Optimizing dose prescription in stereotactic body radiotherapy for lung tumours using Monte Carlo dose calculation

Purpose: To define a method of dose prescription employing Monte Carlo (MC) dose calculation in stereotactic body radiotherapy (SBRT) for lung tumours aiming at a dose as low as possible outside of the PTV. Methods and materials: Six typical T1 lung tumours - three small, three large - were constructed centrally, peripherally in the lung, and nearby the thoracic wall, respectively. For each of these, five treatment plans employing dynamic conformal arc technique were made in which the dose was prescribed to encompass the PTV with the prescription isodose level (PIL) set in a range between 50% and 80% of the isocenter dose. Three shells of respectively 10 mm thickness around the PTV were constructed to assess the dose in the tissues directly adjacent to the PTV. Results: The PTV was nicely covered (mean 98.8% ± 0.9%) with favourable conformity indices (mean 1.09 ± 0.1). Mean doses around the PTVs were 73% (±1.3%), 76% (±3.5%), and 85% (±5.1%) of the prescribed dose in shell 1 for PIL50%, PIL65%, and PIL80%, respectively; 40% (±2.6%), 44% (±5.1%), 54% (±9.3%) in shell 2; and 24% (±1.9%), 26% (±3.6%), 33% (±6.8%) in shell 3. All normal tissue doses including the integral dose were also consistently worst for PIL80%. Monitor units were 30% higher for PIL65%, and 70% higher for PIL50%, compared with PIL80%. Conclusions: To improve normal tissue sparing the dose should be prescribed at an isodose lower than 80% of the isocenter dose in SBRT when using conformal arc technique with MC dose calculation.