Beam monitor calibration in scanned light-ion beams.

PURPOSE: To propose a formalism for the reference dosimetry of scanned light-ion beams consistent with IAEA TRS-398 and Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)]. To identify machine-specific reference (msr) fields and plan-class specific reference (pcsr) fields consistent with the definitions given by Alfonso et al. To review the literature of beam monitor calibration in scanned beams using three different methods in terms of this common formalism. METHODS: Four types of msr fields are identified as those that are meant to calibrate the beam monitor for scanned beams with particular energies. Two types of pcsr fields are identified as those that are meant to apply one or more tuning factors to the entire delivery chain. RESULTS: The formalism establishes the energy-dependent relation between the number of particles incident on the phantom surface and the beam monitor reading and distinguishes three routes to determine the beam monitor calibration function: (i) the use of a calibrated reference ionization chamber in a single-layer scanned beam, (ii) the use of a cross-calibrated large-area parallel plate ionization chamber in a single-energy beamlet, and (iii) the use of a calibrated reference ionization chamber in a box field to adjust a calibration curve obtained by a Faraday cup or an ionization chamber. Examples of all three methods and comparisons between them from the literature are analysed. CONCLUSIONS: The formalism can form the basis of future dosimetry recommendations for scanned particle beams and the analysis of the literature data in terms of this formalism can form the basis of data compilations for the application of the dosimetry procedures.

Comment on 'Proton beam monitor chamber calibration'.

We comment on a recent article (Gomà et al 2014 Phys. Med. Biol. 59 4961-71) which compares different routes of reference dosimetry for the energy dependent beam monitor calibration in scanned proton beams. In this article, a 3% discrepancy is reported between a Faraday cup and a plane-parallel ionization chamber in the experimental determination of the number of protons per monitor unit. It is further claimed that similar discrepancies between calorimetry and ionization chamber based dosimetry indicate that [Formula: see text]-values tabulated for proton beams in IAEA TRS-398 might be overestimated. In this commentary we show, however, that this supporting argument misrepresents the evidence in the literature and that the results presented, together with published data, rather confirm that there exist unresolved problems with Faraday cup dosimetry. We also show that the comparison in terms of the number of protons gives a biased view on the uncertainty estimates for both detectors while the quantity of interest is absorbed dose to water or dose-area-product to water, even if a beam monitor is calibrated in terms of the number of protons. Gomà et al (2014 Phys. Med. Biol. 59 4961-71) also report on the discrepancy between cylindrical and plane-parallel ionization chambers and confirm experimentally that in the presence of a depth dose gradient, theoretical values of the effective point of measurement, or alternatively a gradient correction factor, account for the discrepancy. We believe this does not point to an error or shortcoming of IAEA TRS-398, which prescribes taking the centre of cylindrical ionization chambers as reference point, since it recommends reference dosimetry to be performed in the absence of a depth dose gradient. But these observations reveal that important aspects of beam monitor calibration in scanned proton beams are not addressed in IAEA TRS-398 given that those types of beams were not widely implemented at the time of its publication.

Assessment of improved organ at risk sparing for meningioma: light ion beam therapy as boost versus sole treatment option.

PURPOSE: To compare photons, protons and carbon ions and their combinations for treatment of atypical and anaplastical skull base meningioma. MATERIAL AND METHODS: Two planning target volumes (PTVinitial/PTVboost) were delineated for 10 patients (prescribed doses 50 Gy(RBE) and 10 Gy(RBE)). Plans for intensity modulated photon (IMXT), proton (IMPT) and carbon ion therapy ((12)C) were generated assuming a non-gantry scenario for particles. The following combinations were compared: IMXT+IMXT/IMPT/(12)C; IMPT+IMPT/(12)C; and (12)C+(12)C. Plan quality was evaluated by target conformity and homogeneity (CI, HI), V95%, D2% and D50% and dose-volume-histogram (DVH) parameters for organs-at-risk (OAR). If dose escalation was possible, it was performed until OAR tolerance levels were reached. RESULTS: CI was worst for IMXT. HI<0.05±0.01 for (12)C was significantly better than for IMXT. For all treatment options dose escalation above 60 Gy(RBE) was possible for four patients, but impossible for six patients. Compared to IMXT+IMXT, ion beam therapy showed an improved sparing for most OARs, e.g. using protons and carbon ions D50% was reduced by more than 50% for the ipsilateral eye and the brainstem. CONCLUSION: Highly conformal IMPT and (12)C plans could be generated with a non-gantry scenario. Improved OAR sparing favors both sole (12)C and/or IMPT plans.