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Investigation of Prostate Intrafractional Motion During External Beam Radiotherapy for a Large Patient Population

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X Tong

X Tong1,2*, X Chen2, J Li2, M Lin2, L Chen2, Q Xu1,2, C Ma2, (1) 3rd Affiliated Hospital of Qiqihar Medical University, Qiqihar, (2) Fox Chase Cancer Center, PHILADELPHIA, PA

SU-D-BRA-6 Sunday 2:15:00 PM - 3:00:00 PM Room: Ballroom A

To analyze and report prostate intrafractional motion recorded online during external beam radiotherapy and to provide guidance for advanced prostate IMRT.

Prostate intrafractional motion during IMRT and RapidARC treatments was tracked and recorded using a Calypso 4D-localization system. Data for 8600 treatment fractions of 230 prostate patients was analyzed. The percentage of treatment fractions, the fractional treatment time, and the fractional time of any individual minute were analyzed separately for prostate movements greater than the thresholds (2, 3, 5 and 7mm).

The percentages of fractions in which prostate shifted more than the thresholds for a duration >10 seconds are 66.2%, 34.9%, 8.9% and 2.6% (57.9%, 27.9%, 5.2% and 0.8% for >30 seconds duration), respectively. For 10 patients who exhibited the largest motion, the percentages are 95.2%, 76.0%, 43.2% and 14.7% (91.3%, 72.4%, 36.3% and 6.0% for >30 seconds duration). The fractional time for motion larger than the thresholds is 27.8%, 10.7%, 1.6% and 0.3% (56.2%, 33.7%, 11.2% and 2.1% for these patients). The posterior-direction motion is significantly higher than those in other directions. The fractional time of an individual minute with shift >3mm is higher at the 10th minute than at the 5th minute (20% vs. 10%) after the initial setup. For treatments completed within 5 minutes, the average fractional time with motion >3mm was lower than those within 10 minutes (4% vs. 12%). Irregular movements were observed most likely in the first minute after patient's setup, which were 12% and 7% among all fractions with movements >2mm and >3mm, respectively.

The prostate intrafractional motion was within 3mm for most treatment fractions. However, larger movements were observed for some patients, who require real-time corrections or larger treatment margins. The results indicate that it is beneficial if the treatment can be completed within 5 minutes.

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