Sensitivity test of a 3D diode array system for MLC and GANTRY errors in IMRT delivery

• Published in: Physica medica 2013-06, Vol.29, p.e36-e37
• Main Authors: Pignata, B, Udrescu, C, Chapet, O, Sotton, M.-P, Kerneur, G, Michel-Amadry, G, Ribouton, J, Jalade, P
• Format: Article
• Language: English
• Subjects: Radiology
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2013.08.114

Introduction Delta4 device is designed for three-dimensional dose verification and shows excellent reproducibility, linearity and doserate independence. The purpose of this work was to verify the sensitivity of the Delta4 (ScandiDos) for small MLC and gantry errors in intensity modulated radiotherapy (IMRT). Materials and methods Ten IMRT plans (5 prostate and 5 head and neck) were used. Systematic MLC errors of (–0.2), (–0.5), (–0.7), (–1), (–2), (–3), (–5) and (–10) mm, were introduced for one leaf. Gantry errors of 0.2°, 0.5°, 0.7°, 1°, 2°, 2.5°, 3°, 4° and 5° were introduced for the same plans. The Delta4 system was used to acquire and analyze the data. All the measurements were acquired consecutively for each patient. To eliminate the repeatability errors, standard deviations (SD) of the maximum gamma (Gmax) were calculated for 10 successively beams with no offsets. Gmax values were noted for both global gamma (normalization dose) and local gamma (local detector dose) with pass/fail criteria of 2%/2 mm. Gmax was evaluated for each modified field when compared to the measured field with no introduced error (0 mm or 0°). Because we had a single leaf with a small offset we evaluated only the Gmax and not the mean gamma or the percentage of area gamma < 1. Results For local gamma evaluation (LGE), the maximum SD for Gmax was of 0.06. For global gamma evaluation (GGE), the maximum SD was of 0.02. The MLC and gantry offsets had a noticeable impact on the plan evaluation. For all the ten plans modifications in Gmax were observed starting with 0.2 mm (from 0.2 to 4.75 for LGE and from 0.13 to 2.45 for GGE) for MLC plans and with 0.2° for the gantry errors (from 0.54 to 9.23 for LGE and from 0.37 to 4.71 for GGE). Conclusion The Delta4 system seems to be a very sensitive instrument for the quality assurance of IMRT plans. In our study it detected submilimetric and subdegree errors for MLC and gantry, respectively. In clinical use these small errors may not be detected and could be hidden by the residual error between planned and delivered dose.