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 Gy/5 fractions, 17 by 30 Gy/5 fractions, one by 25 Gy/5 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) ± 0.36 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. Radiation therapy is a common and effective treatment modality for managing skeletal metastases. The possible irradiated dose to vertebral metastases is limited for tumors located near the spinal cord when conventional external beam techniques are used. Recent advances in technology have improved the therapeutic threshold, permitting delivery of an ablative radiation dose using an image-guided stereotactic approach (1). In addition, in the era of modern radiotherapy techniques, the planned target volume (PTV) to vertebral metastases can be reduced by precise positioning, i.e. by image-guided and stereotactic intensity-modulated radiotherapy (IGRT-IMRT) (2). Because helical tomotherapy has the ability to acquire megavoltage computed tomographic (MVCT) images of a patient in the treatment position before each treatment, daily imaging can minimize daily positional uncertainties (2). This precise positioning allows the PTV to be reduced, directly improving conformal avoidance of nearby critical structures. Therefore, we initiated an assessment of the required PTV margin in IGRT-IMRT for a few vertebral metastases using helical tomotherapy.