Tumor Occupancy Ratio-An Imaging Characteristic Prognosticating the Surgical Outcome of Benign Intradural Extramedullary Spinal Cord Tumors

INTRODUCTION

Intradural extramedullary (IDEM) spinal cord tumors constitute approximately two thirds of spinal tumors.^1–3 The most common IDEM tumors are derived from nerve sheath cells covering the spinal nerve roots (schwannoma and neurofibroma) or meningeal cells located along the spinal cord surface (meningioma).³ The natural history of both of the common IDEM tumors were studied, and it was suggested that schwannoma has a varied growth pattern while meningioma has a constant growth pattern.⁴

Both common IDEM tumors (schwannoma and meningioma) have specific imaging features of different T1 and T2 intensity along with the homogeneous or heterogeneous contrast enhancement, but dural tail and extradural extension differentiate both of them, and one author also suggested that the longitudinal diameter of the schwannoma is higher than of the meningioma in IDEM tumors.^5,6 None of the studies have evaluated any radiological parameters other than location to establish their role in prognosticating the surgical outcome. Hence, in this paper, we present our single institution's surgical experience to prospectively analyze the effect of tumor occupancy ratio on the surgical outcomes of patients with IDEM tumors at 1-year follow-up.

MATERIAL AND METHODS

Preoperative Evaluation

Diagnostic evaluations included magnetic resonance imaging (MRI) (spine) with contrast demonstrating the tumor and the respective spinal column (sagittal and axial) location of the lesion. The axial location of the tumor was categorized as anterior, posterior, or lateral with respect to the spinal cord and was described to correlate with a clock face. Tumor position according to the clock was divided into anterior with 10 o'clock and 2 o'clock positions, while 4 o'clock to 8′o clock was defined as posterior, and the rest were considered as lateral.

The tumor occupancy ratio was also measured in axial MRI (spine) as described by Ahn et al⁷ and was done by the surgical team.

The tumor occupying ratio was measured as [(a + b)/(A + B)] × 100, where a is the transverse diameter of the tumor; b is the longitudinal diameter of the tumor; A is the transverse diameter of the intradural space; and B is the longitudinal diameter of the intradural space (Figure 1).

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Figure 1

Magnetic resonance imaging (MRI; dorsal spine) of a ventrally located dorsal tumor showing the method of measurement of tumor occupancy ratio on an axial T2-weighted MRI image.

Clinical disability was assessed by the gait disability scale and Frankel scale at preoperation, time of discharge, 3 months, and 12 months follow-up by meeting or telephone. We have used a newer disability score system named the gait disability score, which mainly describes the ambulatory status of the patient. The gait disability score is a simplified version of the Frankel score and may be very useful and easy for mentioning and monitoring the ambulatory status of the patient. The following are the specifications of the gait disability score: 1, nonambulatory; 2, ambulate with 2-men support/walker; 3, ambulate with moderate assistance/1-man support; 4, ambulate with minimal assistance/stick support; and 5, ambulate without support.

RESULTS

In this study, the age of all patients ranged from 16 to 75 years with a mean (standard deviation) of 42.6 (17.25) years. The patient population was 70.45% male, while only about one third (29.55%) were female. In the sagittal location of the tumor, involved spinal levels were thoracic (63.65%) followed by cervical (20.45%) and lumbar (15.90%) spine. In the axial location, tumors were distributed less in dorsal (6.82%) and ventral (13.64%), while the lateral (79.55%) location was the most common. Out of 44 patients, 28 (63.6%) had schwannoma, while 16 (36.63%) patients had meningioma. Demographic, pathological types, and the final outcome of the study population are shown in Table 1. No significant difference was found in the mean tumor occupancy ratio measured at different sagittal (P = .19) and axial locations (P = .92), and there was also no significant difference (P = .72) in mean tumor occupancy ratio in both tumor types (schwannoma and meningioma) (Table 2).

The tumor occupancy ratio is associated with the preoperative gait disability score, and our results showed that tumors with poor gait score (more disability) had a higher occupancy ratio, suggesting that it directly affects the disability of the patient and also shows a direct correlation (r = 0.66, P < .001) (Table 3 and Figures 2 and 3). When we compared tumors with <80 and >80 tumor occupancy ratios, we elucidated that the mean gait disability score changed more in tumors with a <80 tumor occupancy ratio and finally have a better outcome on the disability score at 1 year (P < .001) (Table 4 and Figure 4).

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Figure 2

Bar diagram showing the relationship between tumor occupancy ratio and preoperative gait disability score.

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Figure 3

Linear correlation between tumor occupancy ratio and gait disability score.

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Figure 4

Correlation between preoperative and postoperative gait disability score at a different tumor occupancy ratio.

