Spinal cord tolerance in the age of spinal radiosurgery: lessons from preclinical studies

Sensitization of Endothelial Cells to Ionizing Radiation Exacerbates Delayed Radiation Myelopathy in Mice

Delayed radiation myelopathy is a rare, but significant late side effect from radiation therapy that can lead to paralysis. The cellular and molecular mechanisms leading to delayed radiation myelopathy are not completely understood but may be a consequence of damage to oligodendrocyte progenitor cells and vascular endothelial cells. Here, we aimed to determine the contribution of endothelial cell damage to the development of radiation-induced spinal cord injury using a genetically defined mouse model in which endothelial cells are sensitized to radiation due to loss of the tumor suppressor p53. Tie2Cre; p53FL/+ and Tie2Cre; p53FL/- mice, which lack one and both alleles of p53 in endothelial cells, respectively, were treated with focal irradiation that specifically targeted the lumbosacral region of the spinal cord. The development of hindlimb paralysis was followed for up to 18 weeks after either a 26.7 Gy or 28.4 Gy dose of radiation. During 18 weeks of follow-up, 83% and 100% of Tie2Cre; p53FL/- mice developed hindlimb paralysis after 26.7 and 28.4 Gy, respectively. In contrast, during this period only 8% of Tie2Cre; p53FL/+ mice exhibited paralysis after 28.4 Gy. In addition, 8 weeks after 28.4 Gy the irradiated spinal cord from Tie2Cre; p53FL/- mice showed a significantly higher fractional area positive for the neurological injury marker glial fibrillary acidic protein (GFAP) compared with the irradiated spinal cord from Tie2Cre; p53FL/+ mice. Together, our findings show that deletion of p53 in endothelial cells sensitizes mice to the development of delayed radiation myelopathy indicating that endothelial cells are a critical cellular target of radiation that regulates myelopathy.

Brachial Plexus Tolerance to Single-Session SAbR in a Pig Model

PURPOSE

The single-session dose tolerance of the spinal nerves has been observed to be similar to that of the spinal cord in pigs, counter to the perception that peripheral nerves are more tolerant to radiation. This pilot study aims to obtain a first impression of the single-session dose-response of the brachial plexus using pigs as a model.

METHODS AND MATERIALS

Ten Yucatan minipigs underwent computed tomography and magnetic resonance imaging for treatment planning, followed by single-session stereotactic ablative radiotherapy. A 2.5-cm length of the left-sided brachial plexus cords was irradiated. Pigs were distributed in 3 groups with prescription doses of 16 (n = 3), 19 (n = 4), and 22 Gy (n = 3). Neurologic status was assessed by observation for changes in gait and electrodiagnostic examination. Histopathologic examination was performed with light microscopy of paraffin-embedded sections stained with Luxol fast blue/periodic acid-Schiff and Masson's trichrome.

RESULTS

Seven of the 10 pigs developed motor deficit to the front limb of the irradiated side, with a latency from 5 to 8 weeks after irradiation. Probit analysis of the maximum nerve dose yields an estimated ED₅₀ of 19.3 Gy for neurologic deficit, but the number of animals was insufficient to estimate 95% confidence intervals. No motor deficits were observed at a maximum dose of 17.6 Gy for any pig. Nerve conduction studies showed an absence of sensory response in all responders and absent or low motor response in most of the responders (71%). All symptomatic pigs showed histologic lesions to the left-sided plexus consistent with radiation-induced neuropathy.

CONCLUSIONS

The single-session ED₅₀ for symptomatic plexopathy in Yucatan minipigs after irradiation of a 2.5-cm length of the brachial plexus cords was determined to be 19.3 Gy. The dose-response curve overlaps that of the spinal nerves and the spinal cord in the same animal model. The relationship between the brachial plexus tolerance in pigs and humans is unknown, and caution is warranted when extrapolating for clinical use.

