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

Ashley Smith; Siyong Kim; Christopher Serago; Kathleen Hintenlang; Stephen Ko; Laura Vallow; Jennifer Peterson; David Hintenlang; Michael Heckman; Steven Buskirk

doi:10.14316/pmp.2014.25.4.288

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Smith, Kim, Serago, Hintenlang, Ko, Vallow, Peterson, Hintenlang, Heckman, and Buskirk: Use of Flattening Filter Free Photon Beams for Off-axis Targets in Conformal Arc Stereotactic Body Radiation Therapy

Original Article

Progress in Medical Physics 2014;25(4):288-297.

Published online: 20 January 2014

DOI: https://doi.org/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

Ashley Smith^∗, Siyong Kim^†, Christopher Serago^∗, Kathleen Hintenlang^∗, Stephen Ko^∗, Laura Vallow^∗, Jennifer Peterson^∗, David Hintenlang^‡, Michael Heckman^§, Steven Buskirk^∗

^∗Division of Radiation Oncology, Mayo Clinic, 4500 San Pablo Road South, Jacksonville, FL, 32224

^†Department of Radiation Oncology, Virginia Commonwealth University, 401 College St, Richmond, VA 23284

^‡J Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, 1275 Center Drive, Gainesville, FL 32611

^§Division of Biomedical Statistics and Informatics, Mayo Clinic, 4500 San Pablo Road South, Jacksonville, FL, 32224

Correspondence: Ashley Smith (smith.ashley@mayo.edu) Tel:1-904-953-7903, Fax:1-904-953-1010

Received 18 November 2014 Revised 23 November 2014 Accepted 14 December 2014

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Keywords: Flattening-filter-free, Lateral targets, Conformal arc, SBRT

References

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Fig. 1.

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

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.

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.

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.

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.

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.

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.

Table 1a.

Lateral target, off-axis geometry evaluation results for 6 MV.

	Target Diameter(cm)	Axis		3 cm Off-Axis		6 cm Off-Axis
		Flattened	FFF	Flattened	FFF	Flattened	FFF
HI	2	1.19	1.17	1.16	1.15	1.14	1.13
	4	1.11	1.11	1.11	1.12	1.12	1.10
	6	1.11	1.13	1.11	1.13	1.12	1.11
CI-100	2	1.11	1.11	1.06	1.06	1.04	1.04
	4	0.98	0.99	0.99	0.99	1.00	1.00
	6	0.98	0.99	0.99	1.00	1.00	1.01
CI-50	2	4.23	4.12	4.33	4.28	4.33	4.30
	4	3.36	3.36	3.36	3.29	3.39	3.38
	6	3.11	3.17	3.17	3.17	3.21	3.24
BOT ratio	2		0.46		0.47		0.50
	4		0.47		0.48		0.50
	6		0.48		0.49		0.51

HI: homogeneity index, CI: conformity index, BOT: beam on time. Note, FFF offers faster BOT than flattened beams.

Table 1b.

Lateral target, off-axis geometry evaluation results for 10 MV.

	Target Diameter(cm)	Axis		3 cm Off-Axis		6 cm Off-Axis
		Flattened	FFF	Flattened	FFF	Flattened	FFF
HI	2	1.23	1.25	1.20	1.21	1.17	1.19
	4	1.13	1.17	1.14	1.19	1.14	1.17
	6	1.12	1.20	1.13	1.23	1.13	1.21
CI-100	2	1.12	1.13	1.06	1.08	1.03	1.07
	4	0.99	0.99	0.99	1.04	0.99	1.03
	6	0.98	1.00	0.99	1.04	0.99	1.04
CI-50	2	4.59	4.49	4.73	4.64	4.71	4.65
	4	3.48	3.40	3.54	3.53	3.56	3.52
	6	3.13	3.12	3.24	3.29	3.25	3.29
BOT ratio	2		0.26		0.27		0.31
	4		0.27		0.29		0.32
	6		0.29		0.30		0.32

HI: homogeneity index, CI: conformity index, BOT: beam on time. Note, FFF offers faster BOT than flattened beams.

Table 1b.

Comparisons of prescription isodose surface coverage, conformality indices, and lung V20 between OAG and CAG.

Mean (minimum, maximum) value
Variable	Original plan)	1 degree collimator rotation	1 degree couch rotation	1 degree collimator, 1 degree couch rotation	Absolute difference: Collimator – Original	Absolute difference: Couch – Original	Absolute difference: Collimator/ Couch – Original
Prescription Isodose
Surface Coverage (%)
OAG (N=5 patients)	95.28	94.89	93.94	93.72	0.39	1.34	1.56
	(95.11, 95.56)	(94.57, 95.49)	(92.46, 95.00)	(92.31, 94.68)	(0.07, 0.76)	(0.11, 2.87)	(0.47, 3.02)
CAG (N=5 patients)	95.22	95.18	95.22	95.19	0.05	0.15	0.11
	(95.11, 95.30)	(95.15, 95.22)	(94.95, 95.53)	(95.03, 95.32)	(0.00, 0.09)	(0.06, 0.24)	(0.01, 0.21)
Comparison of					P=0.024	P=0.068	P=0.028
differences from
original values
CI 100
OAG (N=5 patients)	1.33	1.32	1.33	1.33	0.01	0.02	0.02
	(1.20, 1.54)	(1.19, 1.54)	(1.19, 1.54)	(1.19, 1.53)	(0.00, 0.01)	(0.00, 0.05)	(0.01, 0.04)
CAG (N=5 patients)	1.38	1.38	1.38	1.38	0.00	0.00	0.00
	(1.19, 1.60)	(1.19, 1.60)	(1.19, 1.60)	(1.19, 1.60)	(0.00, 0.01)	(0.00, 0.01)	(0.00, 0.02)
Comparison of					P=0.18	P=0.21	P=0.21
differences from
original values
CI 50
OAG (N=5 patients)	5.07	5.06	5.08	5.06	0.01	0.01	0.01
	(4.05, 5.74)	(4.05, 5.72)	(4.04, 5.76)	(4.05, 5.73)	(0.00, 0.03)	(0.00, 0.02)	(0.00, 0.01)
CAG (N=5 patients)	5.52	5.54	5.53	5.54	0.01	0.02	0.02
	(4.75, 7.06)	(4.76, 7.08)	(4.74, 7.08)	(4.75, 7.09)	(0.00, 0.03)	(0.01, 0.03)	(0.00, 0.03)
Comparison of					P＞0.99	P=0.016	P=0.24
differences from
original values
Lung V20 (%)
OAG (N=5 patients)	3.78	3.77	3.75	3.74	0.01	0.03	0.04
	(1.22, 5.99)	(1.21, 5.99)	(1.20, 5.95)	(1.19, 5.96)	(0.00, 0.01)	(0.01, 0.08)	(0.02, 0.08)
CAG (N=5 patients)	3.71	3.72	3.71	3.71	0.00	0.01	0.00
	(1.19, 6.16)	(1.19, 6.16)	(1.18, 6.15)	(1.18, 6.16)	(0.00, 0.01)	(0.00, 0.01)	(0.00, 0.01)
Comparison of					P=0.37	P=0.11	P=0.027
differences from
original values

Absolute differences are the absolute values of the given difference. P-values result from a paired t-test. OAG: off-axis geometry, CAG: central axis geometry, CI: conformity index.

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