Assessment of planning target volume margin for a small number of vertebral metastatic lesions using image-guided intensity-modulated radiation therapy by helical tomotherapy.

Aim: To evaluate an appropriate planning target volume (PTV) margin in for one to three vertebral metastases using megavolt computed tomography (MVCT) images during the course of image-guided and stereotactic intensity-modulated radiotherapy (IGRT-IMRT) by use of helical tomotherapy. Patients and Methods: A total of 25 lesions in 24 patients with vertebral metastases who received IGRT-IMRT were analyzed. MVCT images were acquired before and after radiation therapy. Intra-fractional movement and PTV margin were calculated by comparing treatment planning images and these 310 MVCT images for right-left (RL), superior-inferior (SI), and anteroposterior (AP) dimensions. Five patients were treated by 35 Gy15 fractions, 17 by 30 Gy15 fractions, one by 25 Gy15 fractions, and one by 60 Gy/30 fractions. A margin to compensate for these variations was calculated with the formula of vanHerk's equation. Results: The intra-fractional motion was 0.02 (-1.3 to 1.4) +/- 0.34 mm in the RL direction, -0.09 (-1.8 to 0.28) +/- 0.44 mm in the SI direction, and 0.20 (-1.8 to 1.8) +/- 036 mm in the AP direction. The required PTV margin was 0.98 mm in the RL direction, 0.69 mm in the SI direction, and 1.26 mm in the AP direction. No patient showed a deviation greater than 2 mm. Conclusion: The PTV margin in hypofractionated IGRT IMRT, using helical tomotherapy for a few vertebral metastases, was 2 mm or less and our tentative PTV margin of 5 mm was sufficient and reducible.