Spatial descriptions of radiotherapy dose: normal tissue complication models and statistical associations

For decades, dose-volume information for segmented anatomy has provided the essential data for correlating radiotherapy dosimetry with treatment-induced complications. Dose-volume information has formed the basis for modelling those associations via normal tissue complication probability (NTCP) models and for driving treatment planning. Limitations to this approach have been identified. Many studies have emerged demonstrating that the incorporation of information describing the spatial nature of the dose distribution, and potentially its correlation with anatomy, can provide more robust associations with toxicity and seed more general NTCP models. Such approaches are culminating in the application of computationally intensive processes such as machine learning and the application of neural networks. The opportunities these approaches have for individualising treatment, predicting toxicity and expanding the solution space for radiation therapy are substantial and have clearly widespread and disruptive potential. Impediments to reaching that potential include issues associated with data collection, model generalisation and validation. This review examines the role of spatial models of complication and summarises relevant published studies. Sources of data for these studies, appropriate statistical methodology frameworks for processing spatial dose information and extracting relevant features are described. Spatial complication modelling is consolidated as a pathway to guiding future developments towards effective, complication-free radiotherapy treatment.

Effects From Nonuniform Dose Distribution in the Spinal Nerves of Pigs: Analysis of Normal Tissue Complication Probability Models

PURPOSE

Our purpose was to evaluate normal tissue complication probability (NTCP) models for their ability to describe the increase in tolerance as the length of irradiated spinal nerve is reduced in a pig.

METHODS AND MATERIALS

Common phenomenological and semimechanistic NTCP models were fit using the maximum likelihood estimate method to dose-response data from spinal nerve irradiation studies in pigs. Statistical analysis was used to compare how well each model fit the data. Model parameters were then applied to a previously published dose distribution used for spinal cord irradiation in rats under the assumption of a similar dose-response.

RESULTS

The Lyman-Kutcher-Burman model, relative seriality, and critical volume model fit the spinal nerve data equally well, but the mean dose logistic and relative seriality models gave the best fit after penalizing for the number of model parameters. The minimum dose logistic regression model was the only model showing a lack of fit. When extrapolated to a 0.5-cm simulated square-wave-like dose distribution, the serial behaving models showed negligible increase in dose-response curve. The Lyman-Kutcher-Burman model and relative seriality models showed significant shifting of NTCP curves due to parallel behaving parameters. The critical volume model gave the closest match to the rat data.

CONCLUSIONS

Several phenomenological and semimechanistic models were observed to adequately describe the increase in the radiation tolerance of the spinal nerves when changing the irradiated length from 1.5 to 0.5 cm. Contrary to common perception, model parameters suggest parallel behaving tissue architecture. Under the assumption that the spinal nerve response to radiation is similar to that of the spinal cord, only the critical volume model was robust when extrapolating to outcome data from a 0.5-cm square-wave-like dose distribution, as was delivered in rodent spinal cord irradiation research.

Existence of a Dose-Length Effect in Spinal Nerves Receiving Single-Session Stereotactic Ablative Radiation Therapy

PURPOSE

The spinal nerves have been observed to have a similar single-session dose tolerance to that of the spinal cord in pigs. Small-animal studies have shown that spinal cord dose tolerance depends on the length irradiated. This work aims to determine whether a dose-length effect exists for spinal nerves.

METHODS AND MATERIALS

Twenty-seven Yucatan minipigs underwent computed tomography and magnetic resonance imaging for treatment planning, followed by single-session stereotactic ablative radiation therapy. A 0.5 cm length of the left-sided C6, C7, and C8 spinal nerves was targeted. The pigs were distributed into 6 groups with prescription doses of 16 Gy (n = 5), 18 Gy (n = 5), 20 Gy (n = 5), 22 Gy (n = 5), 24 Gy (n = 5), or 36 Gy (n = 2) and corresponding maximum doses of 16.7, 19.1, 21.3, 23.1, 25.5, and 38.6 Gy, respectively. Neurologic status was assessed with a serial electrodiagnostic examination and daily observation of gait for approximately 52 weeks. A histopathologic examination of paraffin-embedded sections with Luxol fast blue/periodic acid-Schiff's staining was also performed.

