Design of experiments in medical physics: Application to the AAA beam model validation.

PURPOSE: The purpose of this study is to evaluate the usefulness of the design of experiments in the analysis of multiparametric problems related to the quality assurance in radiotherapy. The main motivation is to use this statistical method to optimize the quality assurance processes in the validation of beam models. METHOD: Considering the Varian Eclipse system, eight parameters with several levels were selected: energy, MLC, depth, X, Y1 and Y2 jaw dimensions, wedge and wedge jaw. A Taguchi table was used to define 72 validation tests. Measurements were conducted in water using a CC04 on a TrueBeam STx, a TrueBeam Tx, a Trilogy and a 2300IX accelerator matched by the vendor. Dose was computed using the AAA algorithm. The same raw data was used for all accelerators during the beam modelling. RESULTS: The mean difference between computed and measured doses was 0.1±0.5% for all beams and all accelerators with a maximum difference of 2.4% (under the 3% tolerance level). For all beams, the measured doses were within 0.6% for all accelerators. The energy was found to be an influencing parameter but the deviations observed were smaller than 1% and not considered clinically significant. CONCLUSION: Designs of experiment can help define the optimal measurement set to validate a beam model. The proposed method can be used to identify the prognostic factors of dose accuracy. The beam models were validated for the 4 accelerators which were found dosimetrically equivalent even though the accelerator characteristics differ.

[Dosimetric study of the different techniques to deal with respiratory motion for lung stereotactic radiotherapy]. / Étude dosimétrique des différentes techniques de gestion du mouvement respiratoire pour l'irradiation thoracique en conditions stéréotaxiques.

PURPOSE: To evaluate the different respiratory movement management techniques during irradiation of lung tumours. PATIENTS AND METHODS: Seven patients with one or more primary or secondary lung lesions less than 5 cm (11 tumours in total) had three computed tomographies (CT): free-breathing, deep-inspiration breath-hold using a spirometer, and 4-dimensional (4D). From these three acquisitions, five treatment plans were performed: free-breathing (reference method), deep-inspiration breath-hold, and three from the 4D CT: two breathing synchronized treatments (inspiration and expiration) and one treatment taking into account all the tumour motions (definition of the internal target volume [ITV]). Planning target volume (PTV) size and dose delivered to the lungs were compared. RESULTS: Mean PTV with the free-breathing modality was 83 ± 28 cm(3), which was significantly greater than any of the other techniques (P<0.0001). Compared to the free-breathing PTV, PTV defined with the ITV was reduced by one quarter (63 ± 31 cm(3)), and PTV with the deep-inspiration breath-hold, breathing synchronized inspiration and breathing synchronized expiration techniques were reduced by one third (50 to 54 ± 24 to 26 cm(3)). Deep-inspiration led to significantly increase the healthy lung volume compared to other methods (mean volume of 5500 ± 1500 cm(3) versus 3540 to 3920 cm(3), respectively, P<0.0001). The volume of healthy lungs receiving at least 5 and 20 Gy (V5 and V5) were significantly higher with the free-breathing method than any of the other methods (P<0.0001). The deep-inspiration breath-hold modality led to the lowest lung V5 and V20. CONCLUSION: Deep-inspiration breath-hold technique provides the most significant dosimetric advantages: small PTV and large lung volume. However, patients must be able to hold 20 seconds of apnea. Respiratory gating also reduces the PTV, but its application often requires the implantation of fiducial, which limit its use. A 4-dimensional CT allows for a personalized and reduced PTV compared to free-breathing CT.