Commercial multileaf collimator (MLC) systems can employ leaves with rounded ends. Treatment planning beam modelling should consider the effects of transmission through rounded leaf ends to provide accurate dosimetry for IMRT treatments delivered with segmented MLC. We determined that an MLC leaf gap reduction of 1.4 mm is required to obtain an agreement between calculated and measured profile 50% dose points. A head and neck dosimetry phantom, supplied by the Radiological Physics Center (RPC), was planned and irradiated as a necessary credentialing requirement for the RTOG H-0022 protocol. The agreement between the RPC TLD measurements and treatment planning calculations was within experimental error for the primary and secondary planning target volumes (PTVs); however, the calculated mean dose for the critical structure was approximately 9% lower than the RPC TLD measurements. RPC radiochromic film profile measurements also indicated significant discrepancies (>5%) with calculated values especially in the high dose gradient region in the vicinity of the critical structure. These results substantiate our own in-house phantom measurements, performed with the same IMRT fields as for the RPC phantom experiment, using Kodak EDR2 film to measure absolute dose. Our results indicate a maximum underestimate of calculated dose of 12% with no leaf gap reduction. The discrepancy between measured and calculated phantom values is reduced to +/- 5% when a leaf gap reduction of 1.4 mm is used. A further improvement in the accuracy of dose calculation is not possible without a more accurate modelling of the leaf end transmission by the planning system. In the absence of published dosimetric criteria for IMRT our results stress the need for stringent in-house dosimetric QA and validation for IMRT treatments. We found the dosimetric validation service provided by the RPC to be a valuable component of our IMRT validation efforts.
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 kQA,Q0fA,fref 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 ( kQ,Q0fnormalref) tabulated in TRS‐483 are provided only for large reference chambers, we calculated the kQ,Q0fnormalref 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 kQA,Q0fA,fref 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 kQA,Q0fA,fref 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 kQAFFF,QFFFfA,fref values introduced in TRS‐483 were taken into account. Conclusions In this study, applying the kQA,Q0fA,fref correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absor...
Purpose The purpose of this study is to provide data for the calibration of the recent RefleXionTM 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 kQA,Q0fA,fref was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The kQA,Q0fA,fref is a beam quality factor that corrects ND,w,Q0fnormalref (absorbed dose to water calibration coefficient in a calibration beam quality Q0) for the differences between the response of the chamber in the conventional reference calibration field fref with beam quality Q0 at the standards laboratory and the response of the chamber in the user’s A field fA with beam quality QA. 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 10×20em0em0.277778emcm2 and 10×30em0em0.277778em cm 2. Knowing the equivalent field size, the beam quality specifier for the hypothetical 10×10cm2 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 kQA,Q0fA,fref 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 10×2cm2 and 10×3cm2 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 kQA,Q0fA,fref values as a function of the beam quality specifier at the RefleXion BgRT setup (TPR20,10false(normalSfalse) and %dd(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...
PurposeWe 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.MethodsVolumetric 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).ResultsFor 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.ConclusionsBgRT has the potential to reduce the radiation dose to proximal critical structures but requires further detailed investigation.
Purpose: To evaluate effectiveness of using six dosimetric indices in evaluating Cyber Knife SRS treatment plans. To investigate the dependence of these indices on the target volume and the histology of the treated tumor. To examine the effect of treating large volume lesions on the dosimetric properties of a treatment plan. Method and Materials: 154 treatment plans for Cyber Knife SRS of acoustic neuroma (AN), melanoma, meningioma, NSCLC and pituitary adenomas (PAs) were analyzed using six dosimetric indices: prescription isodose line PI, tumor isodose and volume coverage indices TI100 and TV100, homogeneity index HI, conformality index CI and a modified CI (mCI). These indices and their averages for each tumor type were examined for dependence on the size of the treated tumor and its histology. Results: TV100, PI and TI100 showed a decrease with tumor size, HI showed a slight increase with tumor size, while CI and mCI showed little dependence on the tumor size. CI for all five treated tumor types was closely clustered about 1.44, while HI showed greater dispersion for melanomas and NSCLC, but closer clustering about 1.39 for PAs, meningiomas and ANs. Conclusion: AN and melanoma plans showed the best on average tumor coverage while NSCLC showed the worst. Modified CI and CI indices were the lowest for the ANs and meningiomas, while HI performance was the best for ANs, intermediate for melanomas and PAs, and the worst for NSCLC. Modified CI showed very little dependence on the tumor size. PI and TV100 showed a trend towards rapid decrease with tumor size and therefore less coverage beyond 10 cc volumes for melanoma, meningioma and PA and beyond 100 cc for NSCLC. All six indices were found to be a useful tool for routine use in evaluating stereotactic treatment plans at our institution.
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