Room: AAPM ePoster Library
Purpose: A gantry-mounted pencil beam scanning system features ultra-fast energy layer switching and an adaptive aperture (AA), capable of layer-by-layer collimation. In mobile targets, fast energy layer switching could reduce interplay effects and enable treatment using deep inhalation breath hold (DIBH). However, despite the dosimetric advantage of the AA, the finite time to collimate energy layers increases field delivery time. The purpose of this study was to evaluate the feasibility of using a static aperture (SA) for lung treatments in lieu of layer-by-layer collimation.
Methods: Ten lung patients were selected with varying target volumes. For each patient, two plans were created using the Monte Carlo capabilities of the treatment planning system; one using an AA and one using a SA. Each plan was optimized to achieve target coverage and to reduce OAR dose. For each patient, field delivery times for both plans were calculated, and dose was evaluated at various points for the heart, lung and esophagus. To investigate the clinical implications of the two collimation methods, OAR normal tissue complication probabilities (NTCP) were compared.
Results: Field delivery times were much lower (ranging from 6 to 39 seconds faster, depending on target volume) when using a SA. The absolute average difference in the mean heart and esophagus dose, lung V5 and V20 between the plans were 17.5 cGy and 62 cGy, 1.8% and 0.9%, respectively. Target coverage of 98% prescription showed an average difference of 0.64%. Calculated NTCP values showed absolute average differences of 0.6%, 0.2% and 1.3% for lung, heart and esophagus, respectively.
Conclusion: The advantage of using layer-by-layer collimation shrinks compared to a SA when the same target coverage is achieved in lung tumors, suggesting that the use of a SA is acceptable to take full advantage of fast energy switching by reducing interplay and enabling DIBH treatment.