Interpretation of In-air Output Ratio of Wedged Fields in Different Measurement Conditions.

BACKGROUND: The collimator scatter factor (Sc) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact Sc values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of Sc with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam.
METHODS: In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm3 and Farmer-type ion chamber with a sensitive volume of 0.65 cm3. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on Sc.
RESULTS: It was found in large field sizes, Sc values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on Sc values. The presence of wedge influenced Sc values significantly. Variation of Sc in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively.
CONCLUSION: It was found that the presence of wedges had a significant influence on Sc and increases with wedge angles. As such, it should be taken into account in manual MU calculations.

Introduction

In radiation therapy, what is of importance is accurate monitor unit (MU) calculation to provide the dose given to the planning target volume and decreasing the doses excessive to the critical organs at risks.[123] In general, the absorbed dose at the point inside a phantom or patient involves two components, the primary (original photons released from the source) and scatter radiation (photons and electrons scattered from the linear accelerator treatment head and phantom), to reach the point of interest.[14567] The total scattering factor (Scp) is one of the components that affect MU calculation.[489] The Scp involves the collimator scatter factor (Sc) and the phantom scatter factor (Sp). The Sc, also known as the head scatter factor or in-air output ratio,[41011] is defined as the ratio of collision with water kinetic energy released per unit mass in the free space of an arbitrary field to that of a reference field size (10 cm × 10 cm).[10] According to the American Association of Physics in Medicine (AAPM) Task Group number 74 (TG74) recommendations, the Sc can be determined in air using miniphantoms in cylindrical shapes (buildup caps) with usually the ionization chamber located at 10 g/cm2 water equivalent depth.[10] This depth is enough to stop contaminating electrons from getting the detector volume. In general, Sc measurements using low-Z miniphantom materials (with atomic number close to water) for large field sizes and high-Z miniphantom materials for small field sizes are recommended.[610] Several factors influence Sc values such as primary collimator, flattening filter, secondary collimator, tertiary collimators (MLCs), and beam-modifying devices such as wedges.[71112] The wedges are generally used as beam modifier devices, to optimize the distribution of target volume dose in radiation therapy.[413] When placed in the path of radiation beam, the wedges decrease the intensity of the beam.[13] Usually, a single Sc value is used for the patients' MU calculations, which are usually measured for open-beam fields, whereas the presence of the wedge changes Sc value; if this variation of Sc value is not taken into account in MU calculations, it may ultimately lead to major tumor dose differences for the patient, especially for MU calculation of palliative cases which dose calculation is performed manually.[51114] To conduct Sc measurements, the standard solid-state drive (SSD) (100 cm) is used, but in small field sizes, to ensure that the buildup cap is completely covered by small fields and electron equilibrium condition, the extended SSD should be used.[15] Several studies have investigated the variation of Sc in different conditions. Jun et al. studied the variation of Sc with different miniphantom materials and reported that the total correction factor increased with increases in field size and miniphantom longitudinal thickness.[16] In another study, Jun et al. showed that at small longitudinal thicknesses, the in-air output ratio correction factors for polymethyl methacrylate (PMMA) and graphite are close to 1. The maximum magnitudes of the in-air output ratio correction factors occur at the largest collimator setting (40 cm × 40 cm) and the largest miniphantom longitudinal thickness (30 g/cm2): 1.008 ± 0.001 for 6 MV and 1.012 ± 0.001 for 15 MV, respectively.[8] Iftikhar assessed the output factors using different ionization chambers and miniphantoms and reported that the use of miniphantoms with Zeff close to water and ionization chamber is the best choice for Sc measurements for large field sizes.[6] Appasamy et al. studied Sc using indigenously designed columnar miniphantom. They found that Sc at 1.5 cm depth is >10 cm, and the miniphantom material, chamber volume, and difference of SSD had no significant impact on Sc, whereas the existence of internal wedge has a significant effect on Sc. They suggested the use of PMMA miniphantom for greater field sizes and brass for smaller field sizes.[51114] Chegeni et al. studied Sc using Electronic Benefits Transfer (EBT2) film dosimeter with water and Cerrobend buildup caps and Farmer-type ionization chamber with polystyrene miniphantom. They found that EBT2 film with water buildup cap can be a good substitute for the ionization chamber.[17] Jomehzadeh et al.[1] studied the effect of material and wall thickness of miniphantoms on Sc measurements in irregular fields shielded by Cerrobend. They reported that using brass compared to copper and Perspex is a good choice for the measurement of Sc.[1] In most treatment planning systems, entering Scp, Sc, and Sp are necessary.[9] What is important is measuring Sc in different conditions with and without wedges to calculate the accurate MU and reduce the error in achieving the desired dose to the target volume. This study aimed to design miniphantoms and investigate the variation of Sc with different miniphantom materials, wall thickness, SSDs, ionization chambers, and Sc variation with wedges in 6 MV photon beam.

