Program Information

Cherenkov Dosimetry Via Stopping-To-Cherenkov Power Ratios: Protocol and Experimental Feasibility

Y Zlateva

Y Zlateva¹*, J Seuntjens² , I El Naqa³ , (1) McGill University, Montreal, QC, (2) McGill University, Montreal, QC, (3) University of Michigan, Ann Arbor, MI

Presentations

SU-K-FS2-8 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: Four Seasons 2

Purpose: To develop a Cherenkov emission (CE)-based dosimetry method, which in contrast to ionization-based dosimetry, employs CE detection in water, converted to dose via charged-particle spectrum-averaged restricted collision stopping power-to-CE power ratios (SCRs). We Monte Carlo-calculate clinical electron beam SCRs, address beam quality specification, and study experimental feasibility.

Methods: 20 validated electron beam models were simulated with the EGSnrc code BEAMnrc. The SPRRZnrc code was modified to calculate Spencer-Attix SCRs in water by summing contributions from charged particle steps on-the-fly. An experimental feasibility study was performed with an optical fiber input on the side of a water tank, using a non-response-calibrated optical system including a water-soluble fluorophore to remove CE anisotropy (fluorescein, 0.5 g/L, excitation mean free path<2 mm). Non-normalized SCRs were computed as the ratio of detector counts to beam-axis PDD.

Results: As expected from the theory, SCRs are strongly beam quality and depth-dependent and independent of below-threshold contamination of typical electron beams. Experiments were in agreement with simulation. R50-based methods were proposed for beam quality specification (R50 rms/maximum deviation: 0.08/0.22 mm) and reference depth selection (PDD rms/maximum deviation: 0.19%/0.40%), which minimize the dosimetric uncertainty and are unaffected by below-threshold contamination. Experiments were in agreement with these simulation findings within uncertainty. For depths2.3 cm, a 7-coefficient SCR fit in terms of R50 and depth is proposed (PDD rms/maximum deviation: 0.4%/1.5%). For depths>R50, the PDD can be predicted from percent-depth CE (PDC) shifted downstream by the R50-C50 (depth of 50% CE) difference (PDD maximum deviation: 1.3%/1.7% for photon contamination<5%/<8%). Experimental validation of these methods was in good agreement with the simulations considering experimental limitations (PDD error<5%).

Conclusion: The proposed CE-based method is promising for high-resolution in-field dosimetry of electron-beam radiotherapy employing out-of-field detection, contingent on comprehensive experimental validation with a response-calibrated detector and well-defined measurement volume.

Funding Support, Disclosures, and Conflict of Interest: FRQNT B2 (grant #: 184385); CIHR (grant #: MOP-114910 and MOP-136774); NSERC (grant #: RGPIN 397711-11); YZ acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council of Canada (grant #: 432290).


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