Design and evaluation of a novel methodology for stereotactic ablative radiotherapy for lung cancer

Objectives 
Clinical adoption of methods to reduce the volume of non-cancerous tissue irradiated in lung stereotactic ablative radiotherapy (SABR) is limited by many centres' inability to continuously monitor tumour motion during treatment and validate their approach. This work developed and validated a novel solution. 

Methods 
Ten patients were recruited to a prospective study. Three planning target volumes (PTV) were investigated, clinical PTV1 (Internal Target Volume (ITV)+5mm), PTV2 (ITV+3mm) and PTV3 (MidVentilation). Affordable depth camera technology (KinectVn2) was used to monitor patients’ chest surface and was combined with 4DCBCT to determine the level of correlation with internal tumour motion. Dose measurement validation was performed in a dynamic phantom with variable real tumour motions, simulating multiple treatment deliveries to assess efficacy using gafchromic EBT3 film. 

Results
KinectVn2 acquisitions were successfully performed at CT and treatment for all patients without impacting workflow. Correlation between chest motion and internal tumour motion had a mean RMS error ≤1mm. PTV2 and PTV3 were significantly (p <0.0001, paired t-test) smaller than PTV1 with a mean reduction in volume of 34.3% (22.6%-50.6%) and 22.4% (1.9%-46.9%) respectively. For all PTV approaches mean geometric target coverage (TC) was >99.5%, however mean TC by the 110% dose dropped from 98.2% (PTV1) to 96.0% and 78.1% for PTV2 and PTV3, respectively. End-to-end patient-pathway) film measurements for three sampled patients showed that dosimetric target coverage was acceptable for all three PTVs.

Conclusion 
This work demonstrates that ITV-based PTV margins can be safely reduced to 3mm while maintaining adequate geometric and dosimetric tumour coverage. The PTV3 (MidVentilation) approach did not yield smaller volumes than the simpler PTV2 (ITV+3mm) method and gave inferior 110% geometric TC. A gamma criterion of 5%/5mm is recommended for dynamic measurements of SABR distributions. Combining continuous surface monitoring and comprehensive dosimetric validation, addresses key barriers to the clinical implementation of reduced PTV volumes.