Quantification, evaluation and audit of the impact of inter-fractional baseline shifts in SABR treatment for multiple lung lesions

Abstract

Purpose: Investigate the magnitude and dosimetric impact of baseline shifts (BLS) while treating multiple lung lesions with stereotactic ablative radiotherapy (SABR) through planning simulation and audit measurements. To evaluate the results from these methods for informing risk to patients. Methods: Retrospective use of SABR plans and on-treatment imaging from thirty-three SABR patients, each with two thoracic cancer lesions, were used to calculate the BLS between lesions. Seventeen patients with a range of BLS and lesion separation distances were selected for a planning simulation to assess the combined effect on target and OAR dosimetry. A method to assess treatment accuracy in the presence of BLS was developed in the form of an end-to-end audit using bespoke inserts containing EBT3 Gafchromic film positioned in a commercially available lung phantom. A planning insert and two further inserts with lesion positional changes were used to measure film dose to each lesion from both single and dual plan Volumetric Modulated Arc Therapy (VMAT) techniques. Median dose within the lesion and low dose region between the lesions, prescription isodose centre of dose (COD) positional changes and local and global gamma pass rates were assessed for suitability as audit metrics. Results: The combined vector BLS between all paired lesions was 0.8cm ± 0.5cm (mean ± standard deviation), with a maximum of 2.7cm. Systematic and random errors were calculated, corresponding to a required margin of 0.53cm for each lesion due to BLS only. PTV D0% significant increases were found with a BLS of at least 0.8cm and the closest distance between PTV edges was < 5cm and significant OAR mean D0.1cc in patients for whom the shortest distance from the PTV edge to an OAR boundary was < 2cm. Audit validation found that largest median film doses in the high and low dose regions between lesions to be 1.4% (0.15Gy) and -3.2% (0.35Gy) compared to the calculated plan dose prescription. The overall difference in COD film measurement compared to expected S/I shifts was 0.6 ± 0.8 mm and 0.3 ± 0.4 mm for the single and dual techniques. Finally, global and local gamma pass rate was suitable for assessing change in the dose delivery in the presence of BLS. Conclusions: While the majority of BLS changes were not found to be significant for patients, in situations of risk, recommendations are given for the planning and treatment stages to mitigate against BLS. Treatment accuracy in the presence of BLS can also be verified using the audit methodology developed, and found to be effective in verifying dosimetric and geometric accuracy.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stephen Bolton (Supervisor)