radiosurgery System.J Can Res Ther 2021;17:870-874

How to cite this URL:
Manavalan M, Durai M, Narayanasamy G, Stathakis S, Godson HF, Subramani V. Output factor measurements with multiple detectors in CyberKnife® Robotic Radiosurgery System. J Can Res Ther [serial online] 2021 [cited 2022 Jan 26 ];17:870-874
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Full Text


The selection of detectors in small fields can pose challenges. The protocols, standards, and code of practice followed in conventional linac are not applicable to the small fields used in CyberKnife. The ion chamber, which is the gold standard for the measurements in conventional radiotherapy, is not suitable for dosimetry in small fields as we deviate from the reference conditions. For small field dosimetry, we require the detectors with high spatial resolution, and the detector has to be water equivalent. The detector should have minimum volume averaging and perturbation effects. The solid-state detectors have high spatial resolution with less volume averaging effect than the micro ion chambers. However, these detectors overestimate in small fields due to density perturbation. Das et al. have explored small fields dosimetry that depends on dose disequilibrium, source size, and selection of detectors.[1]

Currently, many small field detectors are available in the market, each with its advantages and disadvantages. Recent literature affirm that preference should be given to a microdiamond or diode detector for small field output factor (OF) measurement. These detectors have smaller corrections than microchambers and less sensitive to inter-unit variations in beam profiles. Studies show the deviations in the OF measured with different detectors are as high as 30%, which is not acceptable in clinical radiotherapy.[2],[3] With technological advancements, many new detectors have been introduced to the market for small field dosimetry. A recent multi-site study by Russo et al. has examined the dosimetric parameters of CyberKnife using a stereotactic diode.[4]

While the determination of the field OFs is based on a relative measurement, it is still required to correct all the detectors reading to account for the quantities which influence the detector reading. The significant influence will be temperature, dose rate, and dose per pulse dependence of the detectors. The over response due to the high mass density of the detector's active volume should also be considered while correcting the OF. IAEA Technical Report Series (TRS) 483 has come out with the output correction factors for various small field detectors as a function of field size for different treatment machines.

Manavalan et al. have measured compared and analyzed various dosimetric parameters measured with eight different small field detectors in CyberKnife.[5] Our study used eight different detectors from two different vendors (IBA and PTW) for field OF measurements in a CyberKnife unit. We corrected the measured reading with the output correction factors listed in Table 23 of TRS 483 to compare and validate the OF derived from all the eight different detectors in the CyberKnife Robotic Radiosurgery System. Cross-validation of the measurements against values published in multi-site studies also increases confidence in the numbers.

 Materials and Methods

CyberKnife® (Accuray Inc., Sunnyvale, California) is the first frameless radiosurgery system with real-time tracking accounting for tumor motion and correcting for tumor and patient movement.[1] All measurements in this study were performed on a G4 CyberKnife® unit. The CyberKnife® consists of an X-band linac mounted on a robotic manipulator with 6° of freedom. It has a nominal energy of 6 MV with a quality index between 0.62 and 0.67 for a 60 mm collimator at a source-to-axis (SAD) distance of 800 mm. The machine can produce different dose rates, and our system was operated at 800 MU/min dose rate. The CyberKnife® system is capable of kV X-ray imaging for localizing the target during the treatment. The imaging system consists of two X-ray sources mounted on the ceiling, and the imaging detector panels are mounted in the floor. The images are taken throughout the delivery at regular intervals to localize the target. In this study, we used eight different small field detectors from PTW and IBA Dosimetry.

CyberKnife has secondary collimators, also known as cones, to achieve field sizes of 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35, 40, 50, and 60 mm at an SAD of 800 mm. Apart from cones that come in fixed diameters, the G4 model CyberKnife also comes with variable aperture collimators known as IRIS. It contains 12 collimator segments that are divided into two banks of six segments each to define dodecagon-shaped beam aperture. The segments in each bank are mounted in series, and the bank is rotated to 30° to each other to effectively have a circular beam similar to that of fixed collimators.[6] Even though with IRIS collimator we can have any field size, it is restricted to have only 12 field sizes replicating the fixed collimator.


