Background/Objectives: Spatial fractionation of proton fields as sub-millimeter beamlets to treat cancer has shown better sparing of healthy tissue whilst maintaining the same tumor control. It is critical to ensure primary standard dosimetry is accurate and ready to support the modality’s clinical implementation. Methods: This work provided a proof-of-concept, using the National Physical Laboratory’s Primary Standard Proton Calorimeter (PSPC) to measure average absorbed dose-to-water in a pMBRT field. A 100 MeV mono-energetic field and a 2 cm wide SOBP were produced with a spot-scanned proton beam incident on a collimator comprising 15 slits of 400 µm width, each 5 cm long and separated by a center-to-center distance of 4 mm. Results: The results showed the uncertainty on the absorbed dose-to-water in the mono-energetic beam was dominated by contributions of 1.4% and 1.1% ($k$ = 1) for the NPL PSPC and PTW Roos chambers, respectively, originating from the achievable positioning accuracy of the devices. In comparison, the uncertainty due to positioning in the SOBP for both the NPL PSPC and PTW Roos chambers were 0.4%. Conclusions: These results highlight that it may be more accurate and reliable to perform reference dosimetry measuring the Dose-Area Product or in an SOBP for spatially fractionated fields.

Accurate dosimetry in Ultra-High Dose Rate (UHDR) beams is challenging because high levels of ion recombination occur within ionisation chambers used as reference dosimeters. A Small-body Portable Graphite Calorimeter (SPGC) exhibiting a dose-rate independent response was built to offer reduced uncertainty on secondary standard dosimetry in UHDR regimes. The aim of this study was to quantify the effect of the geometry and material properties of the device on the dose measurement. A detailed model of the SPGC was built in the Monte Carlo code TOPAS (v3.6.1) to derive the impurity and gap correction factors, $k_{\mathrm{i}\mathrm{m}\mathrm{p}}$ and $k_{\mathrm{g}\mathrm{a}\mathrm{p}}$. A dose conversion factor, $D_w^{\mathit{MC}}/D_g^{\mathit{MC}}$, was also calculated using FLUKA (v2021.2.0). These factors convert the average dose to its graphite core to the dose-to-water for a 249.7 MeV mono-energetic spot-scanned clinical proton beam. The effect of the surrounding Styrofoam on the dose measurement was examined in the simulations by substituting it for graphite. The $k_{\mathrm{i}\mathrm{m}\mathrm{p}}$ and $k_{\mathrm{g}\mathrm{a}\mathrm{p}}$ correction factors were 0.9993 ± 0.0002 and 1.0000 ± 0.0001, respectively when the Styrofoam was not substituted, and 1.0037 ± 0.0002 and 0.9999 ± 0.0001, respectively when substituted for graphite. The dose conversion factor was calculated to be 1.0806 ± 0.0001. All uncertainties are Type A. Impurity and gap correction factors, and the dose conversion factor were calculated for the SPGC in a FLASH proton beam. Separating out the effect of scatter from Styrofoam insulation showed this as the dominating correction factor, amounting to 1.0043 ± 0.0002.