ESTRO 2020 Abstract book

S926 ESTRO 2020

We found no statistically significant correlation (p > 0.05) between the shoulder shift that we measured and the daily variations of the PTV mean dose and V95. We also found no statistically significant correlation (p > 0.05) between the shoulder shift and the gamma passing rate calculated by our platform. (Fig.2)

1 Hospital de la Santa Creu i Sant Pau, Radiofísica y Protección Radiológica, Barcelona, Spain Purpose or Objective The daily position of the shoulders during treatment might have an impact in cases where, in order to deliver the prescribed dose to the PTV, the beam could pass through them. During the treatment course of a Head and Neck (HN) patient, we co-registered a CBCT/CT focusing on the relevant PTV area. Thus, the daily position of the shoulders may differ from the position in the CT plan (Fig.1).

In our institution, all patients are treated with a VMAT technique. Optimization is performed using an avoidance strcuture, enclosing the shoulders, to decreased beam shoulder penetration. This work investigates the use of PerFRACTION TM (a 3D in vivo dosimetry software) as a tool to asses our immobilization technique as well as our VMAT planning strategy, and to evaluate the dosimetric impact caused in the PTV by the shoulder positioning. Material and Methods We collected all the images performed for 13 HN patients during their course of treatment. Only sessions having a CBCT and an EPID-integrated image were selected. In our institutions, HN patients are immobilized with a 3-points mask while grabbing special straps at a fixed position that help them push their shoulders down. To assess the daily variation of the shoulder position, we edited manually the co-registered CBCT/CT focusing on the shoulder area and measuring shoulder displacement. The effect of shoulder variation was also evaluated with PerFRACTION TM using a 3%-3mm gamma analysis between the expected and delivered dose fluences images for each treatment arc. To account for the dosimetric impact, we determined PTV daily doses with the reconstructed 3D dose distribution calculated by PerFRACTION TM in the co- registered CBCT/CT. We extracted the PTV mean dose and V95 as the dosimetric estimators of the whole 3D dose distribution. Results We examine a total of 13 patients and 30 sessions with co- registered CBCT/CT and EPID-integrated images. The average shoulder shift was 4.2 mm. 63.3% of the observed shifts (19 out of 30) were less than 5 mm (range between 0.0 mm and 17.4 mm). Per-Fraction detected 4 sessions with a poor 3%-3mm gamma performance: 92.3%, 87.41%, 89.26% and 95.9%. The rest of the sessions performed with gamma passing rates higher than 98%.

Conclusion We found that there was no correlation between the daily shoulders shift that occurred during the radiotherapy session and the gamma analysis performed between the plan and the delivered dose fluence image. More surprisingly, the change in the position of the shoulders during the radiotherapy course had no dosimetric impact on the PTV coverage and dose mean. Our preliminary results reassure us of the use of VMAT for treating HN cancer patients in our department. PO-1609 Dosimetric effect of intrafraction motion on hypofractionated prostate radiation therapy plans M. Roch 1 , P. Castro 1 , C. Anson 1 , D. Hernandez 1 , A. Zapatero 2 , F. Garcia de Vicente 3 , A. Viñals 1 , R. Fayos- Sola 1 , L. Perez 1 1 Hospital Universitario La Princesa, Radiophysics / Radiation Oncology, Madrid, Spain ; 2 Hospital Universitario La Princesa, Radiation Oncology, Madrid, Spain ; 3 Hospital Universitario Ramón y Cajal, Radiophysics, Madrid, Spain Purpose or Objective The objective of this study is to quantify the dosimetric impact of intrafractional prostate motion during a hypofractionated prostate radiation therapy Material and Methods The prospective study consisted of 15 prostate carcinoma patients that were treated with a hypofractionated