The detectors used in our study are PTW 31016 PinPoint 3D, IBA PFD shielded diode, IBA EFD unshielded diode, IBA SFD unshielded diode (stereotactic), PTW 60008 shielded diode, PTW 60,012 unshielded diode, PTW 60018 unshielded diode (stereotactic), and PTW 60019 CVD diamond. For field OF measurements, these detectors were positioned in a 3D motorized PTW MP3-M water phantom with a scanning dimension of 500 × 500 × 408 mm. The water tank is used to set the detector at the measurement depth, and the detectors are directly connected to the PTW Unidos E electrometer kept in the console. The field OF measurements were taken at SAD of 800 mm and at depth of 15 mm. For precise positioning of the detectors along the center of the radiation field, both in-plane and cross-plane profiles are acquired at the measurement depth. Based on the full width at half maximum (FWHM), the center check program finds and corrects the shift along the center axis. After running center-check, measurement was acquired using a Unidos E electrometer and MEPHYSTO mc2 software version 3.3 was used in recording the data. The technical details of the detectors are listed in [Table 1].{Table 1}

Output factor measurements

In this study, we obtained the field OF with eight different small field detectors for all the 12 IRIS field sizes. For finding the field OF, we followed the formalism proposed by Alfonso et al.


Where [INSIDE:1] is the output correction factor and is the [INSIDE:2] is ratio of the detector readings M in the clinical (fclin) and the machine specific reference field (fmsr). For CyberKnife, the fmsr is the 60 mm collimator size.


The output correction factors for various small field detectors in CyberKnife are listed in Table 23 of Technical Report Series TRS-483.[7] Some of the correction factors stated in TRS-483 include that of recombination, polarity, and temperature dependence of diodes were taken into consideration in the estimation of field output correction factors (FOCFs). In addition, PTW Pinpoint chamber 31016 and PTW shielded diode 60008 has a minimum collimator diameter size of 10 mm. Values of FOCF for few collimator diameters sizes (namely, 7.5 mm and 12.5 mm) that were not given in TRS-483 were derived by linear interpolation of 2 neighborhood data points introducing a potential source of error. The FOCF for CyberKnife for the detectors in our study is listed in [Table 2].{Table 2}

The output reading was measured with these eight detectors for all 12 field sizes of the IRIS collimators. OF value is the ratio of measured reading at various field sizes normalized to the reading at maximum collimator field size (60 mm) for the same detector. The measurements were repeated thrice at each field size for every detector. The average, standard deviation (SD), and range of the OF values were calculated for every collimator size. Measured OF values of each detector was compared against the data published in the literature.


With the IRIS collimator, the segments effectively define the circular field size. The segments are split into two banks and mounted in series. The bank is rotated to 30° to have a circular beam-like fixed collimators. While the IRIS collimator provides dynamic field shaping, the collimator is burdened by field size errors. The effective field size (EFS) arrived from the FWHM of the measured profile differs from that of the nominal field size (NFS). In our measurements, the SD between NFS and EFS has been found to be <0.1 mm. For comparison, manufacturer tolerances on field size reproducibility are 0.2 mm at 800 mm SAD.

The OF values measured using the 8 small-field detectors estimated at 12 IRIS field sizes are shown in [Figure 1]. The mean value of three measurements was taken for each detector at each field size setting. The values of FOCF are incorporated in the data shown in [Figure 1]. Also shown are the corrected OF values of PTW MicroDiamond (PTW 60019) stated in the multi-site and multi-detector study by Masi et al.[8] The values shown here are comparable with the multi-site OF measurement based on Razor stereotactic diode study by Russo et al.[4] Average values of our measurements differ by <2%, with largest error around 4% for 5 mm collimator.{Figure 1}

