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

Abstract

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 (MRIadapt), verification prior to (MRIver) and after irradiation (MRIpost) of 5 treatment fractions per patient, the primary tumor GTV (GTVprim) and mesorectum CTV (CTVmeso) were delineated. The structures on MRIadapt were expanded to corresponding PTVs. We determined the required expansion margins such that on average over 5 fractions, 98% of CTVmeso and 95% of GTVprim on MRIpost 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 MRIver and MRIpost. A threshold to assess the need for a secondary adaptation was determined by considering the overlap between MRIadapt and MRIver. 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 CTVmeso and 5.0 mm isotropically for GTVprim. A 2nd adaptation, applied for all fractions where the motion between MRIadapt and MRIver exceeded 1 mm (36% of the fractions) would result in a reduction of the PTVmeso margin to 3.2 mm/2.0 mm. For PTVprim 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 CTVmeso and GTVprim respectively, could result in a 30–50% margin reduction with limited reduction of dose to the bowel.

Intra-fraction motion of pelvic oligometastases and feasibility of PTV margin reduction using MRI guided adaptive radiotherapy

“…Traditional margin recipes such as the Van Herk formula may not be applicable for MRIgRT SBRT treatments due to the small number of fractions and the ability to utilize workflows which mitigate a portion of the intra-fraction motion, such as a baseline shift plan after an initial adaptive plan (28,30,40,41). Recently, Kensen et al proposed that PTV margins would be considered adequate for rectal MRIgRT patients at the point in which 95% of the primary GTV would receive the prescription dose in 90% of patients (42). Based on this definition, a 2 mm PTV margin expansion would be sufficient for all patients in our cohort as the minimum GTV coverage for all patients and delivered fractions was greater than or equal to 95%.…”

“…Recently, Kensen et al. proposed that PTV margins would be considered adequate for rectal MRIgRT patients at the point in which 95% of the primary GTV would receive the prescription dose in 90% of patients ( 42 ). Based on this definition, a 2 mm PTV margin expansion would be sufficient for all patients in our cohort as the minimum GTV coverage for all patients and delivered fractions was greater than or equal to 95%.…”

Adaptive margins for online adaptive radiotherapy

“…The choices for the penumbra and the cases used in this paper were made to be comparable to previous literature. The systematic and random standard deviations were chosen such that the resulting margins were within the range of margins reported for online adaptive radiotherapy 2-7 mm (de Bohoudi et al 2017, Winkel et al 2019, Muinck Keizer et al 2020, Intven et al 2021, Kensen et al 2022. As a consequence, the resulting considered systematic errors in the range 0-2 mm were relatively small.…”

Experimental demonstration of real‐time cardiac physiology‐based radiotherapy gating for improved cardiac radioablation on an MR‐linac

“…[20][21][22][23] The integration of a linac within an MRI enables superior soft-tissue visualization during radiotherapy and can potentially facilitate the safe reduction of PTVs. 24,25 During these procedures, MRI-guidance may be used to gate the radiation delivery on either a predefined target window or extracardiac structures. 20,26 In a recent experimental study, we demonstrated that the dose delivery can be gated in a predefined cardiac phase in parallel with respiratory MLC-tracking based on real-time 2D cine MRI, which greatly reduced cardiorespiratory motion-induced dosimetric uncertainty.…”