Feasibility of Gold Fiducial Markers as a Surrogate for Gross Tumor Volume Position in Image-Guided Radiation Therapy of Rectal Cancer

Online Adaptive MRI-Guided Radiotherapy for Primary Tumor and Lymph Node Boosting in Rectal Cancer

The purpose of this study was to characterize the motion and define the required treatment margins of the pathological mesorectal lymph nodes (GTV_ln) for two online adaptive MRI-guided strategies for sequential boosting. Secondly, we determine the margins required for the primary gross tumor volume (GTV_prim). Twenty-eight patients treated on a 1.5T MR-Linac were included in the study. On T2-weighted images for adaptation (MRI_adapt) before and verification after irradiation (MRI_post) of five treatment fractions per patient, the GTV_ln and GTV_prim were delineated. With online adaptive MRI-guided radiotherapy, daily plan adaptation can be performed through the use of two different strategies. In an adapt-to-shape (ATS) workflow the interfraction motion is effectively corrected by redelineation and the only relevant motion is intrafraction motion, while in an adapt-to-position (ATP) workflow the margin (for GTV_ln) is dominated by interfraction motion. The margin required for GTV_prim will be identical to the ATS workflow, assuming each fraction would be perfectly matched on GTV_prim. The intrafraction motion was calculated between MRI_adapt and MRI_post for the GTV_ln and GTV_prim separately. The interfraction motion of the GTV_ln was calculated with respect to the position of GTV_prim, assuming each fraction would be perfectly matched on GTV_prim. PTV margins were calculated for each strategy using the Van Herk recipe. For GTV_ln we randomly sampled the original dataset 20 times, with each subset containing a single randomly selected lymph node for each patient. The resulting margins for ATS ranged between 3 and 4 mm (LR), 3 and 5 mm (CC) and 5 and 6 mm (AP) based on the 20 randomly sampled datasets for GTV_ln. For ATP, the margins for GTV_ln were 10-12 mm in LR and AP and 16-19 mm in CC. The margins for ATS for GTV_prim were 1.7 mm (LR), 4.7 mm (CC) and 3.2 mm anterior and 5.6 mm posterior. Daily delineation using ATS of both target volumes results in the smallest margins and is therefore recommended for safe dose escalation to the primary tumor and lymph nodes.

Effect of intrafraction adaptation on PTV margins for MRI guided online adaptive radiotherapy for rectal cancer

Purpose

To determine PTV margins for intrafraction motion in MRI-guided online adaptive radiotherapy for rectal cancer and the potential benefit of performing a 2nd adaptation prior to irradiation.

Methods

Thirty patients with rectal cancer received radiotherapy on a 1.5 T MR-Linac. On T2-weighted images for adaptation (MRI_adapt), verification prior to (MRI_ver) and after irradiation (MRI_post) of 5 treatment fractions per patient, the primary tumor GTV (GTV_prim) and mesorectum CTV (CTV_meso) were delineated. The structures on MRI_adapt were expanded to corresponding PTVs. We determined the required expansion margins such that on average over 5 fractions, 98% of CTV_meso and 95% of GTV_prim on MRI_post was covered in 90% of the patients. Furthermore, we studied the benefit of an additional adaptation, just prior to irradiation, by evaluating the coverage between the structures on MRI_ver and MRI_post. A threshold to assess the need for a secondary adaptation was determined by considering the overlap between MRI_adapt and MRI_ver.

Results

PTV margins for intrafraction motion without 2nd adaptation were 6.4 mm in the anterior direction and 4.0 mm in all other directions for CTV_meso and 5.0 mm isotropically for GTV_prim. A 2nd adaptation, applied for all fractions where the motion between MRI_adapt and MRI_ver exceeded 1 mm (36% of the fractions) would result in a reduction of the PTV_meso margin to 3.2 mm/2.0 mm. For PTV_prim a margin reduction to 3.5 mm is feasible when a 2nd adaptation is performed in fractions where the motion exceeded 4 mm (17% of the fractions).

Conclusion

We studied the potential benefit of intrafraction motion monitoring and a 2nd adaptation to reduce PTV margins in online adaptive MRIgRT in rectal cancer. Performing 2nd adaptations immediately after online replanning when motion exceeded 1 mm and 4 mm for CTV_meso and GTV_prim respectively, could result in a 30–50% margin reduction with limited reduction of dose to the bowel.