The dosimetric impact of implants on the spinal cord dose during stereotactic body radiotherapy

Abstract

BackgroundThe effects of spinal implants on dose distribution have been studied for conformal treatment plans. However, the dosimetric impact of spinal implants in stereotactic body radiotherapy (SBRT) treatments has not been studied in spatial orientation. In this study we evaluated the effect of spinal implants placed in sawbone vertebra models implanted as in vivo instrumentations.MethodsFour different spinal implant reconstruction techniques were performed using the standard sawbone lumbar vertebrae model; 1. L2-L4 posterior instrumentation without anterior column reconstruction (PI); 2. L2-L4 anterior instrumentation, L3 corpectomy, and anterior column reconstruction with a titanium cage (AIAC); 3. L2-L4 posterior instrumentation, L3 corpectomy, and anterior column reconstruction with a titanium cage (PIAC); 4. L2-L4 anterior instrumentation, L3 corpectomy, and anterior column reconstruction with chest tubes filled with bone cement (AIABc). The target was defined as the spinous process and lamina of the lumbar (L) 3 vertebra. A thermoluminescent dosimeter (TLD, LiF:Mg,Ti) was located on the measurement point anterior to the spinal cord. The prescription dose was 8 Gy and the treatment was administered in a single fraction using a CyberKnife® (Accuray Inc., Sunnyvale, CA, USA). We performed two different treatment plans. In Plan A beam interaction with the rod was not limited. In plan B the rod was considered a structure of avoidance, and interaction between the rod and beam was prevented. TLD measurements were compared with the point dose calculated by the treatment planning system (TPS).Results and discussionIn plan A, the difference between TLD measurement and the dose calculated by the TPS was 1.7 %, 2.8 %, and 2.7 % for the sawbone with no implant, PI, and PIAC models, respectively. For the AIAC model the TLD dose was 13.8 % higher than the TPS dose; the difference was 18.6 % for the AIABc model. In plan B for the AIAC and AIABc models, TLD measurement was 2.5 % and 0.9 % higher than the dose calculated by the TPS, respectively.ConclusionsSpinal implants may be present in the treatment field in patients scheduled to undergo SBRT. For the types of implants studied herein anterior rod instrumentation resulted in an increase in the spinal cord dose, whereas use of a titanium cage had a minimal effect on dose distribution. While planning SBRT in patients with spinal reconstructions, avoidance of the rod and preventing interaction between the rod and beam might be the optimal solution for preventing unexpectedly high spinal cord doses.

Essential Concepts for the Management of Metastatic Spine Disease: What the Surgeon Should Know and Practice

“…Cobalt chrome causes significant beam scatter thus altering the dose delivered to the tumor and organs at risk whereas titanium has been shown to cause minimal radiation beam scatter. 100,101 Carbon fiber implants may provide a superior stabilization option for patients with planned postoperative proton beam therapy treatment, since the presence of metallic implants has been shown to have a detrimental effect on local tumor control after proton therapy. 102,103 Implants based on from carbon fiber also offer superior MR image quality, which may be beneficial for long-term surveillance/monitoring of local tumor control.…”

Sparing the surgical area with stereotactic body radiotherapy for combined treatment of spinal metastases: a treatment planning study

“…Furthermore, the spinal implants cause backscattering of electrons increasing the dose in front of the material while limiting the dose behind the spinal implants [8]. Yazici et al [26] demonstrated that the dose to the spinal cord increased up to 18.6% when spinal implants were present in the SBRT treatment field. Avoiding interaction between the radiation beam and the spinal implant to prevent high doses to the spinal cord is therefore important but also complicates post-operative SBRT planning [26].…”

“…Yazici et al [26] demonstrated that the dose to the spinal cord increased up to 18.6% when spinal implants were present in the SBRT treatment field. Avoiding interaction between the radiation beam and the spinal implant to prevent high doses to the spinal cord is therefore important but also complicates post-operative SBRT planning [26]. Reversing the order of surgery and SBRT may overcome these technical challenges.…”

Evaluation of computed tomography artefacts of carbon-fiber and titanium implants in patients with spinal oligometastatic disease undergoing stereotactic ablative radiotherapy

“…Conventional external beam radiation therapy (EBRT) to the entire spine has been the golden standard for decades due to its excellent palliative effect. However, EBRT doses are too low to ensure long term local control, and raising the dose is not an option because the spinal cord is often at risk 8 . Fortunately, stereotactic ablative radiotherapy (SABR) is an emerging noninvasive approach for the treatment of spinal OMD 9 .…”