Small field dosimetry: Chamber selection and its validation using technical report series-483

INTRODUCTION

Radiotherapy is one of the major treatment modalities for the treatment of cancer. For conventional radiotherapy like 2DRT and 3DRT, the dosimetry is based on Code of Practice (COP) like TRS-398^[1] by International Atomic Energy Agency (IAEA) and TG-51 by American Association of Physicists in Medicine (AAPM). Here, the dosimetry is done in tissue equivalent material, mainly a water phantom, using ionization chambers. Output factors (OF) for large fields are measured keeping 10 x 10 cm² as the reference field where reference conditions stipulated by Code of Practice(COP) s are met.^[2] New techniques like IMRT, IGRT, VMAT, SRS, SRT, SBRT, Tomotherapy, Cyberknife, etc deliver dose distribution to small targets where a steep dose gradient is required. Therefore, in these techniques, small fields are widely used.^[3] Conventionally, a field size less than 3 × 3 cm² is considered as small field.^[4] Small field dosimetry is complex due to the following reasons.^[5,6,7]

1. Loss of lateral charge particle equilibrium (LCPE) in the beam axis.

2. Partial occlusion of the primary photon source by the collimating devices or the beam axis.

3. The size of the detector is similar to or larger compared to the beam dimensions. This causes a volume-averaging effect.

In 2017, the IAEA proposed Technical Report Series TRS-483 for the dosimetry of small fields.^[8] The protocol recommends the ideal setup for measuring output in small fields and identifies suitable detectors.

The characteristic of radiation detectors, like size, shape, and material, plays an essential role in their response to photon beams. Therefore, selecting a suitable detector for small-field dosimetry is important for accurate dose measurement. A detector with high spatial resolution, low noise, water equivalence, and low directional and energy dependence is considered the ideal one.^[9]

MATERIAL AND METHODS

True Beam STx (Varian Medical Systems, Palo Alto, CABrainlab AG, Munich, Germany) is a state-of-the-art machine that is capable of treating large as well as small fields. High-definition multileaf collimator (HD MLC) and Flattening Filter Free (FFF) beams are used for stereotactic treatments. The photon beam energies available in the machine are 6 MV, 10 MV, and 15 MV with a Flattening Filter (FF) and 6 MV and 10 MV as FFF beams. The measurements for this study were carried out with 6 MV-FF and 6 MVFFF beams. The Linear Accelerator (LINAC) was calibrated to deliver 1cGy/MU to water at a depth of maximum dose (d-max) for 10 ×10 cm² at Source to Surface Distance (SSD) of 100 cm as per the recommendations of TRS- 398. All field sizes were defined with jaws as per the recommendation of Varian Medical Systems for the commissioning of the Eclipse treatment planning system, version 15.1.^[16] Field sizes from 0.8 x 0.8 cm² to 3 x 3 cm² are considered as a small field for this work. 10 x 10 cm²was considered as the reference field size, and 4 x 4 cm² was considered as the intermediate field size as per the recommendations of TRS -483.

Detectors used in the study were the electron field diode detector (EFD, IBA Dosimetry GmbH, Schwarzenbruck, Germany), photon field diode detector (PFD, IBA Dosimetry GmbH, Schwarzenbruck, Germany), and CC01 ionization chamber (IBA Dosimetry GmbH, Schwarzenbruck, Germany) for small field measurements^[16] and FC65 (IBA Dosimetry GmbH, Schwarzenbruck, Germany) for the large fields. All measurements were done in the IBA Blue Phantom-2 (IBA Dosimetry GmbH, Schwarzenbruck, Germany) Radiation Field Analyzer and Dose 1 electrometer (IBA Dosimetry GmbH, Schwarzenbruck, Germany). The procedure for measuring the output and output factors was according to the protocol TRS-483. The solid-state detectors, like EFD and PFD, were mounted vertically to the water surface for measurements. CC01 was placed parallel to the water surface.^[17] Output factors for different square and rectangular fields were measured. Gafchromic films (KODAK EBT3) were used for comparing the output factor with different detectors. Better accuracy can be achieved by film dosimetry since Gafchromic films do not possess any of the shortcomings listed for ionization or diode detectors.

