[Errors in six degree-of-freedom pose estimation of spine tumors assessed by image guided radiotherapy].

OBJECTIVE: To evaluate the six-degree setup errors of tumors of cervical vertebra, thoracic vertebra and lumbar vertebra by image guided radiotherapy.

METHODS: From May 2013 to June 2014, 30 patients with spinal malignant tumors(10 patients of cervical vertebra, thoracic vertebra and lumbar vertebra respectively) were treated with Elekata Synergy accelerator (Elekta company,Sweden). Six-degree set up errors were corrected using HexaPODTMevoRT bed under image of on board cone beam computed tomography (CBCT) guided. All the patients received kilovoltage CBCT before receiving radiotherapy and after correction. The acquired images were co-registered with planning CT with bone window. The data of 838 CT images were analyzed and the errors of translational directions X(lateral), Y(lngitudinal),Z(vertical)and rotational directions RX(pitch), RY(roll), RZ(yaw) were recorded. The data were compared by t-test using SPSS 13.0.

RESULTS: The absolute translational setup errors in X, Y and Z axes of cervical vertebra before correction were (1.71 ± 0.10) mm, (1.81 ± 0.11) mm and (1.94 ± 0.09) mm respectively: (3.17 ± 0.19) mm, (4.26 ± 0.28) mm and (2.18 ± 0.12) mm for thoracic vertebra, and (2.69 ± 0.24) mm, (3.33 ± 0.26) mm and (2.86 ± 0.21) mm for lumbar vertebra. The residual setup errors in X, Y and Z axes of cervical vertebra were (0.5 ± 2.4) mm,(0.01 ± 2.4) mm and (2.4 ± 1.4) mm, respectively after correction;(1.17 ± 0.11) mm,(0.26 ± 0.30) mm and (0.08 ± 0.12) mm for thoracic vertebra and (1.09 ± 0.24) mm,(2.03 ± 1.26) mm and (0.06 ± 0.51) mm for lumbar vertebra. The t-test of paired data of set up errors before and after CBCT showed significant difference in three translational directions of cervical vertebra and thoracic vertebra, only Z(t=-3.518,P<0.001) for lumbar vertebra. The absolute rotational setup errors in RX,RY and RZ axes of cervical vertebra before correction were 0.67° ± 0.04°,1.06° ± 0.06° and 0.78° ± 0.05° respectively. 0.62° ± 0.05°, 0.75° ± 0.06°, and 0.84° ± 0.06° for thoracic vertebra, 0.59° ± 0.06°, 0.80° ± 0.07°, and 0.73° ± 0.06° for lumbar vertebra. The rotational directions RX, RY and RZ axes of cervical vertebra were 0.27° ± 0.14°, 1.20° ± 0.04° and 0.28° ± 0.05° respectively; 0.02° ± 0.20°, 0.05°±0.26° and 0.64° ± 0.16° for thoracic vertebra and 0.09° ± 0.26°, 0.50°±0.05°,and 0.03°±0.16° for lumbar vertebra. The t-test of paired data of set up errors before and after CBCT showed significant difference in three rotational directions of cervical vertebra and lumbar vertebra, only RY(t=7.106, P<0.001)for thoracic vertebra. All the patients acquired pain relief and there was no radiation-induced toxicity detected clinically during a median follow-up of 6 months.

CONCLUSION: Six-degree set up errors of spine tumors were corrected effectively with HexaPODTMevoRT bed under CBCT image guided and its feasibility in day-to-day clinical practice has been demonstrated.