Study of Rotational Setup Errors and Their Dosimetric Impacts on Head and Neck IMRT Treatments Using Kilovoltage Cone-Beam Computed Tomography (kV CBCT)

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

With the on-board kV CBCT available, translational and rotational (six degree) setup errors can be obtained through a 3D/3D registration between the planning CT and CBCT images. The purpose of this study is to determine the magnitude of the rotational setup errors in head & neck patients and investigate the dosimetric impact of the residual rotational setup errors on head & neck IMRT to evaluate whether the full six degree setup correction is necessary.

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Materials/Methods 

kV CBCT images were acquired on the first day of treatment and weekly thereafter for eleven head & neck patients treated with IMRT. A total of 72 CBCT image sets were acquired. The CBCT images were registered with the planning CT images using two 3D rigid registration methods. Method 1 determines the translational errors only and method 2 determines all six degree errors. The dosimetric impact of residual rotational errors can be evaluated by comparing the dose distribution and DVHs from translational errors (from method 1) with those from six degree errors (from method 2). In this study, the dose distribution for the 3D translational correction is obtained by shifting beam isocenter. The dose distribution for the six degree correction is obtained using the method proposed by Yue et al⁽¹⁾. In their method, the rotational errors are corrected by adjusting the gantry, collimator and couch angles of treatment beams. For each patient, the dosimetric impact of residual mean and maximum rotational errors was evaluated.

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Results 

For these 11 head & neck patients, the average translational errors determined by method 1 were 0.6 ± 3.8, 1.4 ± 2.8, 0.1 ± 2.5 mm in LR, AP, and SI directions respectively and the values determined by method 2 were 0.5 ± 4.4, 1.3 ± 2.9, 0.4 ± 2.3 mm in LR, AP and SI directions respectively. The average rotational errors determined by method 2 were 0.7° ± 1.0°, 0.1° ± 1.9°, 0.3° ± 0.8° and the average maximum errors were 1.0° ± 1.6°, 0.1° ± 2.6°, 0.4° ± 0.7° around LR, AP and SI axes respectively. The dosimetric impact of residual rotational errors is shown in Table 1.

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Conclusions 

This study found that rotational errors contribute significantly to the treatment setup error for head & neck IMRT patients. With only translational error correction, the PTV prescription dose coverage decreases significantly because the large volume PTV is more susceptible to rotational errors. The GTV dose coverage decreases little because of the large margin between GTV and PTV. The dose increase to spinal cord is not significant. Our CTV volume is very close to PTV, so to improve the CTV dose coverage, the full six degree setup correction is needed.

Table 1. The impact of residual mean and maximum rotational errors on dosimetric quantities for 11 head & neck patients IMRT plans

	Mean rotation error		Maximum rotation error
	Mean	SD	Mean	SD
GTV
ΔVp (%)	−0.15	0.28	−1.12	2.56
PTV
ΔVp (%)	−5.43	4.70	−10.08	5.19
Cord
ΔD1cc (Gy)	0.98	1.65	2.39	4.12

ΔV_p: the difference of the target prescription dose coverage between method 1 and method 2.

ΔD_1cc: the difference of the maximum dose to 1 cc spinal cord between method 1 and method 2.