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Towards Best Practice Guidelines for Reference Dosimetry in a 1.5 T MR-Linac

V Iakovenko1*, B Keller1, H Nusrat1, A Sahgal2, A Sarfehnia1, (1) Sunnybrook Health Sciences Centre, Odette Cancer Centre, Department of Medical Physics; University of Toronto, Department of Radiation Oncology, Toronto, ON, CA, (2) Sunnybrook Health Sciences Centre, Odette Cancer Centre, Department of Radiation Oncology, ON, CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To develop guidelines for reference dosimetry within TG-51 formalism for a 1.5 T MR-linac and to analyze parameters and procedures impacting reference dosimetry in a strong magnetic field.


Methods: In the absence of a formalized TG report for reference dosimetry in MR-integrated linacs, the current suggested formalism follows TG-51 protocol with a modified quality conversion factor k(Q)???=k(Q)k(B) where k(B) further corrects k(Q) by accounting for the effects of magnetic field on ionization chamber response and beam quality. In this work, we identify and quantify various sources of uncertainty that impact each of the parameters in the currently proposed formalism, and evaluate their overall contribution to final dose. Measurements are done in a 1.5T MR-linac (Unity, Elekta AB, Stockholm, Sweden) which integrates a 1.5T Philips magnet and a 7MV-FFFlinac. Several reference-class small volume (Exradin:A1SL, IBA:CC13, PTW:Semiflex-3D) and Farmer type (Exradin:A19, IBA:FC65-G) ionization chambers along with an in-house built MR-compatible water tank were used.


Results: A translational displacement of ±3mm results in dose variation of ?<0.1%; while ±5° rotation gives (?<0.45%) for all 5 ionization chambers. We measured a larger dose variation for horizontal rotation (?<0.45%) than vertical (?<0.28%), which we associate with the gradient of k(Q)??? as a function of chamber orientation with respect to direction of B-field. Uncertainty in P(ion) (for two depths), P(pol) (with various sub-studies including effects of cable length, connector type, etc in magnetic environment), and P(rp) were determined. k(Q)??? was measured for two reference depths and at four cardinal angle orientations. Long-term reproducibility and stability of beam quality (TPR²°10=0.669±0.01%) was also measured in 2 Elekta Unity MR-linacs.


Conclusion: The results of this work can be used 1) to identify best practice guidelines for reference dosimetry in the presence of magnetic fields, and to 2) evaluate an uncertainty budget for future reference dosimetry protocols of MR-linac.

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Keywords

Dosimetry, Quality Assurance, Ionization Chamber

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined: experimental dosimetry (ion chamber)

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