Absolute dosimetry for FLASH proton pencil beam scanning radiotherapy

Abstract

A paradigm shift is occurring in clinical oncology exploiting the recent discovery that short pulses of ultra-high dose rate (UHDR) radiation-FLASH radiotherapy-can significantly spare healthy tissues whilst still being at least as effective in curing cancer as radiotherapy at conventional dose rates. These properties promise reduced post-treatment complications, whilst improving patient access to proton beam radiotherapy and reducing costs. However, accurate dosimetry at UHDR is extremely complicated. This work presents measurements performed with a primary-standard proton calorimeter and derivation of the required correction factors needed to determine absolute dose for FLASH proton beam radiotherapy with an uncertainty of 0.9% (1[Formula: see text]), in line with that of conventional treatments. The establishment of a primary standard for FLASH proton radiotherapy improves accuracy and consistency of the dose delivered and is crucial for the safe implementation of clinical trials, and beyond, for this new treatment modality.

Figure 1

Measured temperature in Kelvin using the NPL PSPC for a FLASH irradiation. The symbols in blue mark approximately the start (*) and end (x) of irradiation as well as the end and start of the pre- and post- irradiation temperature drift curves, respectively.

Figure 2

Measured and simulated dose distributions for the field 12 × 5 cm². Depth- (a) and lateral-dose distributions (b,c). Solid circles represent the measured data, dashed curves represent the simulated data, and open squares and crosses the percentage difference between simulated and measured data using TOPAS and FLUKA, respectively. The field sizes were defined at the 90% of the isodose line. For simplification, the fields were named to the field size rounded down to the nearest integer.

Figure 3

Normalised saturation curves measured for various ionisation chamber types. Data were normalised to the averaged measured signal for the maximum applied voltage for each ionisation chamber type. Note that the PTW Farmer operated at 600 V has not reached saturation.

Figure 4

Ratio between the dose determined by the NPL PSPC and the dose derived from ionisation chambers for the various fields tested. The dashed lines represent the mean ratio value for each ionisation chamber type and the error bars represent type A standard uncertainties.

Figure 5

Experimental setup mounted on the patient couch and radiograph of the NPL PSPC. (a) (1) Gantry, (2) NPL PSPC, (3) measurement instrumentation for the NPL PSPC, (4) vacuum pump, (5) ionisation chamber setup. (b) Radiograph of the front view of the NPL PSPC.