Abstract Introduction Radiotherapy planning systems require many percentage depth dose (PDD) and profile measurements and there are various dosimeters that can be used to obtain these scans. As dose perturbation is particularly troublesome in smaller photon fields, using a low-perturbation, unshielded electron field diode (EFD) in these fields is of interest. The aim of this work was to investigate the suitability of an unshielded diode for beam scanning in 3×3 cm 2 , 5×5 cm 2 , and 10×10 cm 2 , 6 MV fields. Materials and Methods An EFD was used for all the scans. For comparison, in profile measurements, a tungsten-shielded photon field diode (PFD) was also used. PDDs were measured using the PFD and an RK ionization chamber. Results Very good agreement (0.4%) was found between the PDDs measured with EFD and PFD for the two larger fields. However, the difference between them exceeded 1.0% slightly for the smallest field, which may be attributed to the effect of the larger PFD perturbation. The RK chamber PDDs around 10 cm depth were 1-2% lower than those measured with the diodes. There was good agreement (&lt;1 mm) between EFD-and PFD-measured penumbra widths. Conclusion The EFD generally agrees well with the PFD and may even perform better in smaller fields

Ali Ketabi

Mohammad Amin

Mohammad Amin Mosleh-Shirazi

Mosleh-Shirazi

Reza Faghihi

Sareh Karbasi

CiteSeerX

Iranian Journal of Medical Physics 
Vol. 10, No. 1-2, Winter & Spring 2013, 51-57  
Received: November 22, 2012; Accepted: March 12, 2013 
                                                                               Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 51 
 
 Original Article 
 
Assessment of an Unshielded Electron Field Diode Dosimeter for Beam 
Scanning in Small- to Medium-Sized 6 MV Photon Fields 
 
Mohammad Amin Mosleh-Shirazi
1,2
, Ali Ketabi
3
, Sareh Karbasi
2*
, Reza Faghihi
4
 
 
 
Abstract 
 
Introduction 
Radiotherapy planning systems require many percentage depth dose (PDD) and profile measurements and 
there are various dosimeters that can be used to obtain these scans. As dose perturbation is particularly 
troublesome in smaller photon fields, using a low-perturbation, unshielded electron field diode (EFD) in 
these fields is of interest. The aim of this work was to investigate the suitability of an unshielded diode for 
beam scanning in 3×3 cm
2
, 5×5 cm
2
, and 10×10 cm
2
, 6 MV fields.  
Materials and Methods 
An EFD was used for all the scans. For comparison, in profile measurements, a tungsten-shielded photon 
field diode (PFD) was also used. PDDs were measured using the PFD and an RK ionization chamber. 
Results 
Very good agreement (0.4%) was found between the PDDs measured with EFD and PFD for the two larger 
fields. However, the difference between them exceeded 1.0% slightly for the smallest field, which may be 
attributed to the effect of the larger PFD perturbation. The RK chamber PDDs around 10 cm depth were 1-
2% lower than those measured with the diodes. There was good agreement (<1 mm) between EFD- and 
PFD-measured penumbra widths.  
Conclusion 
The EFD generally agrees well with the PFD and may even perform better in smaller fields. 
 
Keywords: Electron Field Diode, Photon Field Diode, Radiotherapy Beam Scanning, RK Ionization 
Chamber, Small and Medium Fields 
 
 
 
 
 
 
 
 
 
1- Center for Research in Medical Physics and Biomedical Engineering, Shiraz University of Medical Sciences, Shiraz, 
Iran 
2- Physics Unit, Department of Radiotherapy and Oncology, Namazi Hospital, Shiraz University of Medical 
Sciences, Shiraz, Iran 
*Corresponding author: Tel: +98 711 6125811; Fax: +98 711 6474320; Email: 
karbasis@sums.ac.ir & karbasi59@yahoo.com 
 