Materials and Methods

Photon beam, chambers, and miniphantom design

In this study, 6 MV photon beam of Elekta Compact Linac is used. The Farmer-type ion chamber FC65-P (Scanditronix, Wellhofer) with a sensitive volume of 0.65 cm3, outer diameter of 7 mm, inner diameter of 6.2 mm, and total active length of 23.1 mm with an inner electrode of aluminum was used. The second chamber was RK ionization chamber (Scanditronix, Wellhofer) with an active volume of 0.12 cm3, inner diameter of 4 mm, outer diameter of 7 mm, and air cavity length of 10 mm; the material for the central electrode and inner wall was a mixture of graphite and epoxy resin, and the material for the outer wall was PMMA. To investigate the influence of miniphantom material and wall thickness on Sc measurement, copper and Perspex were fabricated. The copper homemade buildup cap was designed with a wall thickness of 6.5 mm and 8 mm for farmer ion chamber and a wall thickness of 12 mm and 16 mm for RK ion chamber. Homemade Perspex buildup cap was designed with wall thicknesses of 15 mm and 18 mm for Farmer ion chamber and wall thicknesses of 14 mm and 19 mm for RK ionization chamber.

Head scatter factor (Sc) measurement

To measure Sc, the fabricated miniphantoms were fixed, using a stand, perpendicular to the beam central axis and the measurements were carried out for (5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, 20 cm × 20 cm, and 25 cm × 25 cm) field sizes. The Sc measurements were conducted at a source-to-chamber distance (SCD) of 100 cm and 120 cm. Due to different wall thicknesses, the SCD sets (distance from source to central electrode of chamber) varied for each miniphantom. Figure 1 shows the Sc measurement setup in different materials and wall thicknesses of miniphantoms in open and wedged fields.

Figure 1

The Sc measurement setup in different materials and wall thickness of miniphantoms in open and wedged fields

The field size in each measurement was large enough to cover the buildup cap. All the readings were done three times with 100 MU at 200 MU/min dose rate, and then, the average of the readings was obtained. To calculate Sc, the average of all the readings was normalized to the reference open-field (10 cm × 10 cm) readings average.[1410] All the measurements were conducted in a wedged angle of 30° and 60°.

Results

Effect of miniphantom material and ionization chambers

Table 1 shows Sc in-air values measured for open fields (5 cm × 5 cm–25 cm × 25 cm) using Perspex, copper miniphantoms, and Farmer, RK ionization chambers at SCD = 100 cm. In Farmer ionization chamber measurements, the maximum and minimum deviation of Sc values using two miniphantoms were 0.294% and 0.099%, respectively. The Sc value in copper miniphantom compared to Perspex was higher for field sizes smaller than 10 cm × 10 cm and lower for field sizes larger than 10 cm × 10 cm. In RK ionization chamber measurements, the maximum and minimum deviation of Sc values using two miniphantoms were 0.296% and 0.196%, respectively.

Table 1

Variation of Sc using Perspex, copper miniphantoms, and Farmer, RK ionization chambers (source-to-chamber distances=100 cm)

Field size (cm2)	Farmer chamber (0.65 cm3)		RK chamber (0.12 cm3)		Percentage of deviation
	Perspex 15 mm (A)	Copper 8 mm (B)	Perspex 14 mm (C)	Copper 12 mm (D)	(B-A) × 100/A	(C-A ) × 100/A	(D-C) × 100/C
5	0.972	0.974	0.975	0.977	0.206	0.309	0.205
10	1	1	1	1	0	0	0
15	1.013	1.012	1.012	1.009	−0.099	−0.099	−0.296
20	1.021	1.018	1.015	1.012	−0.294	−0.588	−0.296
25	1.026	1.023	1.021	1.023	−0.291	−0.486	0.196

To assess the effect of ionization chamber on Sc measurements, Farmer and RK ion chambers using Perspex and copper miniphantoms were utilized. An average deviation of 0.371% was observed [Table 1].

Effect of miniphantom thickness

Figure 2 shows that the variation of Sc for 6 MV open fields measured using Perspex and copper miniphantoms with different wall thicknesses and RK and Farmer ionization chambers at SCD = 100 cm. In Perspex cap measurements, with increases in wall thickness, Sc values decreased in field sizes larger than 10 cm × 10 cm for both ionization chambers. In case of Farmer chamber measurements, the percentage of deviation in Sc values measured in Perspex with different wall thicknesses varies from 0.097% to 0.103%; in RK chamber, it varies from 0.098% to 0.390%.

