J. De Pooter

Stereotactic arc therapy for small elongated tumors using cones and collimator jaws; dosimetric and planning aspects.

Stereotactic arc treatment of small intracranial tumors is usually performed with arcs collimated by circular cones, resulting in treatment volumes which are basically spherical. For nonspherical lesions this results in a suboptimal dose distribution. Multiple isocenters may improve the dose conformity for these lesions, at the cost of large overdosages in the target volume. To achieve improved dose conformity as well as dose homogeneity, the linac jaws (with a minimum distance of 1.0 cm to the central beam axis) can routinely be used to block part of the circular beams. The purpose of this study was to investigate the feasibility of blocking cones with diameters as small as 1.0 cm and a minimum distance between the jaw and the central beam axis of 0.3 cm. First, the reproducibility in jaw positioning and resulting dose delivery on the treatment unit were assessed. Second, the accuracy of the TPS dose calculation for these small fields was established. Finally, clinically applied treatment plans using nonblocked cones were compared with plans using the partially blocked cones for several treatment sites. The reproducibility in dose delivery on our Varian Clinac 2300 C/D machines on the central beam axis is 0.8% (1 SD). The accuracy of the treatment planning system dose calculation algorithm is critically dependent on the used fits for the penumbra and the phantom scatter. The average deviation of calculated from measured dose on the central beam axis is -1.0%+/-1.4% (1 SD), which is clinically acceptable. Partial cone blocking results in improved dose distributions for elongated tumors, such as vestibular schwannoma and uveal melanoma. Multiple isocenters may be avoided. The technique is easy to implement and requires no additional workload.

TH-CD-BRA-03: Direct Measurement of Magnetic Field Correction Factors, KQB, for Application in Future Codes of Practice for Reference Dosimetry

PURPOSE With an MR-linac, radiation is delivered in the presence of a magnetic field. Modifications in the codes of practice (CoPs) for reference dosimetry are required to incorporate the effect of the magnetic field. METHODS In most CoPs the absorbed dose is determined using the well-known kQ formalism as the product of the calibration coefficient, the corrected electrometer reading and kQ, to account for the difference in beam quality. To keep a similar formalism a single correction factor is introduced which replaces kQ, and which corrects for beam quality and B-field, kQ,B. In this study we propose a method to determine kQ,B under reference conditions in the MRLinac without using a primary standard, as the product of:- the ratio between detector readings without and with B-field (kB),- the ratio between doses in the point of measurement with and without B-field (rho),- kQ in the absence of the B-field in the MRLinac beam (kQmrl0,Q0),The ratio of the readings, which covers the change in detector reading due to the different electron trajectories in the detector, was measured with a waterproof ionization chamber (IBA-FC65g) in a water phantom in the MRLinac without and with B-field. The change in dose-to-water in the point of measurement due to the B-field was determined with a Monte Carlo based TPS. RESULTS For the presented approach, the measured ratio of readings is 0.956, the calculated ratio of doses in the point of measurement is 0.995. Based on TPR20,10 measurements kQ was calculated as 0.989 using NCS-18. This yields a value of 0.9408 for kQ,B. CONCLUSION The presented approach to determine kQ,B agrees with a method based on primary standards within 0.4% with an uncertainty of 1% (1 std.uncert). It differs from a similar approach using a PMMA-phantom and an NE2571 chamber with 1.3%.

SU-E-T-51: A National QA Audit of QA Systems Used for IMRT and VMAT Patient QA

Purpose: To independently validate patient-specific quality assurance for IMRT and VMAT plans using the same set of treatment plans for all institutes. Methods: In February 2014 we devised a set of treatment plans: simple IMRT/VMAT plans; more complex IMRT/VMAT plans and a stereotactic VMAT plan, all 6MV for both Varian and Elekta linacs. In total we used 5 Varian and 8 Elekta plans. The plans were imported in the institute’s treatment planning system for dose computation on the phantom of the audit team and the institute’s phantom. Additionally, 10x10 cm2 fields were made and computed on both phantoms. Next, the audit team performed measurements using the audit equipment. So far, 18 audits have been performed and we expect to have concluded the audits by June 2015. The measurements were performed using an ionization chamber (PinPoint, PTW), Gafchromic film and a 2D ionization chamber array, all in an octagonal phantom (Octavius, PTW, see Figure). Differences between the measured and computed 2D dose distributions were investigated using a gamma analysis with a 5%/1mm criterion for the stereotactic treatment plan and 3%/3mm for the other plans. Additionally, the participating centres performed QA measurements of the same treatment plans according to their local protocol and equipment. Results: For the 10x10 field on the phantom, the first 18 audits showed differences with respect to the planning of −0.21 (range: −1.8; 2.2)%. See Table 1 for more results. The findings compared well with the QA measurement results reported by the institutions according to their local protocols. Conclusion: These preliminary results demonstrate that such a national QA audit is feasible. Importing and computing the prepared treatment plans in the planning systems in use in the country is achievable. The local QA systems provided similar results as found with the audit.

WE-G-17A-06: A Water Calorimeter for Use in MRI Linacs

PURPOSE At VSL, Dutch Metrology Institute, a new water calorimeter was developed with the purpose to replace the existing primary standard for absorbed dose to water in the Netherlands. The new water calorimeter is designed to be operable in medium- to high energy photon beams, electrons, protons as well as MRI integrated linear accelerators. VSL has operated a water calorimeter since 2001. This calorimeter formed the basis for the NCS-18 dosimetry protocol, which is commonly applied by medical physicists in the Netherlands and Belgium. METHODS The unit Gray is the unit of interest for measurement of the absorbed dose to water. Water calorimetry involves the measurement of a small temperature rise (0.24 mK/Gy) with an uncertainty of less than 1 μK/Gy at a temperature of 4 °C. Using extensive multi-physics simulations the new calorimeters thermal performance was simulated before it was constructed at the end of 2013. With the advent of radiotherapy treatment units incorporating MR imaging the performance of the thermistor temperature sensors were characterized in a 1.5 T magnetic field. RESULTS A change of thermistor resistance was observed of less than 0.004% as a Result of the magneto-resistance effect in a 1.5 T magnetic field. Although a magneto-resistance effect was detectable, the effect on the temperature response in the water calorimeter was found to be negligible. CONCLUSION With the realization of the new calorimeter operable in MRI linacs and designed for use in a variety of beam modalities, VSL is ready for accurate dosimetry in new advanced radiotherapy modalities. Due to the small form factor the calorimeter can be used on location in the actual therapy beam inside a 68 cm linac bore. This work was supported by EMRP grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.