3- Student Research Committee, and Department of Medical Physics, School of Medicine, Shiraz University of 
Medical Sciences, Shiraz, Iran 
4- Department of Medical Radiation and Radiation Research Center, School of Mechanical Engineering, Shiraz 
University, Shiraz, Iran 
Mohammad Amin Mosleh-Shirazi et al. 
                                   Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 52
 
1. Introduction 
Treatment planning systems require 
percentage depth dose (PDD) and profile 
measurements for various field sizes as input 
for beam modeling and verification of its 
results [1]. Small- to medium-sized photon 
fields are widely used in many types of 
modern radiotherapy techniques [2-4]. There 
are a variety of radiation dosimeters that can 
be used to obtain these required beam scans. 
Choosing a suitable dosimeter appropriate for 
the required conditions such as beam energy 
and field size is an important task for 
radiotherapy physicists.  
Semiconductor diode detectors can be used for 
beam data commissioning [5, 6]. Clinical in-vivo 
dosimetry using diodes is also well established 
[7, 8]. Diodes have a number of desirable 
characteristics for radiotherapy dosimetry. These 
characteristics include high sensitivity, small 
volume, good mechanical rigidity, and obviation 
of the need for applied external voltage. In 
particular, diodes offer the opportunity to 
perform beam scanning with fairly high spatial 
resolution due to their compact size [9-12]. 
However, diodes often show dependence on 
many factors including energy, dose rate, source-
to-detector distance, direction of incidence, and 
temperature [8, 13-15].  
Stemming from the higher atomic number of 
silicon and therefore increased influence of the 
photoelectric effect, diode sensitivity to low-
energy photons is higher than air-filled 
ionization chambers, which leads to its higher 
energy dependence. As an attempt to 
overcome this problem, low-energy sensitivity 
of photon field diodes (PFDs) is reduced by 
employing a shield; for instance, a small 
amount of tungsten-doped epoxy added in the 
diode construction. The number of low-energy 
photons reaching the detector is reduced by the 
shield thereby improving its energy 
dependence. For example, it was shown that 
addition of a small amount of tungsten behind 
the silicon reduced PDD overestimation from 
3% to 1% for a 20×20 cm
2
 cobalt-60 field 
[16]. Therefore, semiconductor diodes are 
useful radiation detectors, particularly for 
small field dose measurements where there are 
fewer low-energy photons [6, 17, 18]. 
Given the differences between the interaction 
mechanisms of photons and electrons, such a 
shield in the diode detector is not required in an 
electron field diode (EFD), which is an 
unshielded silicon detector designed primarily 
for use in electron beams. Since scatter-to-
primary ratio is relatively low in small photon 
fields, the shield used in PFDs may not be 
necessary and, in fact, may increase dose 
perturbation due to the presence of tungsten near 
the silicon detector. As dose perturbation is 
particularly troublesome in small fields (where 
electronic non-equilibrium conditions exist), 
using a low-perturbation, unshielded electron 
diode in small photon fields is of interest. Monte 
Carlo modeling has been used to demonstrate 
the benefits of employing a low-buildup diode 
utilizing carbon-epoxy instead of tungsten-epoxy 
in correctly measuring small-field output factors 
in megavoltage X-ray beams [19]. It has also 
been reported that an unshielded diode has been 
used experimentally for measurement of small-
field megavoltage photon output factors [17]. 
The aim of this work was to investigate the 
suitability of a commercial unshielded EFD, in 
comparison with a shielded PFD and an 
ionization chamber, for beam-scanning 
purposes to obtain PDDs and profiles in small- 
to medium-sized 6 MV photon fields. 
 