Figure 2

Variations of Sc in Perspex and copper miniphantoms with different wall thickness in open fields using RK ionization chamber including 12 and 16 mm of copper (cu) and 14 and 19 mm of Perspex and Farmer ionization chamber including 6.5 and 8 mm of copper (cu) and 15 and 18 mm of Perspex. (Source-to-chamber distance = 100 cm)

In copper miniphantom measurements, Sc value increased with increases in wall thickness in field sizes larger than 10 cm × 10 cm in both ion chambers. A maximum deviation of 0.393% was observed in 25 cm × 25 cm field size, between different thicknesses compared to 6.5 mm.

Effect of source-to-chamber distance

Figure 3 shows a plot of Sc variation for 6 MV open fields with different SCD. From the data, it was observed that the impact of SCD on Sc, in small field sizes, was negligible but was noticeable in large field sizes. It was found that with SCD increasing, the output factor decreased in RK ion chamber, while it increased in the Farmer.

Figure 3

Variation of Sc in open square fields using Perspex (14 and 15 mm) at different source-to-chamber distances and ionization chambers

Impact of beam-modifying devices on Sc

The Sc values measured in wedged fields compared to open fields are shown in Table 2. Measurements were carried out using Perspex miniphantom with 15 mm wall thickness and Farmer-type ionization chamber at SCD = 100 cm. Results demonstrated that the Sc value measured in open fields compared to the Sc value measured in wedged fields had a maximum and minimum deviation of 0.876% and 0.395% for 30°, and 6.628% and 2.664% for 60° angles of wedge, respectively. Table 3 presents the comparison of the measured data.

Table 2

The variation of Sc with field size for open and wedged fields in 30 and 60 angles

Field size (cm2)	Open field (A)	Wedge 30° (B)	Wedge 60° (C)	Percentage deviation
				(A-B)/A×100	(A-C)/A×100
5	0.972	0.968	0.944	0.411	2.881
10	1	1	1	0	0
15	1.013	1.017	1.040	−0.395	−2.664
20	1.021	1.028	1.072	−0.686	−4.994
25	1.026	1.035	1.094	−0.876	−6.628

Table 3

This study measured Sc for open square fields with Perspex (15 mm) and Farmer chamber (0.65 cm3) in source-to-chamber distances=100 cm compared with published data

Field size (cm2)	Our study (A)	TG-74 (B)*	Appasamy et al. (C)	Percentage deviation
				(A-B) × 100/B	(A-C) × 100/C
5	0.972	0.971	0.972	0.103	0
10	1	1	1	0	0
15	1.013	1.015	1.014	-0.196	-0.099
20	1.021	1.022	1.027	-0.099	-0.583
25	1.026	-	1.032	-	-0.580

*AAPM TG74: American Association Physics in Medicine Task Group number 74

Figure 4 presents Sc values using Perspex miniphantom with 14 and 15 mm thicknesses and Farmer, RK ion chambers at SCD = 100 cm. The data led to the conclusion that the Sc value in wedged beam is greater than open beam in field sizes larger than 10 cm × 10 cm. Furthermore, all in all, Sc in wedged beam is lower than open beam in field sizes lower than 10 cm × 10 cm.

Figure 4

Variation of Sc in open and wedged fields using Perspex (14 and 15 mm) miniphantoms and RK, Farmer ionization chambers. (source-to-chamber distance = 100 cm)

