Prog Med Phys.  2014 Dec;25(4):288-297. 10.14316/pmp.2014.25.4.288.

Use of Flattening Filter Free Photon Beams for Off-axis Targets in Conformal Arc Stereotactic Body Radiation Therapy

• Smith A
• Kim S
• Serago C
• Hintenlang K
• Ko S
• Vallow L
• Peterson J
• Hintenlang D
• Heckman M
• Buskirk S

• KMID: 2315769
• DOI: http://doi.org/10.14316/pmp.2014.25.4.288

Abstract

Dynamic conformal arc therapy (DCAT) and flattening-filter-free (FFF) beams are commonly adopted for efficient conformal dose delivery in stereotactic body radiation therapy (SBRT). Off-axis geometry (OAG) may be necessary to obtain full gantry rotation without collision, which has been shown to be beneficial for peripheral targets using flattened beams. In this study dose distributions in OAG using FFF were evaluated and the effect of mechanical rotation induced uncertainty was investigated. For the lateral target, OAG evaluation, sphere targets (2, 4, and 6 cm diameter) were placed at three locations (central axis, 3 cm off-axis, and 6 cm off-axis) in a representative patient CT set. For each target, DCAT plans under the same objective were obtained for 6X, 6FFF, 10X, and 10FFF. The parameters used to evaluate the quality of the plans were homogeneity index (HI), conformality indices (CI), and beam on time (BOT). Next, the mechanical rotation induced uncertainty was evaluated using five SBRT patient plans that were randomly selected from a group of patients with laterally located tumors. For each of the five cases, a plan was generated using OAG and CAG with the same prescription and coverage. Each was replanned to account for one degree collimator/couch rotation errors during delivery. Prescription isodose coverage, CI, and lung dose were evaluated. HI and CI values for the lateral target, OAG evaluation were similar for flattened and unflattened beams; however, 6FFF provided slightly better values than 10FFF in OAG. For all plans the HI and CI were acceptable with the maximum difference between flattened and unflattend beams being 0.1. FFF beams showed better conformality than flattened beams for low doses and small targets. Variation due to rotational error for isodose coverage, CI, and lung dose was generally smaller for CAG compared to OAG, with some of these comparisons reaching statistical significance. However, the variations in dose distributions for either treatment technique were small and may not be clinically significant. FFF beams showed acceptable dose distributions in OAG. Although 10FFF provides more dramatic BOT reduction, it generally provides less favorable dosimetric indices compared to 6FFF in OAG. Mechanical uncertainty in collimator and couch rotation had an increased effect for OAG compared to CAG; however, the variations in dose distributions for either treatment technique were minimal.

Keyword

Flattening-filter-free; Lateral targets; Conformal arc; SBRT

MeSH Terms

Axis, Cervical Vertebra
Humans
Lung
Prescriptions
Uncertainty

Figure

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  Fig. 1. Central axis geometry (CAG) and off-axis geometry (OAG) for a laterally located target.

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  Fig. 2. Lateral target, off-axis evaluation. Sphere targets of three sizes (2, 4, and 6 cm diameter) were placed at three locations (center of the patient, 3 cm lateral, and 6 cm lateral) for a total of 9 targets.

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  Fig. 3. Mechanical rotation induced dosimetric uncertainty evaluation. Representative axial slice with OAG shown on the left and CAG on the right for each case.

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  Fig. 4. Prescription Isodose Surface Coverage for off-axis geometry (OAG) and central axis geometry (CAG) for the original plan, one degree collimator rotation, one degree couch rotation, and one degree of both collimator and couch rotations. Points and lines for the same patient are shown in the same color.

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  Fig. 5. CI-100 for off-axis geometry (OAG) and central axis geometry (CAG) for the original plan, one degree collimator rotation, one degree couch rotation, and one degree of both collimator and couch rotations. Points and lines for the same patient are shown in the same color.

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  Fig. 6. CI-50 for off-axis geometry (OAG) and central axis geometry (CAG) for the original plan, one degree collimator rotation, one degree couch rotation, and one degree of both collimator and couch rotations. Points and lines for the same patient are shown in the same color.

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  Fig. 7. Lung V20 for off-axis geometry (OAG) and central axis geometry (CAG) for the original plan, one degree collimator rotation, one degree couch rotation, and one degree of both collimator and couch rotations. Points and lines for the same patient are shown in the same color.

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