Abstract
The objective of this study was to evaluate and understand the systematic error between the planned three‐dimensional (3D) dose and the delivered dose to patient in scanning beam proton therapy for lung tumors. Single‐field and multifield optimized scanning beam proton therapy plans were generated for ten patients with stage II‐III lung cancer with a mix of tumor motion and size. 3D doses in CT datasets for different respiratory phases and the time‐weighted average CT, as well as the four‐dimensional (4D) doses were computed for both plans. The 3D and 4D dose differences for the targets and different organs at risk were compared using dose‐volume histogram (DVH) and voxel‐based techniques, and correlated with the extent of tumor motion. The gross tumor volume (GTV) dose was maintained in all 3D and 4D doses, using the internal GTV override technique. The DVH and voxel‐based techniques are highly correlated. The mean dose error and the standard deviation of dose error for all target volumes were both less than 1.5% for all but one patient. However, the point dose difference between the 3D and 4D doses was up to 6% for the GTV and greater than 10% for the clinical and planning target volumes. Changes in the 4D and 3D doses were not correlated with tumor motion. The planning technique (single‐field or multifield optimized) did not affect the observed systematic error. In conclusion, the dose error in 3D dose calculation varies from patient to patient and does not correlate with lung tumor motion. Therefore, patient‐specific evaluation of the 4D dose is important for scanning beam proton therapy for lung tumors.PACS number: 87.55.D
Highlights
48 Li et al.: Evaluation of dose error in 3D dose calculation planning for lung tumors using internal gross tumor volume (IGTV) override, a proper smearing margin, and planning on time-averaged computed tomography (CT; average replacement of the internal gross tumor volume or AVE_RIGTV plan) for passive scatter proton therapy (PSPT)
Scanning beam proton therapy may deliver a lower dose to normal tissue than does passive scatter proton therapy (PSPT) or intensity-modulated radiation therapy (IMRT).(1) In a previous investigation,(2) we developed an effective and practical method for proton therapy a Corresponding author: Heng Li, Department of Radiation Physics, Unit 1150, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77303, USA; phone: [713] 563 2572; fax: [713] 563 2479; email: hengli@mdanderson.org
A major concern in scanning beam proton therapy for lung cancer is the dose uncertainty caused by internal organ motion resulting from breathing, which may lead to a dose distribution that differs drastically from the planned distribution
Summary
48 Li et al.: Evaluation of dose error in 3D dose calculation planning for lung tumors using internal gross tumor volume (IGTV) override, a proper smearing margin, and planning on time-averaged computed tomography (CT; average replacement of the internal gross tumor volume or AVE_RIGTV plan) for PSPT. A major concern in scanning beam proton therapy for lung cancer is the dose uncertainty caused by internal organ motion resulting from breathing, which may lead to a dose distribution that differs drastically from the planned distribution This uncertainty might be larger than PSPT in scanning beam proton therapy because of the lack of smearing margin.[3] Researchers have shown that this dose uncertainty consists of two components: 1) the systematic difference between the conventionally calculated dose using a three-dimensional (3D) CT dataset (3D dose) and the four-dimensional (4D) accumulated dose, which is the time-weighted average of the dose calculated in all respiratory phases (4D dose); and 2) a random component resulting from the difference between dynamically delivered and the stationary-calculated dose.[4,5,6,7,8] The systematic component is sometimes referred to as motion blurring (of the dose), whereas the random component is often referred to as the interplay effect; the definitions were previously ambiguous. In the study described we examined the systematic error using the 3D dose, instead of the 4D dose, for scanning beam proton therapy in the treatment planning stage with dose-volume histogram (DVH)- and voxel-based techniques, and investigated the relationship between the error and lung tumor motion
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