ESTRO 37 Abstract book

S981

ESTRO 37

innovative method involving an optical microscope. This passive detector is used to characterise the dose distribution in key study for possible future clinical cases where the tumour can be irradiated applying interspersed microbeams (MBs) from different ports. The purpose of the experiment was the validation of the dose distribution, highlighting the dose reduction into normal healthy tissues, especially the bone, while delivering a higher dose in depth to the tumour. Material and Methods The European Synchrotron (ESRF) X-ray source was used with dose rates up to 16000 Gy/s and a mean energy of 100 keV to irradiate a human head phantom (CIRS). Parallel microbeams, 50 µm wide and spaced 1200 µm c- t-c, were applied from four different directions and rearranged in the tumour position in an array of MBs spaced 300 µm c-t-c. Gafchromic HD-V2 film was placed and irradiated at 8 cm depth from the phantom surface, at the target position. The human head phantom lacks the possibility to put a film inside its bone material at the appropriate depth. To evaluate the peak and valley dose delivered in bone tissue, a slab phantom was specifically created and irradiated to reproduce the geometry of the head phantom. The optical microscope was used to analyse the film on a micrometric scale. Results The read-out operation performed with the microscope is able to resolve the spatial dose distribution with a resolution of 2 µm and uncertainties of around 3.5%, the most accurate data achievable nowadays. The comparison between the dose profiles showed a peak dose in bone three times higher than in tumour position. The valley dose, i.e. the dose between two consecutive peaks, is 37% higher in target position irradiated with the array of interspersed MBs respect to the valley dose obtained in the bone. Conclusion The use of radiochromic films combined with their read- out using an optical microscope is a reliable and versatile way to perform dosimetry on micrometric scale in MRT. The treatment approach with multi-port irradiation and interspersed MBs at the tumour position results in an increase of the valley dose delivered to the tumour. A reduction of valley dose in the bone guarantees better normal tissue tolerance, while the tighter c-t-c spacing of interspersed microbeams in depth suggests a higher probability of tumour control. EP-1820 Implications of adopting Monte Carlo treatment planning for SBRT-VMAT of lung metastases A. Ianiro 1 , F. Deodato 2 , G. Macchia 2 , K. Di Brita 1 , A. Angelini 3 , M. Buwenge 4 , S. Cammelli 4 , V. Valentini 5 , A. Morganti 4 , S. Cilla 1 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 3 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Medical Physics Unit, Bologna, Italy 4 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Radiation Oncology Department, Bologna, Italy 5 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy Purpose or Objective Clinical implementation of Monte Carlo lung SBRT planning is challenging. This is due to the difficulty of reinterpreting historical outcome data calculated with

Conclusion The HDMLC accuracy (mechanical repeatability is 0.5 mm) together with the variable leaf directional movement, the higher influence of the tongue-and- groove effect, and the increased complexity of the leaf motion in RA may all contribute to the increased ratio of measured to planned dose when the physical DLG is relatively small. Opening the physical DLG for a TBSTx to a value around 0.5 mm allows the use of a modeled DLG value closer to the measured value. This resulted in improvements in the ratio of measured to delivered dose for RapidArc plans without affecting dynamic fluences and IMRT. EP-1819 Microbeam radiation therapy and dose evaluation using an interspersed beams irradiation geometry P. Pellicioli 1 , R. Hugtenburg 2 , F. Esteve 3 , E. Brauer- Krisch 1 1 European Synchrotron Radiation Facility, Experiments Division, Grenoble, France 2 Swansea University, College of Medicine, Swansea, United Kingdom 3 Grenoble Alpes University, EA.RSRM 7442, Grenoble, France Purpose or Objective Microbeam Radiation Therapy (MRT) is an innovative pre- clinical technique, offering new possibilities for more effective treatments with fewer side effects for patients, compared to conventional radiation therapy. MRT is performed using an array of spatially micro-fractionated X-ray beams, typically 50 µm wide and a few hundreds of micron spaced centre-to-centre (c-t-c). In this project, the dose evaluation on micrometric scale was tackled using radiochromic films read out with an