3299 - Evaluation of Intra-Fraction Motion of Prostate during SBRT Using Four-Dimensional Trans Perineal Ultrasound (TPUS) and Analysis of the Data for Calculation of PTV Margins
Presenter(s)
A. Neupane, A. V. R, H. Kunhiparambath, S. Bhasker, S. Gupta, G. Mounika, T. Gupta, A. B. Dutta, B. D. Shukla, S. Shah, S. Sharma, A. Binjola, and J. Yadav; All India Institute of Medical Sciences, New Delhi, India
Purpose/Objective(s): The widely recommended PTV margins for prostate SBRT is 5 mm. A noninvasive 4D transperineal ultrasound (TPUS) system from a precision radiation medicine company monitors the position of the prostate in real-time during treatment. The aim of this study is to monitor the intra-fraction movement of prostate during SBRT in real-time using the TPUS and to analyze the data to calculate average movement in each direction and finally calculate the required PTV margins for prostate SBRT.
Materials/Methods: TPUS monitoring data of 50 treatment sessions from randomly selected 10 patients of carcinoma prostate treated with 40 Gy/5 fractions over 5 alternate days were taken for the study. Planning CT scan and reference TPUS was acquired in same position. Bladder and rectal protocols were reproduced in each session. Treatment was delivered under TPUS guidance. Prostate motion was continuously monitored with TPUS in all three directions: Superio-inferior (SI), Right-Left (RL) and Antero-posterior (AP). Each treatment session lasted for 8 to 15 minutes. Movement data of approximately every second was analyzed to calculate the average movement in each direction followed by systematic and random errors. With the systematic and random error, Van Herk formula (2.5 ? + 0.7s) was used to calculate the PTV margin.
Results: We analyzed a total of 73,001 TPUS monitorings from the 50 treatment sessions of the 10 patients selected. Mean (±SD) movement of prostate superiorly and inferiorly was 0.24 mm (± 0.20 mm) and 0.61mm (± 0.38mm) respectively. For left and right directions, it was 0.65 (± 0.65mm) and 0.25 mm (± 0.19mm) respectively. The values for anterior and posterior was 0.28 mm (± 0.27mm) and 1.12 mm (± 0.477mm) respectively. The systematic and random errors were 0.52 mm and 0.74 mm; 0.82 mm and 0.75 mm; 0.59 mm and 0.89 mm for SI, RL and AP directions respectively. Using van Herk formula, the calculated PTV margin was 1.8 mm for SI; 2.6 mm for RL and 2.1 mm for AP directions.
Conclusion: A tighter PTV margin of 3 mm is sufficient for SBRT of prostate provided the movement is continuously monitored in real time.