ESTRO 36 Abstract Book

S470 ESTRO 36 _______________________________________________________________________________________________

S.H. Jeon 1 , S.Y. Park 1 , J.H. Kim 1 , J.I. Kim 1 , J.M. Park 1 1 Seoul National University Hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To suggest an optimal planning target volume (PTV) margin in stereotactic body radiotherapy (SBRT) of the spine. Material and Methods From December 2014 to July 2016, 40 patients received 42 fractions of SBRT for spinal tumors to thoracic or lumbosacral spines using a volumetric modulated arc therapy technique and patient immobilization. Before treatment, kilovoltage cone-beam CT (CBCT) images were obtained for a 4 degrees of freedom (DoF) correction of patients alignment (translation + yaw). After corrections were made, additional CBCT was acquired just before treatment delivery (pretreatment CBCT). Immediately following SBRT, CBCT was acquired again (posttreatment CBCT). Residual setup errors for pretreatment CBCT was determined by a 6 DoF manual matching. Intrafraction motions were calculated as differences in errors between pretreatment and posttreatment CBCT. Three clinical target volumes (CTVs) were generated by translating and rotating original CTV by residual setup errors alone (CTV_R), intrafraction motions alone (CTV_I), and residual setup errors and intrafraction motions combined (CTV_R+I), respectively. Adding various uniform margins to original CTV generated PTVs. The impact of PTV margins on CTV coverage was evaluated. A provisional criterion of adequate CTV coverage was that PTV encompasses at least 97% of CTV. Results Time interval between pre-treatment and post-treatment CBCTs was 6.8±2.5 min (mean±2SD). The 2SD values of lateral, vertial, longitudinal translations and pitch, roll, and yaw were 0.7mm, 0.8mm, 1.1mm, 1.7°, 1.1°, and 1.6°for residual setup errors and 1.0mm, 0.9mm, 0.9mm, 1.1°, 0.8°, and 1.1°for intrafraction motions, respectively. Without margins, PTV showed adequate coverage for CTV_R, CTV_I, and CTV_R+I in 48% (20/42), 71% (30/42), and 48% (20/42) of fractions, respectively. With 1-mm uniform margins, PTV was adequate for 95% (40/42), 98% (41/42), and 100% (42/42) of fractions, respectively. 2-mm uniform margin was adequate in all fractions for all three CTVs. Conclusion With appropriate immobilizations and 4DoF corrections, a uniform 1-mm PTV margin may ensure an adequate CTV coverage in most treatment sessions of spine SBRT. Combined with a shortened treatment time, the small extent of intrafraction motions may obviate the need of treatment interruption for additional intra-session image guidance. Despite perfect 6 DoF patient alignment, 1-mm PTV margin is still needed to address intrafraction motions. PO-0864 Accuracy of fiducial based correction of target motion in prostate SBRT treatments T. Viren 1 , M. Korhonen 2 , J. Seppälä 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Applied Physics, Kuopio, Finland Purpose or Objective Robotic stereotactic body radiotherapy (SBRT) incorporating a fiducial based motion tracing system has enabled almost real-time correction of intra-fraction motion of a prostate during SBRT treatments of prostate cancer. However, the effect of number and positioning of the fiducials and the amount of prostate movements on the accuracy of the treatment has not been reported. The aim of the present study was to investigate the accuracy of the fiducial based correction of target motion in

prostate SBRT treatments and to evaluate the effect of fiducial number and positioning to the accuracy of the fiducial tracking. Material and Methods CT image was acquired from custom-made phantom incorporating different fiducial configurations (Fig 1). Subsequently, typical prostate SBRT treatment plan (5x7.25Gy) was calculated in the phantom using treatment planning software (Ray Tracing algorithm, Multiplan, Accuray, USA). To measure the dose distribution within the phantom calibrated Gafchormic films (4 x 4 inch, Gafchromic EBT 3 , RPD Inc., USA) were placed inside the phantom. A prostate treatment was irradiated in three different phantom positions: no movement, typical clinical prostate movements, and maximum movements allowed by the automatic fiducial tracing system (Fig 1).The phantom movements were conducted using Robochouch (Accuray, USA).To mimic the suboptimal positioning of the fiducials the measurements were repeated with four different seed configurations (optimal, typical clinical case, clinical case with three fiducials, clinical case with two fiducially). Measurements were conducted in coronal and sagittal planes. Finally, the films were scanned (Perfection V700, Epson, USA) 72 hours after the irradiation and the measured and calculated dose distributions were compared using gamma-analysis (5%/2mm threshold).

Figure 1. A) Custom made phantom used to measure prostate SBRT treatment plans. B) The directions of the prostate movements and rotations. C) Typical clinical and maximum intra-fraction prostate movements used in the present study Results The accuracy of the automatic correction of intra-fraction motion of the target was clinically acceptable when three or four seed configuration was used in the motion tracking (Table 1). No significant changes in gamma pass rates were detected when the amount of phantom movement was increased. Clinically unacceptable gamma pass rates were detected only when two fiducials where used in tracking. Table 1. Gamma pass rates of measured and calculated treatment plan comparisons for different fiducial configurations and phantom movements.

Conclusion Automatic correction of the target movement was reasonably accurate for clinical use when three or four