Abstract
Pencil beam scanning proton therapy makes possible intensity modulation, resulting in improved target dose conformity and organ‐at‐risk (OAR) dose sparing. This benefit, however, results in increased sensitivity to certain clinical and beam delivery parameters, such as respiratory motion. These effects can cause plan degeneration, which could lead to decreased tumor dose or increased OAR dose. This study evaluated the measurements of proton pencil beam scanning delivery made with a 2D ion chamber array in solid water on a 1D motion platform, where respiratory motion was simulated using sine and cosine4 waves representing sinusoidal symmetric and realistic asymmetric breathing motions, respectively. Motion amplitudes were 0.5 cm and 1 cm corresponding to 1 cm and 2 cm of maximum respiratory excursions, respectively, with 5 sec fixed breathing cycle. The treatment plans were created to mimic spherical targets of 3 cm or 10 cm diameter located at 5 cm or 1 cm depth in solid water phantom. A reference RBE dose of 200 cGy per fraction was delivered in 1, 5, 10, and 15 fractions for each dataset. We evaluated dose conformity and uniformity at the center plane of targets by using the Conformation Number and the Homogeneity Index, respectively. Results indicated that dose conformity as well as homogeneity was more affected by motion for smaller targets. Dose conformity was better achieved for symmetric breathing patterns than asymmetric breathing patterns regardless of the number of fractions. The presence of a range shifter with shallow targets reduced the motion effect by improving dose homogeneity. While motion effects are known to be averaged out over the course of multifractional treatments, this might not be true for proton pencil beam scanning under asymmetrical breathing pattern.
Highlights
Conformal proton pencil beam scanning (PBS) dose distributions generated with intensity‐modulated proton therapy (IMPT) improve the therapeutic ratio, achieving highly conformal target doses while reducing toxic doses to surrounding organs‐at‐risk (OARs)
Since there is interference between PBS delivery and moving target, known as the interplay effect,[1,2] and typical breathing cycles are on the same scale as the time required for the switch between two adjacent energy layers,[3] the superior dose distribution is more sensitive to respiration‐induced organ motion for treatment sites such as lung, liver, and mediastinum, which can cause temporal displacement of the target volume and degrade the proton dose distribution significantly.[4,5]
In order to estimate the degradation of target dose coverage due to respiratory motions, we evaluated dose conformity and uniformity of each measurement dataset at the center plane of each size of moving targets simulated by the motion platform
Summary
Conformal proton pencil beam scanning (PBS) dose distributions generated with intensity‐modulated proton therapy (IMPT) improve the therapeutic ratio, achieving highly conformal target doses while reducing toxic doses to surrounding organs‐at‐risk (OARs). Since there is interference between PBS delivery and moving target, known as the interplay effect,[1,2] and typical breathing cycles are on the same scale as the time required for the switch between two adjacent energy layers,[3] the superior dose distribution is more sensitive to respiration‐induced organ motion for treatment sites such as lung, liver, and mediastinum, which can cause temporal displacement of the target volume and degrade the proton dose distribution significantly.[4,5] It was shown that during dynamic proton beam scanning, intrafractional organ motion induces up to 100% of the target to receive a dose outside the International Commission on Radiation Units and Measurements (ICRU) recommended limits with a minimal dose down to 34% of the prescribed dose in the extreme cases.[6] To mitigate the motion interplay effect, several methods including respiratory gating, breath hold, tumor tracking, and repainting have been investigated or clinically implemented.[7,8,9,10,11,12]. Boria et al[16] investigated the interplay effect mitigation of PBS in terms of fractionation on real pediatric patient 4DCT dataset
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