ESTRO 2021 Abstract Book

S1312

ESTRO 2021

Results A linear dose response was found in the investigated dose range for all groups, but with considerable difference in dose response. A greater variation of dose response was observed within Group 1 compared to Group 2 and 3 and the mean dose responses were 0.021, 0.013 and 0.011 cm -1 Gy -1 for Group 1, 2 and 3 respectively.

Conclusion Dosimeters cured at room temperature (for 3-6 days) had a negligible dose-rate dependency, compared to dosimeters cured for 2 days at 15 C. On the other hand, the latter dosimeters had a higher dose response, as the dose response decreased with curing time. Overall, this study showed the importance of curing time and curing temperature and the need to balance demands for dose-rate sensitivity and dose response. PO-1587 Feasibility of using the Scandidos Delta4 phantom+ for performing star-shot test on a MRIdian linac. S.B.N. Biancardo 1 , G. Valdes Santurio 2 1 Herlev and Gentofte Hospital, University of Copenhagen, Department of Oncology, Herlev , Denmark; 2 Herlev and Gentofte Hospital, University of Copenhagen, Department of Oncology, Herlev, Denmark Purpose or Objective In order to guarantee accurate dose delivery in radiotherapy treatments, the imaging and radiation(RT) isocenters must be accurately aligned. For the MRIdian, however, alignment of lasers outside the bore are also of great importance as they are often the only means to positioning dosimetry equipment accurately in the beam. The typical test performed for establishing the relationship between these three coordinate systems is the star-shot. This test is often performed using radiochromic film, as it offers high spatial resolution. However, using film is time consuming and accurate placement of film-markings to indicate the film placement relative to the phantom is challenging. The purpose of this work is to establish whether the laser to RT isocenter alignment can be verified using the Delta4 phantom. This would enable testing the correlation of the imaging, RT and laser isocenters in a two-step process with no need for film Materials and Methods The MRIdian linac and treatment planning system from ViewRay were used for; delivering and calculating the reference dose distribution. The phantom was placed on the couch using lasers for alignment. The Scandidos software was used to compare measurements and reference dose distributions. A 2D gamma analysis with 5% dose difference and 1mm distance to agreement criteria was used. A passrate of 95% was considered satisfactory. The software offers a feature for calculating the phantom displacement that would lead to the highest passing rate of the gamma analysis. The process of placing the phantom and measuring the dose was repeated several times in order to take into consideration any systematic issues in the delivery or in the measurements. The focus of this initial work was on two parameters; the initial passing rate and the calculated displacement. If the initial passing rate is satisfactory, the dose distribution is in agreement within 1mm, and the calculated displacement gives the “off-set” of the distribution with respect to the center of the phantom (aka the lasers) Results Table 1 shows the results of the gamma analysis and the displacement from the isocenter. As shown in the table, all the gamma passing rates are satisfactory before applying position optimization. Displacements are less than 0.1 cm in each direction. The results show good agreement between measured and calculated dose distribution with less than 0.1 cm offset. When the optimization position tool is used then on average the maximum displacement found was 0.061 cm Conclusion The results indicate that the laser to RT alignment can be verified using the Delta4+ in a satisfactory way.