Use of parallel-plate ionization chambers in reference dosimetry of NOVAC and LIAC® mobile electron linear accelerators for intraoperative radiotherapy: a multi-center survey.

PURPOSE: LIAC® and NOVAC are two mobile linear accelerators dedicated to intraoperative radiation therapy (IORT), generating electron beams in the energy range of 3-12 MeV. Due to high dose-per-pulse (up to 70 mGy per pulse), in 2003 the Italian National Institute of Health (ISS) stated that "for the measure of dose to water in reference conditions, ionization chambers cannot be employed and no published dosimetry protocol can be used". As a consequence, ferrous sulphate (or, alternatively alanine) dosimetry was recommended. Based on a retrospective multi-center survey, a comparison with ferrous sulphate dosimetry is now used to validate the parallel-plate ionization chambers for reference dosimetry of NOVAC and LIAC. METHODS: The IAEA TRS-398 dosimetry protocol was applied except for the reference irradiation setup and the determination of the ion-recombination correction factor ks . Moreover the depth of maximum dose (R100 ) instead of zref as measurement depth was chosen by the majority of centers, thus implying a renormalization of the beam-quality correction factor kQ,Qo , based on water-air stopping power ratios. Regarding the ks determination, a previously published method, independent of ferrous sulphate dosimetry, was adopted. All the centers participating in this study had used both ferrous sulphate dosimeters and ionization chambers in water phantoms for dosimetry under reference conditions. RESULTS: The mean percentage difference between ionization chambers and ferrous sulphate dosimetry was -0.5% with a dispersion of 3.9% (2σ). Moreover, the uncertainty analysis allowed the agreement between ionization chambers and ferrous sulphate dosimetry to be verified. These results did not show any significant dependence on electron energy, thus indirectly confirming kQ,Qo renormalization. The results from the centers using zref as the measurement depth were similar to the other data, but further focused studies could aim at investigating possible dependences of the dose differences on the chosen reference depth. CONCLUSION: The present study confirms that parallel-plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external-beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT, so that users may benefit from specific, authoritative and up-to-date recommendations.

Development and optimization of a beam shaper device for a mobile dedicated IOERT accelerator.

PURPOSE: The aim of this study was to design and build a prototype beam shaper to be used on a dedicated mobile accelerator that protects organs at risk within the radiation field and conforms the beam to the target geometry during intraoperative electron radiotherapy (IOERT). A dosimetric characterization of the beam shaper device was performed based on Monte Carlo (MC) simulations, as well as experimental data, at different energies, field sizes, and source to skin distances. METHODS: A mobile light intraoperative accelerator (LIAC(®), Sordina, Italy) was used. The design of the beam shaper prototype was based on MC simulations (BEAMnrc∕OMEGA and DOSXYZnrc code) for a selection of materials and thicknesses, as well as for dosimetric characterization. Percentage depth dose (PDD) and profile measurements were performed using a p-type silicon diode and a commercial water phantom, while output factors were measured using a PinPoint ion chamber in a PMMA phantom. Planar doses in planes of interest were carried out using radiochromic films (Gafchromic(TM) EBT and EBT2) in PMMA and in a Solid Water(®) phantom. Several experimental set-ups were investigated with the beam shaper device fixed on the top of the phantom, varying both the short side of the rectangular field and the air gap between the device and the phantom surface, simulating the clinical situation. The output factors (OFs) were determined using different geometrical set-ups and energies. RESULTS: The beam shaper prototype consists of four blades sliding alongside each other and mounted on a special support at the end of the 10 cm diameter PMMA circular applicator. Each blade is made of an upper layer of 2.6 cm of Teflon(®) and a lower layer of 8 mm of stainless steel. All rectangles inscribed in a 5 cm diameter can be achieved in addition to any "squircle-shaped" field. When one side of the rectangular field is held constant and the second side is reduced, both R(50) and R(max) move towards the phantom surface. Comparing the PDDs obtained with the 5 cm circular applicator and with a 4.4 × 4.4 cm(2) square field (that is the equivalent square of the 5 cm circular field) obtained with the beam shaper, a different behavior was observed in the region extending from the surface to a depth of 50% of the maximum dose. Isodoses measured for rectangular fields used for clinical cases (i.e., 4 × 9 cm(2) 8 MeV) are shown, with different air gaps. For each energy investigated, the normalized OFs slowly increase, when the length of the side decreases down to about 4 cm, and then rapidly decreases for smaller field widths. MC simulation showed an excellent agreement with experimental data (<2%). CONCLUSIONS: The beam shaper device is able to provide square∕rectangular∕squircle fields with adequate dose homogeneity for mobile dedicated accelerators, thus allowing conformal treatment with IOERT. Monte Carlo simulation can be a very useful tool to simulate any clinical set up and can be used to create a data set to calculate MUs, thereby increasing the accuracy of the delivered dose during IOERT procedures.