(P135) Dose Escalation Using Conventional Versus IMRT Planning for Hypofractionated Palliative Radiation of Lumbosacral Bony Metastases

Bevan Ly, MD, Elizabeth Bossart, PhD, Alan Pollack, MD, PhD, William Amestoy, CMD, Nesrin Dogan, PhD, Jean Wright, MD; University of Miami; Jackson Memorial Hospital

Purpose: The standard approach for palliation of bone metastasis (BM) is conventionally planned radiation (CRT). Randomized studies have shown the equivalence of hypofractionated vs conventionally fractionated regimens; yet, reported pain control is poor with either approach, resulting in some degree of pain relief in only 50% to 80% of cases and complete response in 15% to 60% of cases. We performed a dosimetric comparison of CRT vs intensity-modulated radiation therapy (IMRT) using standardized dose constraints to investigate if IMRT allowed hypofractionated dose escalation to bony targets in the pelvis and lumbosacral spine, based on the hypothesis that dose escalation will result in superior pain control if it can be delivered within acceptable dose limits to normal tissues.

Materials and Methods: We retrospectively replanned 10 patients with IMRT who were initially treated with palliative CRT for BM involving the pelvis and/or lumbar spine at our institution, using standardized dose constraints for five- fraction regimens, as published in American Association of Physicists in Medicine (AAPM) Task Group (TG) 101. We defined the planning target volume (PTV) as all bony structures contained within the 50% isodose line based on original conventional plans. For IMRT plans, optimization was performed to achieve 95% PTV coverage with the prescription dose while meeting published dose constraints for five-fraction regimens, including spinal cord maximum 30 Gy, cauda equina maximum 32 Gy, and bowel maximum 35 Gy. IMRT plans were optimized for a prescription of 30 Gy in five fractions, and dosimetric characteristics were extracted for five-fraction regimens starting at 20 Gy and escalating by 1 Gy per fraction to 35 Gy.

Results: IMRT allowed dose escalation to 30 Gy in five fractions using AAPM TG 101 constraints, with a mean maximum spinal cord dose of 29.6 Gy, mean maximum cauda equina dose of 31.5 Gy, and mean maximum bowel dose of 27.9 Gy. For the same targets, CRT five-fraction plans only allowed a dose of 25 Gy: the mean maximum spinal cord dose for a 25-Gy prescription was 29.3 Gy (within constraint) but 35.2 Gy for 30 Gy (above constraint); the mean maximum cauda equina dose for a 25-Gy prescription was 30.4 Gy (within constraint) but 36.4 Gy for 30 Gy (above constraint); and the mean maximum bowel dose was 28 Gy for a 25-Gy prescription and 33.6 Gy for 30 Gy (both within constraints). The bowel V20 was extremely low for five-fraction regimens using IMRT, with a mean V20 of 2 cc for IMRT plans, and much higher, 39.5 cc, for CRT plans.

Conclusions: IMRT resulted in the ability to dose-escalate to bony targets in the pelvis and/or lumbosacral spine while meeting standardized dose constraints to the spinal cord/cauda equina and bowel. IMRT allowed a dose of 30 Gy using a five-fraction regimen, while CRT allowed a dose of only 25 Gy using AAPM TG101 constraints. Prospective study is warranted to determine if the achieved dose escalation using IMRT results in clinically meaningful improvements in pain control and acceptable toxicity.