RESULTS

The measurements of output factors of different field sizes were performed. The field sizes range from 1 x 1 cm² to 5 x 5 cm². The measurements were done in a varian true beam STx machine using three different kinds of detectors. The energies used were 6 MV-FF and 6 MV-FFF. The corrected and uncorrected output factors were calculated using the formula given in the TRS 483 protocol.

where ${\left[M_{Qclin}^{fclin}{k}_{Qcline,Q\mathrm{int}}^{fclin,f\mathrm{int}}\right]}_{\det }$ Represents the corrected field output of clinical small field sizes to intermediate field size, typically 4x4cm². $\frac{M_{Qclin}^{fclin}}{M_{Q\mathrm{int}}^{f\mathrm{int}}}$ Is the Ratio of the detector reading and the term Represents detector-specific output correction factor.

${\left[\frac{M_{Q\mathrm{int}}^{f\mathrm{int}}}{M_{Qmsr}^{fmsr}}{k}_{Q\mathrm{int},Qmsr}^{f\mathrm{int},fmsr}\right]}_{IC}$ Represents the link between the intermediate field size and to machine-specific reference field size, normally 10 x 10 cm² and $k_{Q\mathrm{int},Qmsr}^{f\mathrm{int},fmsr}$ represents a correction factor that accounts for the changes in detecto response due to differences in field size and beam quality.

The Institute of Radio Oncology, Wiener Krankenanstaltenverbund (Weinkav), published their data^[18] for the Truebeam STx machine online in 2015 prior to the publication of TRS- 483. They used the scaling technique by using 4 x 4 cm²as the intermediate reference field size. Since the Kclin correction factor, as in TRS- 483, was not used by them, the uncorrected output factors in the present study were compared to the reference data for assessing the setup adequacy for measurement. Figures 1 and 2 show the uncorrected output factors with field sizes for different detectors and different beams, like 6FF and 6FFF. The output factors corrected using the correction factors provided in TRS-483 for 6FF and 6FFF beams with different detectors like EFD, PFD, and CC01 have been plotted in Figures 3 and 4.

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Figure 1:

Output factors with field sizes for the 6 Mega Volt Flattening Filter beam measured by using different chambers without applying correction factors as per TRS-483. PD : Percentage deviation, PFD : Photon field detector, EFD : Electron field detector

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Figure 2:

Output factors with field sizes for the 6 Mega Volt Flattening Filter beam measured by using different detectors without applying correction factors as per TRS-483. PD : Percentage deviation, PFD : Photon field detector, EFD : Electron field detector

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Figure 3:

Output factors with field sizes for the 6 Mega Volt Flattening Filter Free beam measured by using different chambers after applying a correction factor for chambers, as mentioned in TRS-483. PD : Percentage deviation, PFD : Photon field detector, EFD : Electron field detector

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Figure 4:

Output factors with field sizes for 6 Mega Volt Flattening Filter beam measured by using different chambers after applying a correction factor for chambers as mentioned in TRS-483.

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Percentage deviation from the standard value

We can see that the percentage deviation for all three detectors is less than 2% for all field sizes except for the smallest field size of 1 x 1 cm² [Table 1 and 2]. This trend of percentage deviation is similar for both 6 MV-FF and 6 MV-FFF. Also, the percentage deviation greater than one per cent for the smallest field size is for the PFD and CC01 detectors, whereas the EFD detector gives results within one per cent.