RESULTS

Marked gait change was observed in 8 of 27 irradiated pigs. The latency for responding pigs was 11 to 16 weeks after irradiation. The affected animals presented with a limp in the left front limb, and 62.5% of these pigs had electrodiagnostic evidence of denervation in the C6 and C7 innervated muscles. A probit analysis showed the dose associated with a 50% incidence of gait change is 23.9 Gy (95% confidence interval, 22.5-25.8 Gy), which is 20% higher than that reported in a companion study where a 1.5 cm length was irradiated. All symptomatic pigs had demyelination and fibrosis in the irradiated nerves, but the contralateral nerves and spinal cord were normal.

CONCLUSIONS

A dose-length effect was observed for single-session irradiation of the spinal nerves in a Yucatan minipig model.

Advances in the treatment of metastatic spine tumors: the future is not what it used to be

An improved understanding of tumor biology, the ability to target tumor drivers, and the ability to harness the immune system have dramatically improved the expected survival of patients diagnosed with cancer. However, many patients continue to develop spine metastases that require local treatment with radiotherapy and surgery. Fortunately, the evolution of radiation delivery and operative techniques permits durable tumor control with a decreased risk of treatment-related toxicity and a greater emphasis on restoration of quality of life and daily function. Stereotactic body radiotherapy allows delivery of ablative radiation doses to the majority of spine tumors, reducing the need for surgery. Among patients who still require surgery for decompression of the spinal cord or spinal column stabilization, minimal access approaches and targeted tumor excision and ablation techniques minimize the surgical risk and facilitate postoperative recovery. Growing interdisciplinary collaboration among scientists and clinicians will further elucidate the synergistic possibilities among systemic, radiation, and surgical interventions for patients with spinal tumors and will bring many closer to curative therapies.

DTI and pathological changes in a rabbit model of radiation injury to the spinal cord after 125I radioactive seed implantation

Excessive radiation exposure may lead to edema of the spinal cord and deterioration of the nervous system. Magnetic resonance imaging can be used to judge and assess the extent of edema and to evaluate pathological changes and thus may be used for the evaluation of spinal cord injuries caused by radiation therapy. Radioactive ¹²⁵I seeds to irradiate 90% of the spinal cord tissue at doses of 40–100 Gy (D90) were implanted in rabbits at T₁₀ to induce radiation injury, and we evaluated their safety for use in the spinal cord. Diffusion tensor imaging showed that with increased D90, the apparent diffusion coefficient and fractional anisotropy values were increased. Moreover, pathological damage of neurons and microvessels in the gray matter and white matter was aggravated. At 2 months after implantation, obvious pathological injury was visible in the spinal cords of each group. Magnetic resonance diffusion tensor imaging revealed the radiation injury to the spinal cord, and we quantified the degree of spinal cord injury through apparent diffusion coefficient and fractional anisotropy.

Spinal cord constraints in the era of high-precision radiotherapy : Retrospective analysis of 62 spinal/paraspinal lesions with possible infringements of spinal cord constraints within a minimal volume

OBJECTIVE

Current constraints aim to minimize the risk of radiation myelitis by the use of restrictive maximal spinal cord doses, commonly 50 Gy. However, several studies suggested that a dose-volume effect could exist. Based on these observations, we evaluated patients receiving potentially excessive doses to the spinal cord within minimal volumes.

PATIENTS AND METHODS

Patients receiving radiotherapy between June 2010 and May 2015 using the Novalis^TM (Varian, Palo Alto, CA, USA; Brainlab, Heimstetten, Germany) radiosurgery system were retrospectively analyzed. A total of 56 patients with 62 treated lesions that had been prescribed radiation doses close to the spinal cord potentially higher than the common 50 Gy 2‑Gy equivalent-dose (EQD2) constraint were selected for further analysis. Of these patients, 26 with 31 lesions had no history of previous irradiation, while 30 patients with 31 lesions had been previously irradiated within the treatment field.

