Room: AAPM ePoster Library
To asses the impact of interplay effect on proton pencil beam scanning (PBS) technique for different target motion amplitude (MA), types and number of rescanning and optimisation approaches.
4D CT data sets of CIRS dynamic thorax phantom were acquired with different MAs (5mm, 10mm, 15mm) and internal clinical target volume of the moving 2 cm spherical target were created along with the planning target volume (PTV) at a 5mm margin. Two types of IMPT plans, conventional PTV based single field optimised (SFO) and CTV based 4D robustly (4DR) optimised, were created with a single lateral beam perpendicular to the direction of motion of target using Raystion TPS modelled with Proteus Plus proton therapy system. For each nominal plans, layer(L) and volume(V) with 1, 3, 5,10 and 15 times rescanning (1L,3L, 5L, , 10L, 15L and 1V, 3V, 5V, 10V, 15V) were created. 4D dose distributions for the 9 plans in 3 MAs with two optimisation techniques (total 60 plans) were evaluated by a python scripting by including the time structure data from IBA ScanAlgo and 4D CT. Finally, dosimetric indices (D99%,D98%,D1%, HI) of targets were evaluated.
Volume rescanning showed better results than layer rescanning in both SFO and 4DR planning. The 5-10 rescanning was found to be optimal in any of the planning technique. SFO when combined with re-scanning approach is only suitable for target motion of up to 10mm. The combination of 4DR and volume rescanning approach was found to be the best solution for higher order MA. The worst case decrease in D98% of CTV was -6.06% (5mm), -6.04%(10mm) and -19.22%%(15mm) in SFO and -0.4% (5mm), -3.35%(10mm) and -3.4%(15mm) in 4DR plans.
4D robust optimisation with volume rescanning is the best technique to mitigate interplay effect without using any extra equipment and patient involvement.
Not Applicable / None Entered.