Abstract Book

ESTRO 37

S548

Results

Change in volume between initial MRI and

Volume initial

Volume repeat-

V outside (cc)

d outside (mm)

MRI (cc)

MRI (cc)

repeat- MRI (cc)

2.5 (0.1– 12.6 cc) 0.5 (0.1–

2.5 (1.4– 7.3 mm) 1.8 (0.8– 5.9 mm)

30.4 (6.3– 62.0 cc)

19.3 (4.5– 68.5 cc)

Conclusion No degradation of the performance of the in-beam MR system was found during simultaneous operation with the proton therapy system. Although MR imaging during irradiation did not deteriorate the ACR parameters, there was a sequence-dependent off-resonance image displacement when the beam line magnets were energized. This proof-of-concept justifies further research towards the development of a first prototype for MRiPT. PO-0987 Target volume changes of brain metastases during fractionated SRS E. Hessen 1 , P. Damen 1 , B. Jasperse 2 , L. Dewit 1 , P. Elkhuizen 1 , A. Van Mourik 1 , F. Wittkamper 1 , E. Damen 1 , U. Van der Heide 1 , P. Hanssens 3 , J. Schasfoort 3 , J. Nijkamp 1 , G. Borst 1 1 Netherlands Cancer Institute, Radiation oncology, Amsterdam, The Netherlands 2 Netherlands Cancer Institute, Radiology, Amsterdam, The Netherlands 3 Gamma Knife Centre Tilburg, Radiation oncology, Tilburg, The Netherlands Purpose or Objective For brain metastases, we have shown previously that significant tumor shifts occur over time. For fractionated stereotactic radiosurgery (SRS), not only time but also the treatment itself may cause changes resulting in underdosage of tumor volume and an unintended dose to normal tissue. Therefore, we investigated whether the initially defined target volume covers the target volume during fractionated treatment by repeating the MRI halfway through the course of fractionated SRS. Material and Methods Twenty patients received a repeat-MRI during fractionated SRS for brain metastases in 3 or 5 fractions (overall treatment time 5-12 days), and were analyzed (SRS only n=8, SRS postoperative n=12 patients). The target volume (i.e. T1 contrast enhancing tumor or postoperative cavity) on both the initial MRI and the repeat-MRI were delineated on the T1-weighted sequence (TE= 2.4 ms, TR= 5.1 ms) post gadolinium contrast by one observer. The target volume (V outside ) on the repeat-MRI that was outside the initial target volume and the maximum distance (d outside ) of the target volume on the repeat-MRI outside the initial target volume were analyzed.

2.0 (-6.5– 12.5 cc)

SRS only

(n=8)

15.1 (2.8– 84.1 cc)

15.5 (2.6– 72.9 cc)

-0.6

(-

SRS postoperative (n=12)

11.2–3.3 cc)

3.8 cc)

Table 1. Results are given in median and range The median time between the initial and repeat-MRI was 8 days (5-14 days). The target volumes determined on the initial and repeat-MRI are given in Table 1. The initial volume of the target did not change significantly towards the second MRI for both the SRS only and SRS postoperative group (Wilcoxon Signed Ranks Test p =0.16 and p =0.16). For SRS only , a median V outside of 2.5 cc (0.1-12.6 cc) was observed, which was 24.0% (0.7-72.2%) of the initial target volume. The median d outside was 2.5 mm (1.4-7.3 mm). There was no correlation between d outside and the time between the two MRIs ( r s = 0.012, p =0.98). For SRS postoperative a median V outside of 0.5 cc (0.1-3.8 cc) was observed, which was 4.7% (0.3-27.7%) of the initial target volume. The median d outside was 1.8 mm (0.8-5.9 mm), and was also for this group not correlated with the time between the two MRIs ( r s = -0.046, p =0.89). For 50% of the patients in the whole group the maximum distance between the two target volumes exceeded the 2 mm. Conclusion For all patients, we found that a part of the target volume was outside the initial defined target volume halfway the treatment. For 50% of the patients this volume reached a distance exceeding commonly used margins in SRS. This phenomenon (which was not correlated with time) leads to the risk of partial "underdosing" the tumor. We are currently analyzing the consequences on the tumor- and normal tissue dose, and continue to include patients in our repeated MRI study. A. Pathmanathan 1 , A. Andreychenko 2 , D. De Muinck Keizer 2 , L. Kerkmeijer 2 , A. Tree 1 , C. Van den Berg 2 , D.B. JCJ 2 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, Sutton, United Kingdom 2 University Medical Centre Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective Our ultimate goal is to develop optimal real-time MR- guided radiotherapy (RT) for extreme hypofractionation for prostate cancer. We have therefore collected an extensive, realistic 4D cine-MR dataset to study intrafraction motion of the prostate over a hypofractionated schedule. Here, we present the first step in our analysis: the fast automatic tracking of fiducial markers (FM) to establish ground truth motions. PO-0988 Automatic fiducial tracking on 4D cine-MRI for MR-guided prostate radiotherapy