RESULTS

According to different dose evaluation approaches (spinal canal, spinal cord contour), 16 and 10 out of 31 primary irradiated lesions infringed constraints. For the 16 lesions violating spinal canal doses, the maximum doses ranged from 50.5 to 61.9 Gy EQD2. Reirradiated lesions had an average and median cumulative dose of 70.5 and 69 Gy, respectively. Dose drop-off was steep in both groups. Median overall survival was 17 months. No radiation myelitis or radiomorphological alterations were observed during follow-up.

CONCLUSION

This study adds to the increasing body of evidence indicating that excessive spinal cord doses within a minimal volume, especially in a reirradiation setting with topographically distinct high-point doses, may be given to patients after careful evaluation of treatment- and tumor-associated risks.

Spine Stereotactic Body Radiotherapy: Indications, Outcomes, and Points of Caution

STUDY DESIGN

A broad narrative review.

OBJECTIVES

The objective of this article is to provide a technical review of spine stereotactic body radiotherapy (SBRT) planning and delivery, indications for treatment, outcomes, complications, and the challenges of response assessment. The surgical approach to spinal metastases is discussed with an overview of emerging minimally invasive techniques.

METHODS

A comprehensive review of the literature was conducted on the techniques, outcomes, and developments in SBRT and surgery for spinal metastases.

RESULTS

The optimal management of patients with spinal metastases is complex and requires multidisciplinary assessment from an oncologic team that is familiar with the shifting paradigm as a consequence of evolving techniques in surgery and stereotactic radiation, as well as new developments in systemic agents. The Spinal Instability Neoplastic Score and the epidural spinal cord compression (Bilsky) grading system are useful tools that facilitate communication among oncologic team members and can direct management by providing a baseline assessment of risks prior to therapy. The combined multimodality approach with "separation surgery" followed by postoperative spine SBRT achieves thecal sac decompression, improves tumor control, and avoids complications that may be associated with more extensive surgery.

CONCLUSION

Spine SBRT is a highly effective treatment that is capable of delivering ablative doses to the target while sparing the critical organs-at-risk, chiefly the critical neural tissues, within a short and manageable schedule. At the same time, surgery occupies an important role in select patients, particularly with the expanding availability and expertise in minimally invasive techniques. With rapid adoption of spine SBRT in centers outside of the academic setting, it is imperative for the practicing oncologist to understand the relevance and application of these evolving concepts.

Three-dimensional conformal fractionated radiotherapy for spinal schwannoma with a paravertebral or an intraosseous component

INTRODUCTION

We retrospectively evaluated the efficacy of three-dimensional conformal radiotherapy (3D-CRT) for spinal schwannoma.

METHODS

Nine patients with spinal schwannoma were treated with 3D-CRT. All patients had a paravertebral or intraosseous component. Tumor sizes ranged from 0.8 to 8.7 cm, with a median of 3.5 cm. The prescribed dose was 50 Gy in 25 fractions at the isocenter, except for 1 patient who received 66 Gy in 33 fractions for a large sacral tumor. The follow-up period ranged from 20 to 137 months, with a median of 72 months.

RESULTS

Tumor shrinkage within 3 mm occurred in 4 patients and tumor expansion within 3 mm occurred in 3. One tumor showed neither expansion nor shrinkage at the last follow-up. One patient experienced transient expansion by 8 mm in diameter at 12 months after the completion of radiotherapy (35-43 mm), and then the tumor size remained unchanged for 7 years. No severe late toxicity ≥ grade 3 was observed.

CONCLUSIONS

Only 1 of 9 tumors showed transit expansion over 3 mm after 3D-CRT, and severe late radiation toxicity was not observed. Use of 3D-CRT should be considered a treatment option for spinal schwannoma.

Radiation dose-fractionation effects in spinal cord: comparison of animal and human data

Purpose

Recognizing spinal cord dose limits in various fractionations is essential to ensure adequate dose for tumor control while minimizing the chance of radiation-induced myelopathy (RIM). This study aimed to determine the α/β ratio of the spinal cord and the cord dose limit in terms of BED50, the biological equivalent dose (BED) that induces 50 % chance of RIM, by fitting data collected from published animal and patient studies.

Methods

RIM data from five rat studies; three large animal studies on monkeys, dogs, and pigs; and 18 patient studies were included for the investigation. The α/β ratios were derived, respectively, for rat (group A), large animal (group B), patient (group C), and combined data (group D).

