Exploiting Tumor Position Differences between Deep Inspiration and Expiration in Lung Stereotactic Body Radiation Therapy Planning

We introduce a novel stereotactic body radiation therapy (SBRT) planning technique that takes advantage of tumor motion relative to normal structures between respiratory states. We hypothesize that delivery of half a SBRT fraction dose in deep-inspiration breath hold (DIBH) position and the other half in the expiratory position spares more normal tissue compared to DIBH and expiration-alone plans. Eligible patients had non-small cell lung carcinoma and previous lung SBRT in five fractions with available DIBH and expiratory computed tomography (CT) scans. Utilizing volumetric modulated arc therapy (VMAT), three different radiation plans were generated—one using a DIBH scan, another using an expiratory scan, and a third using a free breathing scan with corresponding target and adjacent chest wall contour. A fourth composite plan was generated delivering half the dose using the DIBH plan and the other half using the expiratory phase plan for each fraction. To generate the composite plan for accurate dose volume histogram (DVH) evaluation, the two scans underlying the two plans were fused based on ribs adjacent to the tumor. Five eligible patients had lesions located close to the chest wall. The median longest tumor diameter was 2.5cm (range: 0.7-2.9cm, n=5). The median superior-inferior tumor movement was 2.05cm (range: 1.4-2.9cm, n=4), and the measured anterior-posterior tumor movement for one patient was 2.9cm. The median reduction in the chest wall V30Gy for the composite plan was 98.3% (range 33.7-100%, absolute reduction: 6.3cc (range: 2.1-17.3cc)) compared to the inspiration phase alone plan and 98.2% (range 32.3%-100%, absolute reduction: 5.9cc (range: 3.2-16.6cc)) compared to the expiration phase alone plan, and 99.1% (range 69.7%-100%, absolute reduction: 25.8cc (range: 5.0-32.7cc)) compared to the free breathing plan. The median reduction in chest wall maximum dose for the composite plan was 32.9% (range 0.270%-46.4%, absolute reduction: 1424cGy (range: 15-2466cGy)) compared to the inspiration phase alone plan and 35.1% (range 3.18%-48.2%, absolute reduction: 1571cGy (range: 182-2650cGy)) compared to the expiration phase alone plan. The composite plan had a greater reduction in chest wall maximum dose if there was no overlap in planning target volumes (PTVs) between the inspiration and expiration target location relative to adjacent ribs (no overlap vs overlap, median 43.1% vs 5.21% reduction). Lung V20Gy were similar within 2.1% between different plans. The new composite DIBH-expiration plan theoretically allows for three of the five patients to receive three fraction SBRT to 54Gy instead of five fraction SBRT to 50Gy. We conclude that composite deep inspiration-expiration SBRT planning has the potential to significantly improve organ at risk (OAR) sparing if there is sufficient tumor movement. This offers opportunities to lower toxicity risk, dose escalation, and for some, treatment in three fractions instead of five.