PTV margin assessment for an online adaptive MR-guided dose-escalation strategy in rectal cancer

Abstract

Purpose or Objective Large PTV margins are needed in the radiotherapy treatment of rectal cancer patients. These large margins are necessary to correct for interfraction displacements caused by daily variations in bladder and rectum dynamics. MR-guided radiotherapy (MRgRT) allows online target definition and replanning at each fraction and has the potential to scale down these margins significantly. This may be of particular value in future organ sparing regimens where the primary tumor will be boosted to a high dose to obtain complete remission. The aim of this study was to assess the GTV-to-PTV margin needed to cover tumor intrafraction motion during an online adaptive MR-guided dose-escalation strategy in intermediate risk rectal cancer. Materials and Methods We included fifteen patients with rectal cancer treated with neoadjuvant short-course radiotherapy, 5x5 Gy, according to an online adaptive workflow on a 1.5T MR-Linac (1). Per patient, we made 26 3D T2 weighted MRI scans; one reference scan preceding treatment, and five repeat scans during each treatment fraction (typically 40 min). For this study, the GTV was delineated on each MRI scan. Target coverage margins were assessed by isotropically expanding the reference GTV until more than 95% of the voxels of the sequential GTVs were covered. Interfraction GTV motion was assessed after registration to the bony anatomy. A margin with a coverage probability threshold of 90% was defined as adequate. Intra- and interfraction margins to cope with the movement of the GTV in the period between scans were calculated to indicate the target volume margins. Furthermore, the margin needed to cover GTV movement was calculated for different time intervals. Results The required margins to cover inter- and intrafraction GTV motion were 17 mm and 6 mm, respectively, and are plotted in Figure 1. The margin analysis based on time intervals between scans is shown in Figure 2. Margins needed for adequate GTV coverage were smaller as time intervals became shorter, with a 4 mm margin required for a procedure of 15 minutes or less. Conclusion The shorter the treatment time, the smaller the margins needed to cover for the GTV movement during an online adaptive MRgRT dose-escalation strategy for intermediate risk rectal cancer. When time intervals between replanning and the end of dose delivery could be reduced to 15 minutes, a 4 mm margin would allow adequate target coverage.