Results

The α/β ratio (and its 95 % confidental interval) was 4.1 (3.2, 5.0) or 3.6 (2.6, 4.6) Gy for group A, depending on fitting algorithms. It was 3.9 (3.0, 4.8), 3.7 (2.2, 8.2) and 3.9 (3.0, 4.9) for groups B, C, and D, respectively. BED50 was 111 Gy for the combined data. It corresponds to a D50 of 73.4 Gy in 2 Gy/FX, or 19.0 Gy in single fraction. BED5, which is the BED to induce 5 % of RIM, was calculated to be 83.9 Gy. It corresponds to D5 of 55.4 Gy in 2 Gy/FX, or 16.2 Gy in single fraction.

Conclusion

The study showed that all four groups had similar α/β ratios close to 3.9 Gy, suggesting that the spinal cord has a similar fractionation effect for different species, including human beings.

Surgical Strategies for Cervical Spinal Neurinomas

Cervical spinal neurinomas are benign tumors that arise from nerve roots. Based on their location, these tumors can also take the form of a dumbbell-shaped mass. Treatment strategies for these tumors have raised several controversial issues such as appropriate surgical indications and selection of surgical approaches for cervical dumbbell-shaped spinal neurinomas. In this report, we review previous literature and retrospectively analyze cervical spinal neurinoma cases that have been treated at our hospital. Surgical indications and approaches based on tumor location and severity are discussed in detail. Thus, with advances in neuroimaging and neurophysiological monitoring, we conclude that appropriate surgical approaches and intraoperative surgical manipulations should be chosen on a case-by-case basis.

Molecular imaging detects impairment in the retrograde axonal transport mechanism after radiation-induced spinal cord injury

PURPOSE

The goal of this study was to determine whether molecular imaging of retrograde axonal transport is a suitable technique to detect changes in the spinal cord in response to radiation injury.

PROCEDURES

The lower thoracic spinal cords of adult female BALB/c mice were irradiated with single doses of 2, 10, or 80 Gy. An optical imaging method was used to observe the migration of the fluorescently labeled nontoxic C-fragment of tetanus toxin (TTc) from an injection site in the calf muscles to the spinal cord. Changes in migration patterns compared with baseline and controls allowed assessment of radiation-induced alterations in the retrograde neuronal axonal transport mechanism. Subsequently, tissues were harvested and histological examination of the spinal cords performed.

RESULTS

Transport of TTc in the thoracic spinal cord was impaired in a dose-dependent manner. Transport was significantly decreased by 16 days in animals exposed to either 10 or 80 Gy, while animals exposed to 2 Gy were affected only minimally. Further, animals exposed to the highest dose also experienced significant weight loss by 9 days and developed posterior paralysis by 45 days. Marked histological changes including vacuolization, and white matter necrosis were observed in radiated cords after 30 days for mice exposed to 80 Gy.

CONCLUSION

Radiation of the spinal cord induces dose-dependent changes in retrograde axonal transport, which can be monitored by molecular imaging. This approach suggests a novel diagnostic modality to assess nerve injury and monitor therapeutic interventions.

Paralysis following stereotactic spinal irradiation in pigs suggests a tolerance constraint for single-session irradiation of the spinal nerve

BACKGROUND AND PURPOSE

Paralysis observed during a study of vertebral bone tolerance to single-session irradiation led to further study of the dose-related incidence of motor peripheral neuropathy.

MATERIALS AND METHODS

During a bone tolerance study, cervical spinal nerves of 15 minipigs received bilateral irradiation to levels C5-C8 distributed into three dose groups with mean maximum spinal nerve doses of 16.9 ± 0.3 Gy (n=5), 18.7 ± 0.5 Gy (n=5), and 24.3 ± 0.8 Gy (n=5). Changes developing in the gait of the group of pigs receiving a mean maximum dose of 24.3 Gy after 10-15 weeks led to the irradiation of two additional animals. They received mean maximum dose of 24.9 ± 0.2 Gy (n=2), targeted to the left spinal nerves of C5-C8. The followup period was one year. Histologic sections from spinal cords and available spinal nerves were evaluated. MR imaging was performed on pigs in the 24.9 Gy group.

