KEY POINTS
- Investigators evaluated TLD-100 powder using 87.2 MeV synchrotron-generated protons across in-pulse dose rates from 0.16 to 1,640 Gy/s. Measurements were performed in the low-LET entrance region to separate dose-rate effects from LET-dependent detector response.
- Fifteen dose-rate conditions and three absorbed doses—3, 9, and 18 Gy—were evaluated across three experimental sessions. After excluding ten outliers, the final analysis included 161 TLD measurements.
- At dose rates below 7 Gy/s, measured-to-expected dose ratios ranged from approximately 1.02 to 1.07, representing a 2%–7% over-response. The authors attributed this to efficient carrier trapping at lower ionization density.
- Between 7 and 300 Gy/s, response remained within approximately 4% of unity. Above 300 Gy/s, TLDs under-responded by roughly 2%–4%, with pooled response ratios of approximately 0.958–0.984.
- Dose rate explained 82.9% of total response variance in the two-way analysis (p < 0.0001), whereas absorbed dose explained only 2.3%. Mean responses at 3, 9, and 18 Gy differed by less than 1.2%, with no significant pairwise differences.
- The conventional clinical spot-scanning beam produced near-unity response at an in-pulse rate of 0.3 Gy/s, despite falling inside the experimental over-response region. This discrepancy suggests that pulse duration, spill structure, repetition rate, and duty cycle matter in addition to nominal dose rate.
- The findings apply to a synchrotron with millisecond-scale spill delivery and one beam energy at low LET. Combined LET and dose-rate effects near the Bragg peak, and applicability to cyclotrons or synchrocyclotrons, remain untested.
CLINICAL TAKEAWAY
TLD-100 may underestimate dose during synchrotron proton FLASH above approximately 300 Gy/s and may require dose-rate-specific correction. The thresholds should not be generalized across proton systems because temporal beam microstructure appears to influence detector response independently of nominal dose rate.