Session Item

Intrafractional respiratory motion is a concern for lung tumor radiotherapy but full evaluation of its impact is hampered by the lack of images representing the motion over the full treatment time. This study presents a novel evaluation using images acquired over realistic treatment times to investigate the dosimetric impact of respiratory motion during free breathing in photon radiotherapy. The image sequences were acquired at three occasions and were combined with simulated patient shifts to create a large set of probability weighted treatment scenarios for evaluation of different treatment planning strategies.

Three planning methods were compared all aiming to deliver 54 Gy as median dose (D50%) in three fractions to the target: one denoted FLU with homogeneous fluence to the PTV, one denoted ISD with isodose prescription to the PTV combined with a high central dose, and one denoted IRN with concurrent renormalization.
As basis for simulated dose tracking in free-breathing we used cine-CT images of 14 patients with small lung tumors. Images were acquired during eight minutes at each occasion and used together with simulated patient shifts to create sets of probability weighted combinatorial treatment scenarios for each patient and treatment planning strategy. Using 15 different patient shifts, including the nominal, allowing repetitions and disregarding the order it can be shown that the number of treatment scenario combinations per patient and planning methods is 16 215 for a treatment in  three fractions. For our cohort of 14 patients this gave a total of 227 010 simulated treatment scenarios per planning method that were used for evaluation.

Both FLU and IRN achieved at least 95% of the intended dose in more than 90% of the simulated treatments, despite tumor motion amplitudes being larger in the cine-CT images than in the 4DCT and patient shifts being applied. The ISD method led to consistently higher D50% doses than the intended one. Confidence intervals at 80% for CTV D50%, in Gy, were [51.9, 54.6] for FLU, [51.7, 54.6] for IRN and [57.1, 63.5] for ISD.Our study shows that the tumor motion during free breathing not covered by the 4DCT gives a risk of underdosage but that this effect is small when planning for an additional PTV margin together with 4DCT derived ITV. Further, it was shown that the PTV prescription level for ISD corresponded poorly to the intended CTV dose and we do not recommend PTV based prescriptions when a certain CTV dose is desired.
Confidence intervals at 80% for CTV HI were [0.08, 0.22] for FLU and [0.16, 0.23] for IRN and ISD.
Further advantage of FLU over IRN and ISD was lower dose to the ipsilateral lung.

Our study shows that the tumor motion during free breathing gives a risk of underdosage but that this effect is small when planning for an additional PTV margin together with 4DCT derived ITV. Further, it was shown that the PTV prescription level for ISD corresponded poorly to the intended CTV dose.