RESULTS

No pig that received a maximum spinal nerve point dose ≤19.0 Gy experienced a change in gait while all pigs that received ≥24.1 Gy experienced paralysis. Extensive degeneration and fibrosis were observed in irradiated spinal nerves of the 24.9 Gy animals. All spinal cord sections were normal. Irradiated spinal nerve regions showed increased thickness and hypointensity on MR imaging.

CONCLUSION

The single-session tolerance dose of the cervical spinal nerves lies between 19.0 and 24.1 Gy for this model.

Stereotactic body radiation therapy in the re-irradiation situation--a review

Although locoregional relapse is frequent after definitive radiotherapy (RT) or multimodal treatments, re-irradiation is only performed in few patients even in palliative settings like e.g. vertebral metastasis. This is most due to concern about potentially severe complications, especially when large volumes are exposed to re-irradiation. With technological advancements in treatment planning the interest in re-irradiation as a local treatment approach has been reinforced. Recently, several studies reported re-irradiation for spinal metastases using SBRT with promising local and symptom control rates and simultaneously low rates of toxicity. These early data consistently indicate that SBRT is a safe and effective treatment modality in this clinical situation, where other treatment alternatives are rare. Similarly, good results have been shown for SBRT in the re-irradiation of head and neck tumors. Despite severe late adverse effects were reported in several studies, especially after single fraction doses >10 Gy, they appear less frequently compared to conventional radiotherapy. Few studies with small patient numbers have been published on SBRT re-irradiation for non-small cell lung cancer (NSCLC). Overall survival (OS) is limited by systemic progression and seems to depend particularly on patient selection. SBRT re-irradiation after primary SBRT should not be practiced in centrally located tumors due to high risk of severe toxicity. Only limited data is available for SBRT re-irradiation of pelvic tumors: feasibility and acceptable toxicity has been described, suggesting SBRT as a complementary treatment modality for local symptom control.

Spinal cord tolerance to single-session uniform irradiation in pigs: implications for a dose-volume effect

BACKGROUND AND PURPOSE

This study was performed to test the hypothesis that spinal cord radiosensitivity is significantly modified by uniform versus laterally non-uniform dose distributions.

MATERIALS AND METHODS

A uniform dose distribution was delivered to a 4.5-7.0 cm length of cervical spinal cord in 22 mature Yucatan minipigs for comparison with a companion study in which a laterally non-uniform dose was given [1]. Pigs were allocated into four dose groups with mean maximum spinal cord doses of 17.5 ± 0.1 Gy (n=7), 19.5 ± 0.2 Gy (n=6), 22.0 ± 0.1 Gy (n=5), and 24.1 ± 0.2 Gy (n=4). The study endpoint was motor neurologic deficit determined by a change in gait within one year. Spinal cord sections were stained with a Luxol fast blue/periodic acid Schiff combination.

RESULTS

Dose-response curves for uniform versus non-uniform spinal cord irradiation were nearly identical with ED(50)'s (95% confidence interval) of 20.2 Gy (19.1-25.8) and 20.0 Gy (18.3-21.7), respectively. No neurologic change was observed for either dose distribution when the maximum spinal cord dose was ≤ 17.8 Gy while all animals experienced deficits at doses ≥ 21.8 Gy.

CONCLUSION

No dose-volume effect was observed in pigs for the dose distributions studied and the endpoint of motor neurologic deficit; however, partial spinal cord irradiation resulted in less debilitating neurologic morbidity and histopathology.

Sparing the posterior surgical site when planning radiation therapy for thoracic metastatic spinal disease

BACKGROUND CONTEXT

Most patients with painful spinal metastases are sufficiently palliated by hypofractionated radiotherapy. However, a small group of patients will need surgical intervention to treat symptomatic spinal cord compression and/or gross mechanical instability. Irradiation of a (prospective) surgical area may lead to postsurgery complications, including wound dehiscence, infection, and chronic wound ulcers. Decreasing the radiation dose to the surgical area could reduce radiation-induced toxicity and associated surgical complications.

