Sarfehnia, Arman

Abstract Background Magnetic Resonance‐guided Radiation Therapy (MRgRT) systems are increasingly used in clinical practice, necessitating a dedicated dosimetry protocol to address the challenges posed by the presence of strong magnetic fields. Purpose AAPM TG351 introduces a reference dosimetry protocol for external beam MRgRT systems, aligning with AAPM TG51 reports and incorporating elements from the IAEA TRS398 code of practice. Methods The protocol offers practical guidance on beam quality and dosimetry setup in the presence of strong magnetic fields. It reviews relevant literature to identify reference‐class chambers suitable for external beam MRgRT systems and determines their beam quality conversion and other relevant correction factors through a detailed analysis of available data and associated uncertainties. Results AAPM TG351 offers a comprehensive dosimetry protocol for external beam MRgRT systems, addressing the unique challenges posed by the presence of magnetic fields. It includes detailed guidance for measurement setups, a list of reference‐class chambers appropriate for reference dosimetry in MRgRT systems, and updated beam quality conversion and correction factors. Additionally, the protocol presents an in‐depth analysis of the uncertainty budget, identifying key contributors and providing strategies to minimize uncertainties, thereby enhancing clinical accuracy and reliability. Conclusions This protocol provides a practical, evidence‐based reference for medical physicists conducting reference dosimetry in external beam MRgRT systems.

AbstractBackgroundMagnetic resonance (MR)‐guided radiation therapy provides capabilities to utilize high‐resolution and real‐time MR imaging before and during treatment, which is critical for adaptive radiotherapy. This emerging modality has been promptly adopted in the clinic settings in advance of adaptations to reference dosimetry formalism that are needed to account for the presence of strong magnetic fields. In particular, the influence of magnetic field on the uncertainty of parameters in the reference dosimetry equation needs to be determined in order to fully characterize the uncertainty budget for reference dosimetry in MR‐guided radiation therapy systems.PurposeTo identify and quantify key sources of uncertainty in the reference dosimetry of external high energy radiotherapy beams in the presence of a strong magnetic field.MethodsIn the absence of a formalized Task Group report for reference dosimetry in MR‐integrated linacs, the currently suggested formalism follows the TG‐51 protocol with the addition of a quality conversion factor kBQ accounting for the effects of the magnetic field on ionization chamber response. In this work, we quantify various sources of uncertainty that impact each of the parameters in the formalism, and evaluate their overall contribution to the final dose. Measurements are done in a 1.5 T MR‐Linac (Unity, Elekta AB, Stockholm, Sweden) which integrates a 1.5 T Philips MR scanner and a 7 MVFFF linac. The responses of several reference‐class small volume ionization chambers (Exradin:A1SL, IBA:CC13, PTW:Semiflex‐3D) and Farmer type ionization chambers (Exradin:A19, IBA:FC65‐G) were evaluated throughout this process. Long‐term reproducibility and stability of beam quality, , was also measured with an in‐house built phantom.ResultsRelative to the conventional external high energy linacs, the uncertainty on overall reference dose in MR‐linac is more significantly affected by the chamber setup: A translational displacement along y‐axis of ± 3 mm results in dose variation of < |0.20| ± 0.02% (k = 1), while rotation of ± 5° in horizontal and vertical parallel planes relative to relative to the direction of magnetic field, did not exceed variation of < |0.44| ± 0.02% for all 5 ionization chambers. We measured a larger dose variation for xy‐plane (horizontal) rotations (< |0.44| ± 0.02% (k = 1)) than for yz‐plane (vertical) rotations (< ||0.28| ± 0.02% (k = 1)), which we associate with the gradient of kB,Q as a function of chamber orientation with respect to direction of the B0‐field. Uncertainty in Pion (for two depths), Ppol (with various sub‐studies including effects of cable length, cable looping in the MRgRT bore, connector type in magnetic environment), and Prp were determined. Combined conversion factor kQ× kB,Q was provided for two reference depths at four cardinal angle orientations. Over a two‐year period, beam quality was quite stable with being 0.669 ± 0.01%. The actual magnitude of was measured using identical equipment and compared between two different Elekta Unity MR‐Linacs with results agreeing to within 0.21%.ConclusionIn this work, the uncertainty of a number of parameters influencing reference dosimetry was quantified. The results of this work can be used to identify best practice guidelines for reference dosimetry in the presence of magnetic fields, and to evaluate an uncertainty budget for future reference dosimetry protocols for MR‐linac.