ESTRO 2024 - Abstract Book

S6036

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada

Purpose/Objective:

Adaptive radiotherapy using MR-linac (MRL) can allow for safe dose-escalation to improve tumor control of liver metastases that was not possible with conventional technology. This will be tested in the planned ‘ULTRAS' (Ultra-high dose for Liver metastasis TReatment with stereotactic Ablative Single fraction) multi-institution phase III randomized clinical trial of 3800cGy/1 fraction for liver metastases. The aim was to quantify the minimum distance required between PTV (Planning Target Volume), and dose-limiting organs at risk (OAR) to meet safe dose constraints while achieving a high target dose.

Material/Methods:

Planning and CT-/MR-simulation data from 25 patients treated for liver metastases on an ethics board-approved imaging study were imported into the MRL treatment planning system (Monaco) for analysis. Simulated PTVs ranging from 6 to 10cc were created and placed at various locations, while systematically varying the proximity (<1cm, 1-1.5cm and 1.5-2cm) to the OARs. The size of the PTV volumes accounted for motion and imaging uncertainties that may be encountered when accruing patients. All scenarios were calculated with the goal the 95% of the PTV would receive >3800cGy/fraction while respecting the doses to the OAR. The OAR dose constraints adhered to in this study were D0.5cc 1100cGy for all luminal structures, D0.5cc < 3000cGy for the great vessels, D0.5cc < 1600cGy for the Biliary tree, D0.5cc < 1800cGy for heart, mean liver dose < 900cGy and Dmax < 1000cGy for the spinal canal. In the Monaco treatment planning system, 20 equally spaced intensity modulated radiotherapy (IMRT) beams were used to simulate volumetric arc therapy (VMAT) technique. VMAT is not yet clinically available for MRL but expected to be a future capability and is beneficial for small liver targets. On the 25 CT/MR anonymized patient data sets, 50 different treatment plans were calculated, each target varying in size and in proximity to luminal structures, the biliary tree, great vessels, spine, chest wall and heart. PTVs ranged in size, and all calculated plans respected the OAR constraints as per the ULTRAS protocol. Our calculations illustrate that when the PTV was 1.5cm - 2cm from OAR, acceptable distributions were achieved in all 26 cases (100%), with no reduction to PTV coverage. In all 26 scenarios 95% of PTVs received > 3800cGys. When the PTV was 1.0cm – 1.5 cm from OAR, acceptable distributions for both OARs and PTV were obtained in 11 of 17 cases (65%). In the 6 remaining scenarios, the optimizations were difficult and took a great deal of time to achieve acceptable OAR results, and the coverage to the PTV was compromised. The 95% PTV coverage for these 6 scenarios was 93-97% of the prescribed dose of 3800cGy. When the PTV was < 1cm from OAR, 7 of the 7 cases had significant reduction to the 95% PTV (ranging from 67-95%) coverage. Indicating the 3800cGy in 1 fraction is not suitable for this cohort of patients. Other findings noted, if the liver volume has been reduced by resection or is nonfunctioning (< 900cc) as seen in 2 patients, the mean liver dose was the limiting factor, regardless of the position of the PTV. Results:

Conclusion:

In this dosmetric evaluation of ultra-high-dose adaptive radiotherapy using an MR-linac, most (76%) single liver metasases lesions < 8cm were able to be escalated to 38Gy/1 fraction. Cases with targets <1cm or <1.5cm with