Work Package 3 (lead by NPL)

Reference for dosimetry with scanned proton and carbon ion beams: consistency and traceability

The aims of this WP are to determine and validate the correction factors for both water and graphite calorimeters in scanned beams and to compare the primary standards in terms of absorbed dose to water, to study the correction factors to be applied to ionisation chamber measurements in scanned beams experimentally and by means of calculations, to study the water equivalence of alanine and radiochromic films and other water equivalent phantom materials in high energy protons beams.

 

Task 3.1: Calorimetry for the realisation of absorbed dose to water in clinical scanned proton and ion beams

Graphite and water calorimeters have been previously established for dosimetry of passively scattered proton and carbon ion therapy beams but have not been thoroughly characterised for use in scanned particle beams. Within this task, the capability to calculate corrections for heat transfer in graphite and water calorimeters in scanned particle beams will be developed by coupling 4D Monte Carlo simulations of the time-dependent absorbed dose deposition patterns for scanned particle beams with finite element heat transfer simulations. The theoretical results from this work will be validated by experiments in both individual pencil beams and complex scanned beams. The chemical heat defect in water calorimeters exposed to scanned particle beams will be investigated by comparing the relative dose response of different chemical systems. These efforts will lead to the establishment of graphite and water calorimeters as primary standard level instruments for scanned particle beam dosimetry and finally, comparisons of both instrument types in scanned particle beams will be performed.

 

Task 3.2: Ionisation chamber dosimetry for scanned proton beams

The lack of characterisation in scanned particle beams applies evenly to ionisation chambers. Ion recombination in ionisation chambers will be studies for time-dependent dose deposition patterns that include partial irradiation conditions encountered in scanned particle beams by Monte Carlo simulations and experimental validation. Monte Carlo simulations will be used to calculate perturbation factors of ion chambers irradiate by abutting beamlets as compared to scattered beams that deliver the same dose distribution in water. Experimental validation of these simulations will be performed by measuring the relative dose response of different ionisation chamber types.

 

Task 3.3: Water equivalence and energy dependence of detector and phantom materials in scanned proton beams

The energy dependence of alanine and radiochromic film in high-energy protons will be characterised by comparing their relative dose response with ionisation chambers and recommendations will be produced on how to use these data for practical dose verification. The water-equivalence of relevant detector materials (alanine and radiochromic film) and water- and tissue-equivalent phantom materials will be evaluated by calculating depth dependent stopping power ratios and fluence correction factors using the Monte Carlo method and by measuring the fluence as a function of depth using plane-parallel ionisation chambers.

 

Work package leader: Hugo Palmans (NPL)