Background and Aims

The FLASH effect occurs in tissue when therapeutic radiation dose is delivered at ultra-high dose-rates (UHDR), greater than 40 Gy/s. At these dose-rates, the probability of sparing healthy tissue is greatly enhanced whilst damage to cancerous tissue remains devastating. In the clinic, accurate determination of absorbed-dose delivered to the target region at UHDR is challenging due to inefficient collection of charge within ionisation chambers (IC), and over-response of radiochromic film (RCF).

National Physical Laboratory (NPL), scientists re-purposed a simple portable graphite calorimeter (SPGC), for use with UHDR particle beams.

Methods

Measurements were carried out using a clinical 250 MeV scanned-proton beam system adapted to deliver FLASH proton radiotherapy beams and compared with the NPL primary-standard level graphite proton calorimeter, IC, RCF and alanine pellets, all in terms of dose-to-water.

Results

Preliminary analysis indicates agreement between the SPGC and primary-standard level calorimeter is within the overall combined measurement uncertainties of 1.5%, k=1. Calculation of calorimeter perturbation factors using Monte Carlo simulations are ongoing, as well as analysis of RCF, IC and alanine data.

Conclusions

This presentation will explain the measurement protocol carried out, present the results obtained and the associated levels of measurement uncertainty to show how a simple, portable calorimeter can be an effective tool in the clinic to accurately determine absorbed-dose delivered to the target at a significantly lower value of measurement uncertainty than current IC measurement protocols, typically >2%, k=1, or to determine correction factors for IC and RCF measurements in the clinic.

This project was funded by EMPIR 18HLT04UHDpulse