According to the gait disability score, on presentation, most of the patients were at grade 4 (38.64%), followed by grade 2 (25%), grade 5 (18.18%), grade 1 (15.91%), and grade 3 (2.27%). There was deterioration in 2 cases (one in the purely ventral upper thoracic location, while the other was a ventrolateral high cervical tumor with extraforaminal extension) in the immediate postoperative period, and both had residual weakness at 1-year follow-up, indicating the role of location in predicting the outcome, although the number of ventral tumors is less predictive of significance. In the postoperative period, we have seen that within 7 days after surgery, approximately 60% of patients were walking, either with minimal support or independently (grade 4 and 5). At 12 months follow-up, 90.91% of patients had a gait disability score >2, showing significant improvement (Table 5 and Figure 5), while 81.82 % were ambulatory and 18.18% were nonambulatory as analyzed by the Frankel score (Table 6 and Figure 6).

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Figure 5

Changes in gait disability score in preoperative period and in follow-up at 12 months (significant is a >2 change in score).

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Figure 6

Final outcome of ambulatory status on the basis of Frankel score.

No immediate complications of cerebral spinal fluid leak, hematoma, or wound infection was noted in our study.

DISCUSSION

Imaging characteristics of IDEM tumors is usually limited to T1, T2 intensity and contrast enhancing changes, and very few studies have also measured the longitudinal and transverse diameter of the tumors and tumor occupancy ratio;^5–7 but none of the studies have ever tried to find the role of this feature in predicting the prognosis of surgical outcome in these patients. We have seen that tumor occupancy ratio not only correlated with preoperative disability of patients but also predicted the final outcome of these IDEM tumors. Patients with tumors with lower tumor occupancy ratios had low preoperative gait scores and recovered better than patients with tumors with high occupancy ratios, as shown in the results. Hence, we can conclude that the tumor occupancy ratio should be used as a routine parameter in the preoperative evaluation of IDEM tumors and can help prognosticate the outcome in such tumors.

Few studies have also shown that tumor location has a significant impact on surgical outcome.^1,6,8 In our case study, we have seen that ventrally located tumors trended toward the development of neurological deficit, as occurred in our 2 patients; but, because of the small number of ventral tumors in our study, it is not statistically significant. Two cases that have deterioration in our study have ventral location, one in the upper thoracic region and the other in the high cervical region with extradural extension. As we know, tumors located in the upper thoracic spine are more likely to develop postoperative neurological deficits because of anatomical limitations at these levels and insubstantial blood supply to that region.^1,6,9 The extradural extension may also have a propensity to cause root/spinal cord-related neurological deficits, and the chances of recurrence are also high in such cases, while cervical spinal schwannoma with extradural extension has more of a chance of recurrence than at any other location.^10–13

The surgical approach is determined primarily depending on the location of tumors in the spinal canal, although the standard posterior approaches through bilateral or unilateral laminectomy with or without unilateral medical facetectomy provide adequate exposure to safely remove the vast majority of these lesions.^8,14–16 Posterior fixation is not required in cases of unilateral facetectomy or part of rib removal, and we have not done it in any case even if we required multilevel laminectomy; but certain studies favored the minimally invasive approach in view of postoperative kyphosis.^6,12,17,18

The majority (90%) of clinical improvement was noted within 3 months of the operative intervention, while at 12 months of follow-up, there was significant improvement in both the gait disability score and Frankel score, as discussed in the results. As we know, complete surgical excision of IDEM tumors is possible with good preoperative planning according to the imaging characteristics and location after following the surgical principle. The complete excision of the tumor helps to prevent recurrence; hence, it is also a good predictor of clinical outcome and should be attempted.^{3,9,12,19–21}

Spine centers have started, as a routine practice, to do neurophysiological monitoring in most cases of spinal surgery across the globe, but still have not proven its utmost importance in IDEM tumors.^9,22 One benefit of intraoperative neuromonitoring is that it helps for intraoperative evaluation of nerve function and thus helps in excision of nonfunctional nerves.²²

Radiosurgery in intradural spinal tumors is evolving and seems to be an effective tool for patients who are not suitable for open surgery as well as with multiple lesions, recurrences, or tumor remnants after microsurgery.^11,23

Our study has some limitations because of the smaller study population and the short follow-up period of only 1 year. Recurrence was also not commented on because of the unavailability of postoperative MRI in all patients and the limited duration of the study. The larger study sample size and randomization of the study may help in further validating the role of tumor occupancy ratios for predicting the surgical outcome of IDEM tumors.

CONCLUSION

MRI findings should be undertaken in a standardized fashion to ensure that important details are not missed for a more accurate evaluation of the location of the tumor. Tumor occupancy ratio should be measured routinely as it is correlated with the early clinical presentation of the patient and also predicts the surgical outcome and thus can be used as an important prognostic factor.

The location of the tumor was important to understanding the nature and course of the disease in these tumors. We have found deterioration in 2 ventrally located tumors; hence, we should be careful when operating on ventrally located tumors and also be cautious in cases of tumors with extradural extension.