PURPOSE

To investigate an alternative radiation technique designed to lower the surgical area dose while delivering an adequate target dose and minimal off-target dose.

STUDY DESIGN

Comparison of radiation doses received by various anatomic structures after simulating irradiation with a routine posteroanterior single field (SF) technique and experimental multiple field (MF) technique in a setting of thoracic metastatic spinal disease.

METHODS

The computed tomography (CT) data from six previously treated patients with a total of 10 thoracic spinal metastases were used to plan four radiation schemes (SF8 Gy; SF20 Gy; MF8 Gy; and MF20 Gy). Discrete anatomic structures were defined on CT data, including a posterior surgical area, and after simulation the doses received were calculated and compared for the 8 Gy and 20 Gy techniques.

RESULTS

With the experimental MF technique, a clinically relevant dose could be delivered to the affected vertebra, whereas the dose received at the (prospective) surgical area could be significantly reduced compared with the SF technique. The dose received at the nontarget tissues fell below the threshold level for clinical relevance.

CONCLUSIONS

This radiation planning study showed the feasibility of sparing the surgical area while delivering an adequate dose to affected vertebrae in thoracic metastatic spinal disease.

Prospective evaluation of spinal cord and cauda equina dose constraints using cone beam computed tomography (CBCT) image guidance for spine radiosurgery

Cone beam computed tomography (CBCT) image guidance has become more widely used for spine radiosurgery delivery.Our center began a dedicated spine radiosurgery program that utilizes CBCT image guidance technology for target localization. This study prospectively evaluated the spinal cord and cauda equina doses received during single fraction spine radiosurgery treatments in order to determine a safety profile for this technique. Two hundred consecutive spine and paraspinal lesions were treated using the Elekta Synergy S 6-MV linear accelerator with a beam modulator and CBCT image guidance combined with a HexaPOD couch that allows patient positioning correction in 3 translational and 3 rotational directions. Lesion location included 33 cervical, 84 thoracic, 60 lumbar, and 23 sacral tumors. Tumor histologies included 164 malignant and 36 benign. One hundred thirty-three lesions (66%) had received prior conventional fractionated radiotherapy. Thirty-three lesions (16%) were intradural. Radiosurgery was used as a primary treatment modality in 60 cases (30%), for radiographic progression after prior conventional radiotherapy in 96 cases (48%), and adjuvant post-surgery therapy in 44 cases (22%). For each case, the maximum point dose to the spinal cord and/or cauda equina as well as the volume of those organs at risk receiving greater than 8, 10, and 12 Gy were recorded. No subacute or long term spinal cord or cauda equina toxicity occurred during the follow-up period (median 21 months). For cases at the level of the spinal cord (117 cases) the mean prescribed dose to the gross tumor volume (GTV) was 14 Gy (range 11-18 Gy). The GTV ranged from 0.37 to 100.24 cm³ (mean 26.1 cm³). The mean maximum point dose to the spinal cord was 10 Gy (range 4-12 Gy), and the mean spinal cord volumes (cm³) receiving greater than 8, 10, and 12 Gy were 0.76 (0-3.45), 0.05 (0-.42), and 0.0, respectively. For cases at the level of the cauda equina (83 cases) the mean prescribed dose to the GTV was 15 Gy (range 10-20 Gy). The GTV ranged from 0.19 to 491.6 cm³ (mean 73.8 cm³). The mean maximum point dose to the cauda equina was 11 Gy (range 5-14 Gy), and the mean cauda equina volumes (cm³) receiving greater than 8, 10, and 12 Gy were 1.4 (0-6.68), 0.26 (0-2.99), and 0.02 (0-.39), respectively. This study demonstrates that limiting the spinal cord dose to 10 Gy and the cauda equina dose to 11 Gy can be achieved with therapeutic GTV prescription doses for single fraction spine radiosurgery using CBCT guidance. Such dose constraints are associated with a safe clinical outcome.