Radiation and Cancer Physics

PD 12 - Physics 6 - Poster Discussion - Adaptive Planning/Delivery and Motion

1101 - Normal Tissue Doses During Isotoxically Dose Escalated Lung Radiation Therapy Using MR-Linacs

Tuesday, October 23
2:57 PM - 3:03 PM
Location: Room 217 C/D

Normal Tissue Doses During Isotoxically Dose Escalated Lung Radiation Therapy Using MR-Linacs
L. Bendall1, J. D. Fenwick2, B. W. Papiez3, and M. A. Hawkins4; 1Department of Oncology, University of Oxford, Oxford, United Kingdom, 2University of Liverpool, Liverpool, United Kingdom, 3Institute of Biomedical Engineering, University of Oxford, Oxford, United Kingdom, 4CRUK/MRC Oxford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom

Purpose/Objective(s): Real-time imaging using MR-linacs during lung radiotherapy may allow PTV margin reduction, permitting increased isotoxically prescribed doses. However, this may affect the doses received by nearby critical organs at risk (OARs) when respiratory motion is considered. It is hypothesized that surrounding OARs may require monitoring to ensure dose tolerances are not exceeded during treatment over the respiratory cycle.

Materials/Methods: Isotoxic plans with a range of PTV margins based on MR-linac tracking accuracies were generated on the mid-ventilation phase of a 4DCT image for 10 patients with locally advanced NSCLC. The dose-distributions were time-integrated over the respiratory cycle of the planning scan (CT1), using a deformable registration algorithm to map distributions back to the mid-ventilation phase. For 5 patients with repeat 4DCT scans, the time-integration was performed on 2 further image sets acquired at a range of 7-22 days (CT2) and 27-50 days (CT3) after CT1 to investigate the effects of changes in respiration/anatomy.

Results: Planning CT results show that for ~50% of plans, small volumes of a single OAR (heart, trachea, pulmonary artery, oesophagus) may exceed the dose tolerance. 17/18 of these OARs were the dose limiting structure at planning; the remaining OAR lay at 99.7% of tolerance but over the respiratory cycle received a small increase of 0.4 Gy. For later image sets, the number of plans with OARs exceeding tolerance ranged from 75-90% of plans, and the maximum number of OARs per plan increased from 1 (CT1), to up to 4 (CT2) or 5 (CT3). The dose violations also increased in magnitude, up to 4.8 and 9.2% of tolerance for CT2 and CT3. The regions of overdose within OARs were small (median volume of 1.72 cm3 for CT1), and generally located at or near areas of PTV and OAR overlap.
Structure Structure composition Number of OARs exceeding tolerance across all plans (max % above tolerance)
CT1 CT2 CT3
PTVstandard ITV + 5 mm (CTV) + 5 mm (PTV) 4/120 (+2.5%) 6/60 (+4.0%) 10/60 (+8.9%)
PTVMRtrack_lo_acc GTVmidV + 5 mm (CTV) + 5 mm (PTV) 4/120 (+2.4%) 7/60 (+4.8%) 10/60 (+9.2%)
PTVMRtrack_hi_acc GTVmidV + 5 mm (CTV) + 2 mm (PTV) 5/120 (+3.2%) 8/60 (+3.1%) 10/60 (+6.3%)
PTVMRtrack_ideal GTVmidV + 5 mm (CTV) + 0 mm (PTV) 5/120 (+2.4%) 11/60 (+3.1%) 16/60 (+8.2%)
Table 1 Results across whole respiratory cycle for OARs that exceeded tolerance.

Conclusion: These results show that for MRI-linac LA-NSCLC plans created on the mid-ventilation phase of that day’s 4DMR image, and delivering high prescribed doses using tight PTV margins, OAR tolerance violations in doses considered over the whole breathing cycle are small and occur at predictable locations. However, these violations grow when dose-distributions are time-integrated over 4D-CT images acquired 1-7 weeks after the planning scan, indicating that frequent re-imaging and plan re-optimisation is required to minimise tolerance violations.

Author Disclosure: L. Bendall: None. J.D. Fenwick: None. B.W. Papiez: None.

Louise Bendall, MS, BS, CSci

Disclosure:
Employment
University of Oxford Oxford: Employee: Employee

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