2. Materials and Methods 
The measurements were performed in 6 MV 
X-ray beams of an Elekta Compact linear 
accelerator. The measurements included 
profiles and central-axis PDDs in small- to 
medium-sized fields (3×3 cm
2
, 5×5 cm
2
, and 
10×10 cm
2
). The profiles were taken at depths 
1.5, 5, and 10 cm. An unshielded diode (EFD) 
(IBA, Sweden) was used for all of the scans. 
For comparison, in profile measurements, a 
tungsten-shielded diode (PFD) from the same 
manufacturer was also used. As for PDD 
comparisons, they were also measured using 
the same PFD as well as an RK ionization 
chamber from the same manufacturer. 
Electron Diode for Photon Beam Scanning 
                                                                               Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 53 
Both shielded and unshielded diodes have a 
detection area of 2.5 mm diameter and a 
sensitive volume thickness of 60 μm. During 
measurements of PDD curves and beam 
profiles, a diode was placed with its long axis 
aligned with the beam’s central axis to give the 
best spatial resolution. 
 
The RK chamber is a cylindrical ionization 
chamber with a fairly small sensitive volume 
of 0.12 cm
3
, outer diameter of 7 mm, inner 
diameter of 4 mm, and length of 10 mm. The 
chamber was positioned with its long axis 
perpendicular to the beam axis. 
The measurements were performed in an RFA-
300 computerized scanning water phantom 
(IBA, Sweden). The measurements were 
performed with 1 mm resolution (data point 
spacing) for all PDD curves and beam profiles. 
All measurements were made at a source-to-
surface distance of 100 cm. A reference diode 
detector was placed in the field to correct for 
beam output variations during scanning. As 
the scans included a field size of 3×3 cm
2
, in 
order to avoid the perturbation effect of the 
reference detector in this small field, the 
detector was placed inside the accelerator’s 
head above the secondary collimators (out of 
the defined field). 
 
3. Results 
The PDDs obtained with the RK chamber as 
well as the shielded and unshielded diodes for 
the three field sizes are shown in Figure 1. The 
PDDs were normalized to 100% at the depth 
of maximum dose (dmax). For all of the 
considered field sizes, disagreements between 
PDDs were specified by local relative 
differences between PDDs averaged over the 
depth range of 9–11 cm. These differences are 
quantified in Table 1. A comparison of the 
PDDs at phantom surface, measured using 
these detectors, are quantified in Table 2. 
Lateral dose profiles obtained with the 
shielded and unshielded diodes at the depth of 
1.5 cm in water for the same three fields are 
shown in Figure 2. 
 
 
 
Table 1. Comparison of the PDDs near the depth of 10 cm in water, measured using the three detectors (EFD, PFD, and 
RK chamber). 
 PDD around depth 10 cm (%) Local difference (%) 
Field size (cm
2
) EFD PFD RK (EFD – PFD)/EFD (EFD – RK)/EFD 
10×10 66.1 66.5 65.0 -0.4 1.7 
5×5 62.0 62.3 61.2 -0.4 1.3 
3×3 59.3 59.9 58.7 -1.1 1.1 
 
 
Table 2. Comparison of the PDDs at phantom surface, measured using the three detectors (EFD, PFD, and RK chamber). 
 
 PDD at surface Local difference (%) 
Field size (cm
2
) EFD PFD RK (EFD – PFD)/EFD (EFD – RK)/EFD 
10×10 37.7 48.2 65.2 -27.8 -72.9 
5×5 35.6 44.7 62.0 -25.6 -74.2 
3×3 32.1 34.5 61.0 -7.4 -89.9 
  
Mohammad Amin Mosleh-Shirazi et al. 
                                   Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 54
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. Percentage depth dose curves for different field sizes: a) 10×10 cm
2
, b) 5×5 cm
2
, and c) 3×3 cm
2
 (EFD: 
electron field detector, PFD: photon field detector, RK: RK ionization chamber). 
 
