The following phantoms are available for the dosimetric audit based on end-to-end tests:
- The homogenous polystyrene phantom (HP phantom)
Seven polystyrene plates, each with a size of 20x20x3 cm3, are stacked together to form a 21 cm long phantom. Four plates, labelled A, B, C and D, hold 20 alanine pellets in total. In addition to the cavities for the alanine pellets, plates A and C have a depression to place a radiochromic film just on the top of the pellets and perpendicular to the beam direction.
Fig. 1, on the left: the four plates of the homogeneous phantom loaded with alanine pellets and EBT3 films; on the right: the homogeneous phantom loaded with a Farmer ionization chamber positioned on the treatment couch during the end-to-end test procedure at MedAustron (Carlino et al., 2018)
Another plate of the same outer dimensions holds an ionisation chamber (IC) (Farmer 0.6 cm3 PTW-TM30013). E2E testing with this phantom allows validating basic steps in planned dose delivery.
- The Proton Therapy Dosimetry head (PTDH) phantom
The phantom has been customized by CIRS (Norfolk, VI, USA) in order to allocate 22 alanine pellets or up to two ionisation chambers (see Figure 2). The phantom is sectioned in 20 mm increments for three film locations in the cranio-caudal direction starting from the approximate centre of the sagittal plane. This phantom allows using alanine pellets, radiochromic films and ionisation chambers for dose measurements in a realistic patient head geometry.
Fig. 2.1: half bulk part of the head phantom with alanine pellets inside; 2.2: alanine pellets placed inside the cubic brain tissue equivalent insert. Each cubic insert is subdivided in three slices. 2.3: CT scan of the head phantom with one Farmer chamber placed in the brain cavity. The second cavity is not filled in this image but during measurements with one chamber it is filled with substitute material. 2.4: half bulk part of the head phantom with one EBT3 film and alanine pellets inside the brain cavity (Carlino et al, 2018).
For alanine measurements the cubes are sliced in 3 parts and 22 alanine pellets are allocated for a single plan irradiation. The posterior insert for ICs accommodates one Farmer chamber PTW-TM30013. One radiochromic EBT3 film is placed on the bulk part of the head phantom on the central sagittal plane perpendicular to the beam axis (see figure 2). E2E testing with this phantom allows validating planned dose delivery for intracranial targets.
- The Pelvis Phantom (PP) was customized by CIRS (Norfolk, VI, USA) following MedAustron specifications. This phantom allows allocating different tissue-equivalent interchangeable cubes that accommodate Farmer IC PTW-TM30013, EBT3 films or alanine pellets. For alanine measurements the cubes are sliced in 3 parts and 22 alanine pellets are allocated for a single plan irradiation E2E test with this phantom allows validating planned dose delivery for abdominal targets.
Fig. 3, on the left: the pelvis phantom on the CT couch; on the right: the pelvis phantom loaded with a Farmer ionization chamber positioned on the treatment couch during the end-to-end to test procedure at Medaustron (Carlino et al, 2018).
The following detectors are used for dosimetric audit based on end-to-end test (E2E) at proton PBS facilities:
- The alanine pellets that are used for the determination of the delivered doses in homogeneous and anthropomorphic phantoms are provided by the National Physical Laboratory (NPL), Teddington, UK. The pellets irradiated during the audit are read out using Electron Paramagnetic Resonance (EPR) by the NPL Chemical Dosimetry group and a report is provided to the auditors with the results. The following characteristics of alanine detectors can be emphasized:
- Alanine is nearly water equivalent in terms of stopping powers and density (pellet densities actually close to PMMA)
- Alanine is an integrating dosimeter with very stable post-irradiation signal and no destructive readout making it suitable as an archiving dosimeter
- There is no dose rate dependence until extremely high dose rates making it very suitable for scanning beams with high instantaneous dose per pulse.
- Similar to other solid-state detectors there is also an LET and energy dependence (quenching effect) which can be corrected for (Carlino et al, 2018). Compared to other solid-state detectors alanine allows for a simpler modeling of the quenching.
- The sensitivity is lower compared to other detectors (a dose of 10 Gy is needed for a 0.5% readout reproducibility)
- A Farmer ionization chamber (PTW-TM30013) coupled with a Unidoswebline electrometer is calibrated in terms of absorbed dose to water in a 60Co reference beam at NPL (UK). The absorbed dose to water in a proton beam is derived using the formalism and beam quality correction factors from the IAEA TRS-398 code of practice.
- EBT3 radiochromic films are employed only for relative dosimetry to provide additional two-dimensional dose distribution information.