ESTRO 2020 Abstract Book

S876 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)

radiotherapy protocol: 67.5 Gy in 25 fractions, 2.7 Gy per fraction, with a simultaneous boost irradiation of 69 Gy. Three fiducial gold markers were implanted into the gland at least two weeks before the CT planning acquisition. At each fraction a pre-treatment CBCT was acquired and a rigid 3D fiducial markers-registration to the planning CT was performed to an accurate positioning of the patient. A post-treatment CBCT was acquired with the patient still in the treatment position on alternate days. The shifts based on the post-CBCT fiducial registration were used for total intrafractional motion determination, as we described in [1]. Organs at risk were re-contoured in each pre and post- CBCT, and CTV, PTV1 and PTV2 were re-contoured only on the post-CBCT. The plan was recalculated on daily pre- CBCT and post-CBCT. The delivered dose based on post- CBCT was calculated without applying the shifts corrections of intrafractional prostate motion, so we could evaluate the dosimetric impact maximum threshold of these displacement. A correlation analysis between intrafractional prostate motion and daily delivered doses was made. [1] Impact of rectum and bladder anatomy in intrafractional prostate motion during hypofractionated radiation therapy. M. Roch. Clin Transl Oncol DOI 10.1007/s12094-018-1960-y Results There is a correlation between rectum delivered doses and vertical intrafraction prostate motion. A prostate displacement in the anterior direction results in higher rectum doses, largest statistically significant differences were observe in the high dose range. Also we found that a prostate displacement in posterior and inferior direction results in higher bladder delivered doses. This dose increase was compensated by bladder filling during the treatment session that results in a reduction in the delivered dose at the end of the session, especially in medium dose range, as we show in Fig 1.

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