 
  
Electron Diode for Photon Beam Scanning 
                                                                               Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 55 
 
 
 
 
Figure 2. Dose profiles at depth of 1.5 cm for different field sizes: a) 10×10 cm
2
, b) 5×5 cm
2
, and c) 3×3 cm
2
 (EFD: 
electron field detector, PFD: photon field detector). 
4. Discussion 
In our measurements, positioning uncertainties 
were less than ±1 mm. The scanning system had 
a positional accuracy of ±0.5 mm and a 
reproducibility of ±0.1 mm. Average differences 
between dosimeter readings in successive 
repeated scans were found to be 0.5%. 
Very good agreement (0.4%) was found 
between the PDDs measured with EFD and 
PFD for the two larger fields. However, the 
difference between them exceeded 1.0% 
slightly for the smallest field, which may be 
attributed to the effect of the larger 
perturbation of the PFD.  
The RK chamber PDDs around 10 cm depth 
were 1-2% lower than those measured with the 
diodes. EFD-to-RK difference was the greatest 
in the case of the 10×10 cm
2
 field. Large 
Mohammad Amin Mosleh-Shirazi et al. 
                                   Iran J Med Phys, Vol. 10, No. 1-2, Winter & Spring 2013 56
differences between the diodes and RK 
chamber can also be seen in the buildup region 
close to the surface, where the RK chamber 
again underestimated dose. Although the 
differences after dmax were not large, these 
results confirm that the size of dosimeters 
normally used for measuring in larger fields, 
such as the RK chamber, makes them less 
suitable for use in a 3×3 cm
2
 (or smaller) field 
due to the perturbation effect of the detector. 
Furthermore, the RK chamber, due to the 
volume averaging effect, can be seen to be 
generally unsuitable for PDD measurements in 
very shallow depths in water (less than 3 mm). 
There is good agreement between EFD-
measured penumbra widths and those 
measured with the PFD in these photon fields. 
The 50–90% penumbra distance obtained with 
EFD and PFD were different by less than 1 
mm. The PFD, however, gave lower relative 
measurements than those of the EFD for the 
out-of-field region of the smallest field, which 
may be due to the presence of its shield, and 
requires further investigation. The trends seen 
at the three depths of measurement were 
similar. The averaging effect of a detector is 
most important in the beam penumbra region. 
Measurements with diode detectors, due to 
their small sensitive volume, are expected to 
give better spatial resolution and be the most 
accurate in the penumbra region.  
Our results show similar trends to those 
obtained previously in a study regarding 
output factor measurements using similar 
detectors [20].  
It is worth pointing out that, in preference to 
the RK chamber, a very small volume (<0.05 
cc) ion chamber would be a better choice for 
comparisons with the diodes in small-field, 
near-surface, and lateral-dose measurements. 
Nonetheless, the RK chamber is a fairly 
widely available chamber in many centers and 
its evaluation is worthwhile. 
Moreover, for the points at or near surface, 
knowing the dose contribution from electron 
contamination is of interest. Investigation of 
electron contamination is, however, outside 
the scope of this paper. 
 
5. Conclusion 
For the beam energy and range of field sizes 
studied here, the EFD generally agreed very 
well with the measurements using the PFD. 
Our results suggest that the EFD may even 
perform better in smaller fields due to its lower 
beam perturbation. The results suggest that an 
unshielded diode is an appropriate choice of 
detector for scanning in small- to medium-
sized radiation fields instead of an RK 
chamber or PFD. The RK chamber is a less 
suitable detector when high spatial resolution 
is required, as it exhibits some degree of 
volume averaging. 
 
Acknowledgements 
The work presented here constitutes a part of 
Mr Ali Ketabi’s MSc dissertation in Medical 
Physics (number 5614), which was carried out 
at and funded by Shiraz University of Medical 
Sciences.  
 
 
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Assessment of an Unshielded Electron Field Diode Dosimeter for Beam Scanning in Small-to Medium-Sized 6 MV Photon Fields

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Assessment of an Unshielded Electron Field Diode Dosimeter for Beam Scanning in Small-to Medium-Sized 6 MV Photon Fields

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