ESTRO 2021 Abstract Book

S784

ESTRO 2021

Figure 1: Illustration of the prostate calculi (pink contour) as internal surrogate for prostate tracking

Materials and Methods 27 patients from two institutions were recruited in two arms (n = 22 in prostate-tracking (PT)) and (n = 15 in calcification-tracking (CT)). Couch corrections were applied based on daily cone-beam computed tomography (CBCT). Either prostate gland boundaries or prostatic calcifications was used as surrogate tracked using 4D TPUS (Figure 1). Post-treatment CBCT was acquired as reference and respectively compared with intra- fraction prostate displacement observed in the corresponding PT and CT sessions. All image registrations were verified by experienced radiation oncologists, physicists and radiation therapists. Results 98 PT and 53 CT monitoring sessions revealed the mean prostate (calcifications) displacements as 0.3 mm (0.5 mm) posteriorly and 0.2 mm (0.5 mm) inferiorly which is expected and agreed in many research groups. When compared to post-treatment CBCT, the mean difference was 0.25 mm right, 0.13 mm inferior and 0.82 mm anterior in the PT group while 0.03 mm left, 0.18 mm inferior and 0.70 mm anterior relative to CBCT were found in the CT group. Statistical insignificant p -values (p>0.05) in paired-t test were obtained in both arms. Conclusion The use of prostatic calcifications as surrogate in TPUS is feasible and comparable to the use of prostate boundaries. Besides, long or multi-foci calcifications can sometimes provide rotational information which spherical prostate cannot. PD-0942 Intrafraction error analysis of homemade mouth-bite masks in linac-based SRS for brain metastasis V. Pisoni 1,2 , S. Trivellato 3 , S. Corbetta 2 , A. Masperi 2 , E. Ierman 1 , S.P. Bianchi 1,2 , R. Lucchini 1,2 , D. Panizza 3,2 , S. Arcangeli 1,2 1 ASST Monza, Department of Radiation Oncology, Monza, Italy; 2 University of Milan Bicocca, School of Medicine and Surgery, Monza, Italy; 3 ASST Monza, Medical Physics Department, Monza, Italy Purpose or Objective To assess the intrafraction accuracy of a frameless stereotactic linac-based radiosurgery (SRS) for brain metastases (BM) using a homemade mouth-bite thermoplastic mask in combination with cone-beam computed tomography (CBCT) and six-degrees of freedom (6-DOF) couchtop. Materials and Methods Covid-19 restrictions resulted in significant delays in the supply of the dedicated devices for BM SRS. A frameless approach using a homemade mouth-bite thermoplastic mask was implemented to offer BM SRS in a period of limited mobility (figure 1). All patients were immobilized and treated at the same institution with 6 MV FFF VMAT radiosurgery, with a 2 mm isotropic expansion from the GTV to the PTV. Before treatment delivery, patients underwent a low-dose CBCT to check position accuracy. Through image co-registration, translational (x, y, z) and rotational errors (roll, pitch, and yaw) were determined. The 6-DOF couchtop was used to automatically relocate the patient with sub-millimetric precision. Immediately after irradiation, patients underwent a second CBCT to verify any translational or rotational errors occurring during therapy (intrafraction motion).