ABSTRACT
Purpose: We investigated the feasibility of biology-guided radiotherapy (BgRT), a technique that utilizes real-time positron emission imaging to minimize tumor motion uncertainties, to spare nearby organs at risk. Methods: Volumetric modulated arc therapy (VMAT), intensity-modulated proton (IMPT) therapy, and BgRT plans were created for a paratracheal node recurrence (case 1; 60 Gy in 10 fractions) and a primary peripheral left upper lobe adenocarcinoma (case 2; 50 Gy in four fractions). Results: For case 1, BgRT produced lower bronchus V40 values compared to VMAT and IMPT. For case 2, total lung V20 was lower in the BgRT case compared to VMAT and IMPT. Conclusions: BgRT has the potential to reduce the radiation dose to proximal critical structures but requires further detailed investigation.
ABSTRACT
PURPOSE: The purpose of this study is to provide data for the calibration of the recent RefleXion TM biology-guided radiotherapy (BgRT) machine (Hayward, CA, USA) following the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans et al. International Atomic Energy Agency, Vienna, 2017) and (Mirzakhanian et al. Med Phys, 2020). METHODS: In RefleXion BgRT machine, reference dosimetry was performed using two methodologies described in TRS-483 and (Mirzakhanian et al. Med Phys, 2020) In the first approach (Approach 1), the generic beam quality correction factor k Q A , Q 0 f A , f ref was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The k Q A , Q 0 f A , f ref is a beam quality factor that corrects N D , w , Q 0 f ref (absorbed dose to water calibration coefficient in a calibration beam quality Q 0 ) for the differences between the response of the chamber in the conventional reference calibration field f ref with beam quality Q 0 at the standards laboratory and the response of the chamber in the user's A field f A with beam quality Q A . Field A represents the reference calibration field that does not fulfill msr conditions. In the second approach (Approach 2), a square equivalent field size was determined for field A of 10 × 2 cm 2 and 10 × 3 cm 2 . Knowing the equivalent field size, the beam quality specifier for the hypothetical 10 × 10 cm 2 field size was derived. This was used to calculate the beam quality correction factor analytically for the six chamber types using the TRS-398. (Andreo et al. Int Atom Energy Agency 420, 2001) Here, TRS-398 was used instead of TRS-483 since the beam quality correction values for the chambers used in this study are not tabulated in TRS-483. The accuracy of Approach 2 is studied in comparison to Approach 1. RESULTS: Among the chambers, the PTW 31010 had the largest k Q A , Q 0 f A , f ref correction due to the volume averaging effect. The smallest-volume chamber (IBA CC01) had the smallest correction followed by the other microchambers Exradin-A14 and -A14SL. The equivalent square fields sizes were found to be 3.6 cm and 4.8 cm for the 10 × 2 cm 2 and 10 × 3 cm 2 field sizes, respectively. The beam quality correction factors calculated using the two approaches were within 0.27% for all chambers except IBA CC01. The latter chamber has an electrode made of steel and the differences between the correction calculated using the two approaches was the largest, that is, 0.5%. CONCLUSIONS: In this study, we provided the k Q A , Q 0 f A , f ref values as a function of the beam quality specifier at the RefleXion BgRT setup ( TPR 20 , 10 ( S ) and % d d ( 10 , S ) x ) for six chamber types. We suggest using the first approach for calibration of the RefleXion BgRT machine. However, if the MC correction is not available for a user's detector, the user can use the second approach for estimating the beam quality correction factor to sufficient accuracy (0.3%) provided the chamber electrode is not made of high Z material.
Subject(s)
Nuclear Energy , Radiometry , Biology , Calibration , Monte Carlo Method , Photons , United StatesABSTRACT
PURPOSE: The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine-specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS-483 code of practice (COP) (Palmans et al. TRS-483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6 MV flattening filter free (FFF) beam and with field sizes down to 10 × 2 cm2 . METHODS: Two methods of calibration were provided following the TRS-483. In calibration Method I, the generic correction factors k Q A , Q 0 f A , f ref were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm2 to 10 × 10 cm2 . In calibration Method II, we extended the methodology in TRS-483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm2 . The beam quality specifier for a hypothetical 10 × 10 cm2 field was derived by extending the methodology provided in the TRS-483. Since the beam quality correction values for the conventional reference field ( k Q , Q 0 f ref ) tabulated in TRS-483 are provided only for large reference chambers, we calculated the k Q , Q 0 f ref values analytically for our beam quality specifier and chambers used, using interaction data in TRS-398 (Andreo, et al. TRS-398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001). RESULTS: The k Q A , Q 0 f A , f ref correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS-483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm2 field size. However, this had a negligible impact on the k Q A , Q 0 f A , f ref values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and k Q A FFF , Q FFF f A , f ref values introduced in TRS-483 were taken into account. CONCLUSIONS: In this study, applying the k Q A , Q 0 f A , f ref correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the k Q A , Q 0 f A , f ref values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high-Z material unless the electrode perturbation effects and k Q A FFF , Q FFF f A , f ref values are taken into account.
