Quality assurance of flattening-filter-free beams (FFF)

Quality assurance of flattening-filter-free beams (FFF)

SFPM Annual Meeting 2012 mean dose. We consider tuning the RA plans optimization objectives and investigating the TomotherapyÒ technique on these pati...

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SFPM Annual Meeting 2012 mean dose. We consider tuning the RA plans optimization objectives and investigating the TomotherapyÒ technique on these patients. Keywords: Esophageal cancer, RapidArc, 3D-Conformal Radiation Therapy

9 A dosimetric comparison of tomotherapy and volumetric modulated arc therapy (VMAT) in the treatment of high risk prostate cancer with pelvic nodal radiotherapy T. Lacornerie1, F. Cavillon2, C. Touzeau3, M. Touleimat3, E. Tresch4, E. Lartigau5 and D. Pasquier3,5 1 Service de Physique Me´dicale du Centre Oscar Lambret Lille, France, 2Faculte´ Libre de Me´decine Lille, France, 3Centre Galile´e, Clinique de la Louvie`re Lille, France, 4Unite´ de Me´thodologie et Biostatistique Centre O. Lambret, France, 5 De´partement Universitaire de Radiothe´rapie Centre O. Lambret, France Introduction: Purpose: To compare the dosimetric results of volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT) in the treatment of high-risk prostate cancer with pelvic nodal radiotherapy. Methods: Plans were generated for ten consecutive patients treated for high-risk prostate cancer with prophylactic whole pelvic radiotherapy (WPRT) using VMAT and HT. After WPRT, a sequential boost was delivered to the prostate. Plan quality was assessed according to the ICRU 83 report’s criteria: the near-minimal (D98%), near-maximal (D2%) and median (D50%) doses, the homogeneity index (HI) and the Dice similarity coefficient (DSC). Beam-on time, integral dose and several organs at risk (OAR) dosimetric indexes were also compared. Results: For WPRT, HT was able to provide a higher D98% than VMAT (44.30.3 Gy and 43.90.5 Gy, respectively; p¼0.032) and a lower D2% than VMAT (47.30.3 Gy and 49.10.7 Gy, respectively; p¼0.005), leading to a better HI. The DSC was better for WPRT with HT (0.890.009) than with VMAT (0.800.02; p¼0.002). The dosimetric indexes for the prostate boost did not differ significantly. VMAT provided better rectum wall sparing at higher doses (V70,V75,D2%). Conversely, HT provided better bladder wall sparing (V50,V60,V70), except at lower doses (V20). The beam-on times for WPRT and prostate boost were shorter with VMAT than with HT (3.10.1 vs. 7.40.6 min, respectively; p¼0.002, and 1.50.05 vs. 3.70.3 min, respectively; p¼0.002). The integral dose was slightly lower for VMAT. Conclusion: VMAT and HT provided very similar and highly conformal plans that complied well with OAR dose-volume constraints. Although some dosimetric differences are statistically significant, they remained small. HT provided a more homogeneous dose distribution, whereas VMAT enabled a shorter delivery time. Keywords: Comparison VMAT Tomotherapy

10 Dose delivery verification in helical tomotherapy using transmission dosimetry: Clinical experience C. Lesven, S. Losa, A. Mazal and P. Franc ¸ois Service de physique me´dicale, Institut Curie, Paris, France, St Cloud, France Introduction: In-vivo dosimetry represents a great challenge for complex techniques such as Tomotherapy. In a previous work, a dose-verification software tool supplied by Tomotherapy was evaluated. This tool uses transmission dosimetry to reconstruct the dose delivered to the patient. Subsequent results lead us to determine a 7% tolerance limit for use in clinical routine. The aim of

S3 this study is to analyse the reconstructed dose from patient treatments delivered and to compare it to previous tolerance limit. Materials/Methods: A group of 50 patients treated on the Tomotherapy Hi-Art system between August and December 2011 was studied. For each patient, specific verification method was established where the number of in-vivo measurements depended on the fractionation schedule. Data were collected the first three fractions and then every seven fractions. After each treatment fraction delivery, exit detector collected signal was used to reconstruct the incoming fluence. This fluence was then re-projected on the daily MVCT scan to calculate the delivered dose which was then compared to the local planned dose. A selection of different points of interest for dose reconstruction was made depending on the number of target volumes for each patient. This constituted the first level of our dose verification. As a second level, if the dose difference obtained was beyond 7%, then different action plans were implemented including a full 3D dose reconstruction. Results/Conclusion: For 80% of the patients controlled, dose differences fall within 7% tolerances. Differences beyond 7% were further investigated. In most cases, discrepancies could be explained by some technical issues related to either MVCT acquisition or also patient positioning and anatomic changes. As an alternative to conventional in-vivo dosimetry, this software resulted to be suitable for in-vivo dosimetry for Tomotherapy system. Keywords: Tomotherapy, transmission dosimetry, In-vivo

11 Quality assurance of flattening-filter-free beams (FFF) R. Garcia, C. Khamphan, V. Bodez, E. Jaegle, M.E. Alayrach and A. Badey Institut Sainte Catherine, Avignon, France Introduction: The new generation of linear accelerators allows the use of X-ray beams without the presence of Flattening Filter. These beams are inhomogeneous and allow dose rates two to four times higher than those currently available. They are to be combined with intensity modulation to homogeneously irradiate a target volume. However, they are radically transforming the measurement methods and quality controls. Several physical aspects need to implement new procedures not yet available. Materials and Methods: To establish specific recommendations to inhomogeneous beams and high dose rates, a questionnaire was then sent to experts physicists from 11 European centers, currently in charge of accelerators equipped with FFF beams. The questionnaire contained all the physical aspects that could potentially require changes to quality assurance protocols. It involved: the energy spectrum, the quality index, the symmetry, the non homogeneity, the penumbra, the surface dose, the output factor, the dose rate, the calibration, the tests frequency, the synchronization and the security. Results: All subjects have been have been the source of comments but without real consensus. However, some of them require special attention because they condition the measurements and analyzes. The penumbra can no longer rely on the 50% position of the dose profile. The measure of symmetry should combine the comparison of points and cross slopes of the profile. The non homogeneity replaces homogeneity. The high dose rates influence the detector use. The energy spectrum is very different but does not seem strongly conditioned dosimetry validations. The tolerances of the new tests stay to be determined. Conclusion: The use of Flattening-Filter-Free beams require adapting quality assurance protocols. Publications exist but do not yet provide a consensus on methods. Keywords: Quality Assurance, Flattening Filter, Dose Rate.