The output factors of 6 MV-FF and 6 MV-FFF beams with different dosimeters like EFD, PFD, and CC01 were measured with field sizes from 0.8 x 0.8 cm² to 4 x 4 cm² Those values were compared with the output factors measured using Gafchromic films [Table 3 and 4].^[19] The results using Gafchromic films are independent of the source occlusion effect and the volume averaging effect. As with the results of the percentage deviation of the measured output factor with the reference data, the percentage deviation of the output factors using various detectors as compared to that measured using Gafchromic films is greater for smaller fields, as compared to the field size of 2 x 2 cm² and above. Figure 5 shows the percentage deviation of the output factors measured using the three detectors with regard to those measured using Gafchromic films. From the results, it is evident that the percentage deviation for all measurements is less than 10 per cent. Deviation is relatively higher for fields less than 1 x 1 cm² and also for 4 x 4 cm². For the 6 MVFF beam, CC01 is more consistent. Figure 6 shows that for 6 MV-FFF, the EFD detector is more suitable for measuring output from 1 x 1 cm² to 3 x 3 cm².

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Figure 5:

Percentage deviation of OFs for the 6 Mega Volt Flattening Filter Free beam for different detectors from reference data. PD : Percentage deviation, PFD : Photon field detector, EFD : Electron field detector

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Figure 6:

Percentage deviation of OFs for the 6 MV-FF beam for different detectors from reference data. PFD : Photon field detector, EFD : Electron field detector, MV-FFF : Mega volt flattening filter free.

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Table 3: Percentage variation of output factors for the three detectors in comparison to output factors measured using Gafchromic films for the 6 MV-FF beam,

Side of square field (cm)	PFD	CC01	EFD
0.8	-5.883197546	1.41538339	-4.969646127
1	-4.647910521	-1.291663341	-4.34491355
2	1.578458951	1.267463575	0.285920157
3	-1.419277387	-1.905927536	-2.588373601
4	5.35100122	5.653521209	5.35100122

PFD : Photon field detector, EFD : Electron field detector

Table 4: Percentage variation of output factors for the three detectors in comparison to output factors measured using Gafchromic films for the 6 MV-FFF beam,

Side of square field (cm)	PFD	CC01	EFD
0.8	-8.18421	0.466369	-2.87048
1	-0.25644	5.572444	3.978324
2	-0.212	1.085464	1.519501
3	-3.16107	-2.75561	-2.38256
4	5.069477	5.069477	5.069477

PFD : Photon field detector, EFD: Electron field detector

DISCUSSION

The present study compared the output factors, measured using different detectors for small field sizes ranging from 1x1 cm2 to 5x5 cm2 with standardized reference data. Since the Kclin correction factor, was not used by published reference data, we used the output factors without this correction factor to compare with the standard. From this study it is found that the detectors like PFD and CCO1 showed percentage deviation greater than 1% for both 6MV-FF and 6MV-FFF beams where as EPD showed variation less than 1% for all field sizes. Additionally, the output factors measured using different detectors for the field sizes from 0.8 x 0.8 cm2 to 4 x 4 cm2 were compared with Gafchromic film data which is not influenced by of source occlusion effect and volume averaging effect. It was found that the percentage deviation of output factors was highest for small field sizes less than 2x2cm2. From tables 4 and figures 7 and 8 it is evident that the detector CC01 is most suitable for small field sizes for both 6MV- FF and 6MV-FFF beams.

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Figure 7:

Percentage variation of output factors for the three detectors in comparison to output factors measured using Gafchromic films for a 6 MV-FF beam. PFD : Photon field detector, EFD : Electron field detector, MV-FFF : Mega volt flattening filter free.

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Figure 8:

Percentage variation of output factors for the three detectors in comparison to output factors measured using Gafchromic films for a 6 MV-FFF beam. PFD : Photon field detector, EFD : Electron field detector, MV-FFF : Mega volt flattening filter free.

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CONCLUSION

In this study, we compared the output factors for small fields of True Beam STX linear accelerator using EFD and PFD detectors,and also CCO1 ionization chambers, with the output factors measured using Gafchromic films. From the comparison, keeping Gafchromic film data as the standard, it was seen that CC01 is more suitable for 6 MV-FF beam measurement, whereas EPD gives the least percentage deviation for the output factors of 6 MV-FFF beams. Hence, the data generated using these detectors for the respective energies can be used for beam modelling in the treatment planning system.