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WE‐G‐BRD‐08: Motion Analysis for Rectal Cancer: Implications for Adaptive Radiotherapy On the MR‐Linac
Author(s) -
Kleijnen J,
van Asselen B,
Burbach M,
Intven M,
Philippens M,
Reerink O,
Lagendijk J,
Raaymakers B
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4926064
Subject(s) - nuclear medicine , radiation therapy , medicine , sagittal plane , voxel , dosimetry , magnetic resonance imaging , image guided radiation therapy , colorectal cancer , radiation treatment planning , cancer , radiology
Purpose: Purpose of this study is to find the optimal trade‐off between adaptation interval and margin reduction and to define the implications of motion for rectal cancer boost radiotherapy on a MR‐linac. Methods: Daily MRI scans were acquired of 16 patients, diagnosed with rectal cancer, prior to each radiotherapy fraction in one week (N=76). Each scan session consisted of T2‐weighted and three 2D sagittal cine‐MRI, at begin (t=0 min), middle (t=9:30 min) and end (t=18:00 min) of scan session, for 1 minute at 2 Hz temporal resolution. Tumor and clinical target volume (CTV) were delineated on each T2‐weighted scan and transferred to each cine‐MRI. The start frame of the begin scan was used as reference and registered to frames at time‐points 15, 30 and 60 seconds, 9:30 and 18:00 minutes and 1, 2, 3 and 4 days later. Per time‐point, motion of delineated voxels was evaluated using the deformation vector fields of the registrations and the 95th percentile distance (dist95%) was calculated as measure of motion. Per time‐point, the distance that includes 90% of all cases was taken as estimate of required planning target volume (PTV)‐margin. Results: Highest motion reduction is observed going from 9:30 minutes to 60 seconds. We observe a reduction in margin estimates from 10.6 to 2.7 mm and 16.1 to 4.6 mm for tumor and CTV, respectively, when adapting every 60 seconds compared to not adapting treatment. A 75% and 71% reduction, respectively. Further reduction in adaptation time‐interval yields only marginal motion reduction. For adaptation intervals longer than 18:00 minutes only small motion reductions are observed. Conclusion: The optimal adaptation interval for adaptive rectal cancer (boost) treatments on a MR‐linac is 60 seconds. This results in substantial smaller PTV‐margin estimates. Adaptation intervals of 18:00 minutes and higher, show little improvement in motion reduction.