ESTRO 2020 Abstract book

S312 ESTRO 2020

PH-0529 Prostate intrafraction motion during the full course of MR-guided radiotherapy sessions D. De Muinck Keizer 1 , L.G.W. Kerkmeijer 1 , T. Willigenburg 1 , M.D. Den Hartogh 1 , J.R.N. Van der Voort van Zyp 1 , E.N. De Groot-van Breugel 1 , B.W. Raaymakers 1 , J.J.W. Lagendijk 1 , H.C.J. De Boer 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective We develop extremely hypofractionated (HF) MR-guided radiotherapy (RT) of the prostate. Here, we present results on intrafraction motion of the prostate during the full course of online adaptive RT fractions on an MR linac (MRL) with online adaptive contouring and treatment planning at each fraction. Material and Methods We treated 5 low and intermediate risk prostate cancer patients with 20 x 3.1 Gy fractions on a 1.5 Tesla MRL (Unity, Elekta). In each fraction a daily pre-treatment MRI (Pre) scan was obtained. Contours from the planning MRI were propagated and manually adapted to this Pre scan after which full plan re-optimization was started in the Monaco MRL TPS. Shortly before the end of plan completion a position verification MRI (PV) scan was acquired. If the CTV target volume coverage was sufficient treatment delivery was started. If not, a virtual couch shift was applied to the dose distribution by the TPS. 3D Cine- MR imaging was started simultaneously with the start of beam-on (Fig. 1) and 3D dynamics were acquired over the full beam-on period. Acquisition times per dynamic ranged from 8.5 to 16.9 seconds. The daily Pre, PV, and post- treatment MRI (Post) scans were all acquired with a T2- weighted 3D sequence with a duration of 2 minutes. The prostate in Post, PV and cine-MR dynamics was rigidly registered to the Pre scan based on soft tissue contrast of the prostate using a validated algorithm based on mutual information (presented at ESTRO 38). Results A total number of 100 Pre, 104 PV, 97 Post scans and 2227 3D cine-MR dynamics were analyzed. Intrafraction prostate motion results over the full course of the RT sessions are provided in Fig 2. After an initial relatively rapid increase in 3D displacements, the magnitude of the motion seems to reach a plateau. The 95% confidence intervals for the translations are within the clinically applied PTV margins of 5 mm. The virtual couch shift was therefore necessary in only 4 out of 100 fractions. Largest intrafraction rotations are observed about the left-right axis with the 95% confidence interval reaching up to 7 degrees. Conclusion This is the first study to investigate prostate intrafraction motions during full MR-guided RT sessions on an MRL. We have shown that high quality full 3D cine-MR imaging and prostate tracking during RT is feasible with beam-on and thus acquired images can be used to quantify prostate intrafraction motion. The clinically applied margins of 5 mm have proven to be sufficient and may potentially be further reduced using intrafraction plan adaptation guided by cine-MR imaging.

Conclusion The study represents an initial step towards clinical implementation of MR-guided UHF prostate treatments. In just under half of 2 and 1 fraction plans target doses were compromised; in two of ten 2 fraction plans PTV D95% was compromised significantly (underdosed by greater than 5%). Further work is needed to verify plan delivery and to evaluate plan robustness via retrospective dose reconstruction of prostate 2D cine MRI acquisitions for simulated UHF treatment deliveries.