ESTRO 2021 Abstract Book

S691

ESTRO 2021

the delivery, the prostate motion was tracked with the EM system and SBRT was interrupted when a 2-mm threshold was trespassed and corrected by a new CBCT, unless the offset was transient. Real-time measurement of prostate displacement was recorded for each treatment fraction. These measurements were used to determine the duration and magnitude of displacement along the three directional axes. Results Total treatment time lasted on average 9.3 minutes [range 6-14], 5.8 minutes [range 3-9] for setup and 3.5 minutes [range 2-5] for beam delivery. In 41% of the monitored fractions, a new CBCT was mandated. The CBCT was repeated during the initial setup phase in 10 out of 44 fractions, while the beam delivery was interrupted only in 8. The transmitter shifts along x, y, and z axes for every treated patient during each phase of setup and beam delivery are reported in table 1. The majority of intra-fractional motion measured by the system occurred in the anteroposterior direction. The mean value of the target average deviation was -0.3 mm, -0.2 mm, and 0.0 mm in vertical, lateral, and longitudinal direction, respectively. The prostate was found within 1 mm from its initial position in 83.9% of the treatment time, between 1 and 2 mm in 13.0%, and exceeds 2 mm only in 3.1%.

Conclusion Our findings show that EM tracking is a reliable technique for real-time non-ionizing prostate monitoring during dose-escalated SBRT, allowing to keep the average target motion within 2 mm. It could be implemented rapidly and resulted well tolerated by the patient. Using FFF VMAT beams shortened the treatment time and significantly contributed to reduce intra-fractional motion. PD-0858 Impact of immobilization on intrafraction motion during CBCT-linac based SBRT of PCa metastases J. Janssen 1 , C. Brouwer 2 , H. van Herpt 2 , S. Both 3 , H. Langendijk 4 , S. Aluwini 4 1 University Medical Center Groningen, Radiation Oncology, Groningen , The Netherlands; 2 University Medical Center Groningen, Medical Physics, Groningen, The Netherlands; 3 Univeristy Medical Center Groningen, Medical Physics, Groningen, The Netherlands; 4 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands Purpose or Objective Metastasis-directed radiotherapy (MDRT) using stereotactic body radiotherapy (SBRT) is an effective treatment for oligo-recurrent prostate cancer (PCa). SBRT is delivered in few fractions, using tight margins and steep dose gradients, requiring a reliable immobilization. We aimed to assess the impact of two immobilization modalities on intrafraction motion during SBRT using cone beam computed tomography (CBCT). Materials and Methods CBCT data of twenty-one oligo-recurrent PCa patients were used to analyse patient intrafraction motion during SBRT. Patients were immobilized with either the customized moulded thermoplastic Klarity Cushion™ R550-L2 (TC, 10 patients/11 plans) or with the conventional Civco Couchtop™ Cover (CC, 11 patients/11 plans). Identical arm, knee and feet fixation devices were used in both groups. Analysis included 21 lymph node targets treated with 5x7Gy and one bony target treated with 3x10Gy. Repeated CBCT was performed pre- and post-fraction delivery, and in 16 fractions a mid-fraction CBCT was performed to assess mid-fraction motion. Online image registration (Elekta XVI, Bone T+R) was used for setup verification. The registration mask included the PTV and adjacent bony structure. Online correction was performed for all couch