A single centre, non-profit, randomised, unblinded pilot study on volume definition for spinal stereotactic body radiation therapy treatment.

Randomization will be 1:1 between two types of SBRT treatment for spinal metastases. The statistical unit will be the individual vertebral metastasis. Multiple metastases will be randomized and evaluated independently.

Patients with spinal oligometastases who will be evaluated for SBRT will have to sign informed consent before any study-related procedure. Afterwards, a simulation computed tomography (CT) will be performed, and the metastases will be randomized to:

• Control arm: volume definition according to Cox contouring guidelines
• Experimental arm: volume definition based on the pathological lesion at Magnetic Resonance Imaging (MRI).

The treatment will be delivered at the center in either three or five fractions.

The planned follow-up consists of routine visits and exams, as per common clinical practice, according to primary histology (including an MRI to evaluate response of the vertebral lesion treated) at 3 months, 6 months, and subsequently every 6 months until the 2-year follow-up period. During these visits, physicians will assess the adverse events and NRS (a unidimensional measure of pain intensity in adults, from 0 to 10, and the result will be compared to the baseline).

The gross tumor volume (GTV) will be defined by MRI and PET uptake, if available, and the simulation CT. The treatment volume will be delineated based on MRI coregistered with the planning CT. The MRI protocol will include T1, T2-weighted, and diffusion-weighted imaging (DWI) sequences. The Planning Target Volume (PTV) will be obtained by adding an isotropic margin of 2 mm from the GTV, avoiding the spinal canal.

The clinical target volume (CTV) will be defined by the vertebral region according to the Cox contouring guidelines, and the PTV will be obtained by adding an isotropic margin of 2 mm from the CTV, avoiding the spinal canal.

Radiotherapy dose:

According to the current guidelines, available fractionations are 30 Gy in 3 fractions or 35 Gy in 5 fractions.

Planning:

The plan evaluation ensures at least 95% of the prescribed dose to 95% of the PTV without exceeding 120% of the prescribed dose. A partial PTV uncovering will be accepted if necessary to preserve mandatory constraints to OARs, considering a minor deviation if 95% of the PTV receives 85% or more of the prescribed dose, and a major deviation if 95% of the PTV receives less than 85% of the prescribed dose. Any deviation will be recorded but will not impact the patient's participation in the study.

Inclusion Criteria:

• Patients affected by spinal metastases eligible for SBRT
• All histologies are allowed
• Oligometastatic disease (maximum 5 metastases) in every setting (oligopersistent, oligorecurrent and oligoprogressive, according to ESTRO consensus)
• Metastases treated with ablative intent
• Both symptomatic and asymptomatic vertebral metastases
• Spine Instability Neoplastic Score (SINS) score < 7 [18]
• Age > 18 years
• Life expectancy > 12 months
• PS ECOG 0-1
• Vertebral pathological lesion on the diagnostic MRI
• Single active lesion on the target vertebra
• More lesions in different vertebrae can be simultaneously treated
• Informed consent to trial participation and personal data treatment

Exclusion Criteria:

• More than 2 adjoining vertebrae involved
• Palliative intent
• 2 or more lesions on the target vertebra
• Contraindications to MRI
• More than 5 metastases