Discussion

The characteristics of an ideal miniphantom are as follows: (a) the overall width of the miniphantom should be physically smaller than the high-dose region, (b) miniphantom should be sufficiently thick to eliminate electron contamination, and (c) should be able to be situated exactly and reproducibly.[18] Furthermore, in Sc measurement, lateral electron equilibrium is necessary, and the miniphantom must be fully covered by the radiation beam without the penumbra region.[10] For small field measurements, high Z materials, extended SSD, and detectors with small sensitive volume are recommended.[101719] Based on the data [Table 1], the type of miniphantom materials has no significant impact on Sc measurements in 6 MV and is in good agreement with the findings by Iftikhar[6] and Appasamy et al.[14] In higher energies with increasing field size, instances of large scatter and reductions in mean energy of the beam occur, which interact differentially with different buildup cap materials and cause variation in Sc measurements. However, in low energies, this behavior may be reversed and no differential impact with buildup cap materials is occurred. Miniphantoms with wall thicknesses equal to dmax might allow electron contamination to reach the detector sensitive volume and wrongly increase the reading principally in larger field sizes.[10] Increasing field size alters the effective energy of the beam and as a consequence, changes the scatter, transmission, and buildup in the miniphantom.[19] Using a miniphantom with a smaller wall thickness still stops all contamination in the electrons, reducing the amount of scatter and attenuation in the miniphantom.[10] Jursinic suggested that miniphantoms with wall thicknesses <10 cm can be useful for photon energies of 6 and 15 MV.[19] Figure 2 demonstrated that Sc values at wall thicknesses larger than dmax, in copper miniphantom, are greater than those of dmax(in agreement with Li et al.[816] findings). In Perspex buildup cap, with increases in wall thickness beyond the dmax, the Sc decreases (in agreement with the findings by Appasamy et al.[5] and AAPM TG-74[10]). Table 2 depicted that the ionization chamber type had no significant impact on Sc; these results are in good agreement with those by Iftikhar.[6] Chen et al. reported that for the radiation fields of ≥4 cm × 4 cm, no differences were observed in Sc measurements by three ionization chambers (0.6, 0.13, and 0.01 cm3 sensitive volume).[15] However, for small field sizes, particularly for fields as small as 1 cm × 1 cm, great alterations were found in Sc values, even though the CC01 (0.01 cm3) and CC13 (0.13 cm3) chambers have small active volumes.[15] The standard SSD (SSD = 100 cm) setup is usually used to measure Sc for large fields, but extended SSD should be used for small fields to ensure that the fields completely cover the buildup cap and electron equilibrium is established; otherwise, great errors would arise.[15] The impact of SSD on Sc was studied by measuring Sc at different SSD (100 and 120 cm) in Perspex buildup cap and RK, Farmer ionization chambers [Figure 4]. These results show that SSD has no impact on Sc; this is in good agreement with the data in the study by Appasamy et al.[514] In the present study, the influence of wedge on Sc [Figure 4 and Table 2] demonstrated that in field sizes larger than 10 cm × 10 cm, Sc increases in wedged fields compared to open fields. Sc also increases as the wedge angle does. This is due to increased scattering photons from the wedge filter, which increases in proportion to increases in field sizes above 10 cm × 10 cm. This is in agreement with the results of the studies by Appasamy et al.[51114] and Ashokkumar.[712]

The published data concerned Sc with Perspex at 15 mm wall thickness and Farmer chamber.[1014] Compared with Appasamy et al.[14] and AAPM TG-74,[10] the present study showed a maximum deviation of 0.580% and 0.196% for a field size of 25 cm × 25 cm, respectively. This deviation may be due to what Appasamy et al.[14] and AAPM TG-74[10] have measured Sc in 10 cm, but the present study was carried out in 1.5 cm wall thickness.

Conclusions

Sc measurements were carried out in 6 MV square fields using low and high Z buildup caps with different wall thicknesses and ionization chambers. The results revealed that the effect of copper and Perspex buildup cap as well as that of wall thickness in Farmer or RK ionization chambers (with different active volume) on Sc measurements in 6 MV Elekta Linac are not significant; also, different SCDs had no significant impact on Sc measurement in large field sizes. However, it was found that the presence of wedge has significant influence on Sc measurement, increasing as the wedge angle does. Therefore, it should be taken into account in manual MU calculations.

Financial support and sponsorship

This work was financially supported by Tabriz University of Medical Sciences.

Conflicts of interest

There are no conflicts of interest.

BIOGRAPHIES

Parinaz Mehnati received PhD degree in Department of Integrative Biomedical Sciences, Graduate school of Medicine, Kyushu University, Japan, on the March, 2001. Dr Mehnati main interesting is radiobiology as well as radiation protection branches of Medical Physics field. She is currently an Associate Professor at Tabriz University of Medical Sciences, Department of Medical Physics.

Email: parinazmehnati@yahoo.com

Farideh Biglari received B.Sc degree from Tabriz University of Medical Sciences, Iran on Radiology in 2013. She is now M.Sc Candidate of Medical Physics at Tabriz University of Medical Sciences. Research interests include Radiotherapy, Dosimetry, Radiobiology.

Email: Faride.biglari92@gmail.com

Ali Jomehzadeh received B.S. degree in applied physics from Shahid Bahonar University of Kerman, Kerman, IRAN in 1999, M.S. degree in medical physics from Mashhad University of Medical Sciences, Mashhad, IRAN in 2004. Then he got Ph.D. of Medical Physics from Isfahan University of Medical Sciences, Isfahan, Iran in 2014. Since 2004 he has worked as faculty member in Medical Physics and since 2014 as assistant professor at medical physics department in Kerman University of Medical Sciences. He is also head of radiotherapy physics department (Chief medical physicist) in Shafa Kerman Hospital.

Email: a.jomehzadeh@kmu.ac.ir