Inter-detector differences were studied using the range of differences between the highest and lowest readings and SD. [Figure 2] shows the percent range of deviation and percent SD for each IRIS collimator diameter size. Note that both range and SD decreases with increasing collimator size. Percent range as high as 12.5% and 3.5% was recorded for 5 mm and 10 mm collimator, respectively, before falling off to 1% at 20 mm collimator field. SD values were as high as 4.5% for the 5 mm collimator, 3% for 10 mm collimator before dropping below 1% for collimator diameters larger than 10 mm.{Figure 2}


Measurement of dosimetric parameters, including dose output, percent depth dose, beam profiles, and OF, among others, is considered vital in the commissioning of treatment planning systems. The uncertainties in dosimetric measurements of stereotactic sized fields can be significant.[9] ICRU Report-91 deals in length with the dosimetric characterization of small fields in CyberKnife system, including beam profile, field sizes, OF, and penumbra.[10] A study on stereotactic cone OF measurement by Beddar et al. shows that plastic scintillator records higher readings than silicon diode, radiographic film, and ionization chamber.[11] The largest differences were observed in the 5 mm field size. Archambault et al. have shown similar results in using an ionization chamber and plastic scintillator for OF measurements.[12] The multi-site OF study on CyberKnife documents the PTW-60019 microDiamond and Exradin W1 plastic scintillation detector-based OF measurements against silicon diode correction using Monte Carlo factors.[7] A OF measurement study on CyberKnife by Bassinet et al. provides a systematic approach.[13] Our results are consistent with previous studies with a focus on the measured OF of the CyberKnife system.

Small volume water (or tissue) equivalent detectors with a high spatial resolution are absolutely essential for small field dosimetry.[14] Diodes are the recommended detectors in small field dosimetry due to high signal per unit dose, real-time readout, and high spatial resolution. However, among the eight detectors, PTW and IBA photon diodes recorded the highest OF values. This over-response of photon diodes especially at small field sizes up to 10 mm, could be explained by the usage of high density shielding material (Silicon) that is not water equivalent. American Association of Physicists in Medicine task group report 135 suggests using unshielded p-type Silicon diode for not only their good signal to dose, high spatial resolution, and avoid undue response to low energy scatter photons which accompany the shielded diodes.[15] This can be ascertained in [Figure 1] by the unshielded SRS diodes as well as electron diode readings. On a related note, Monte-Carlo correction factor for silicon diode usage in CyberKnife was determined in the study by Francescon et al.[16]

Chemical vapor deposition-based diamond detectors can withstand high electric field gradient across the sub-micron thick diamond layer. These possess a robust behavior over a wide-ranging field sizes 1 cm × 1 cm–40 cm × 40 cm. The newer generation microDiamond detector 60019 from PTW has been reported to have high linearity, low energy, and angular response by Larraga- Lárraga-Gutiérrez et al.[17] The percent depth dose and off-axis profiles measurements by this high-spatial resolution detector were within 1% in both conventional and small fields ≥1 cm. However, variations larger than 2.5% in OF measurements by PTW 60019 microDiamond for very small field sizes have been reported by O'Brien et al.[18]

The OF data measured by the Pinpoint chamber have lower values for all the collimator field sizes. This supports the partial volume averaging effect observed when chambers are large compared to the radiation field size. In high dose gradient regions, the dose values change significantly over the active volume of detector. When irradiating a portion of the active volume, the large detector reading is lower than that by a smaller volume detector.[19] Another study confirms the need to introduce the field correction factors for pinpoint chambers based on MonteCarlo simulations, especially for small fields.[20]

Dosimetry of stereotactic fields can be quite challenging in the modern complex treatment machines, and comparison with multi-site data increases confidence. Accurate OF measurements were performed with 8 small-field detectors on a CyberKnife system with FOCF applied from TRS-483 report. Although large inter-detector differences were expected for small field sizes, our study confirms agreement in large sized fields. This study gives physicists confidence in measuring field OFs while using small field detectors mentioned in this work.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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