2577 - Phase 2 Study of Stereotactic Body Radiation Therapy Using Intensity-Modulated Radion Therapy with Dose Escalation by Simultaneous Integrated Boost (SIB-IM-SBRT) for Spinal Metastases

T. Mitsuyoshi¹, P. J. K. Tokuda¹, Y. Kokubo¹, T. Iwai², R. Ashida¹, R. Nasada³, M. Yamashita³, H. Tanabe³, S. Arizono⁴, T. Imagumbai¹, and M. Kokubo¹; ¹Department of Radiation Oncology, Kobe City Medical Center General Hospital, Kobe, Japan, ²Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, Kyoto, Japan, ³Department of Radiological Technology, Kobe City Medical Center General Hospital, Kobe, Japan, ⁴Department of Diagnostic Radiology, Kobe City Medical Center General Hospital, Kobe, Japan

Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) is a useful treatment option for spinal metastasis; however, some patients experience local recurrence (LR). The purpose of this prospective phase 2 study was to assess whether SBRT using intensity-modulated radiation therapy with dose escalation in the gross tumor volume (GTV) by simultaneous integrated boost (SIB-IM-SBRT) could reduce LR without increasing the rate of adverse events (AEs). Materials/

Methods: Patients with spinal metastasis who were aged =20 years and had life expectancy >1 year were eligible. They received SIB-IM-SBRT in five fractions over one week. The prescribed dose was 30 Gy to planning target volume (PTV) and was escalated by SIB to almost 40-45 Gy to GTV. Patients were followed-up with neurological examinations and magnetic resonance imaging (MRI) at 3, 6, and 12 months, and then subsequently every 6 months. Toxicity was recorded according to the Common Terminology Criteria for Adverse Events Version 5.0. The primary endpoint was the rate of radiographic LR at 1 year after the completion of treatment.

Results: Between October 2020 and March 2023, 28 tumors in 25 patients were enrolled and provided with treatment; one patient did not complete the protocol treatment. Patient/tumor characteristics and dosimetric parameters are shown in the table. Median follow-up period was 26.2 months (range 4.8-39.1), and 23 tumors were followed-up with MRI for >1 year; 18 patients were alive at the time of analysis. The 1- and 2-year overall survival rates were 92.0% and 72.8%, respectively. Only one patient developed LR, at 4 months. The cumulative incidence of LR at 1 and 2 years was 0% and 4.4%, respectively. Six patients developed vertebral compression fractures; these were Grade 2 and Grade 1 AEs in two and four patients, respectively. The other radiation-induced AEs were Grade 1 esophagitis and Grade 1 radiculitis in one patient each. No other AEs of Grade 2 or higher were observed.

Conclusion: Spine SIB-IM-SBRT demonstrates excellent local control with acceptable toxicity, warranting a formal comparative study. Dx = minimum dose delivered to x% of the respective target volume

Patient Characteristics (n = 25 patients)	Data
Median age (range), years	70 (43-86)
Sex
Male	15
Female	10
Tumor Characteristics (n = 28 tumors)
Histology
Breast cancer	9
Prostate cancer	8
Lung cancer	4
Renal cell carcinoma	4
Other	3
Location of treated tumor
Cervical	1
Thoracic	11
Lumbar	10
Sacrum	6
Median PTV, cm3 (range)	70.2 (17.3-468.7)
Median GTV, cm3 (range)	9.5 (1.1-183.4)
Dosimetric Parameters (n = 28 tumors)
Median PTV D90, Gy	30.0
Median GTV D2, Gy (range)	47.4 (45.7-48.1)
Median GTV D50, Gy (range)	43.2 (41.3-45.7)
Median GTV D98, Gy (range)	32.8 (25.3-41.4)