Physics contribution
Stereotactic radiosurgery XVI: Isodosimetric comparison of photon stereotactic radiosurgery techniques (gamma knife vs. micromultileaf collimator linear accelerator) for acoustic neuroma—and potential clinical importance

https://doi.org/10.1016/S0360-3016(03)01580-3Get rights and content

Abstract

Purpose

Two stereotactic photon radiation therapy methods are currently in practice for the treatment of acoustic neuroma. In the 1990s, our data and those of others demonstrated isodosimetric advantages for gamma knife technology over linear accelerator methodology. Since then, the introduction of micromultileaf collimator technology has improved the conformity of the linear accelerator method such that the isodosimetric differences between the two techniques have narrowed.

Methods and materials

In this study, modern gamma knife isodosimetry was compared to that of modern linac technology (conformal fixed fields and dynamic arcs) for the therapy of acoustic neuroma. This is an unusual target in that a special sensory nerve (holding the key to hearing preservation) frequently runs through the targeted volume, unlike the majority of other stereotactic radiation therapy targets. This was a single-dose prescription comparison; the perceived extra benefit of fractionation (a technique not routinely available to the gamma knife) was thereby abrogated.

Results

Although the gamma knife technique maintained a slight, but statistically significant, advantage with regard to dose conformity (p < 0.02) (at the debatable cost of a lower minimum target dose), the much higher internal dose gradient (high maximum dose to prescription dose [MD:PD] ratio) could be interpreted as a disadvantage with respect to hearing preservation, although advantageous with regard to tumor ablation. Of the two linac methods, the dynamic arc method gave a statistically significant advantage over the fixed-field method as regards conformity (p < 0.05), at the expense of a slightly higher brainstem dose (an average of 12.4 Gy, cf. 11.7 Gy for fixed fields), but this result was not statistically significant. No significant difference was seen in the MD:PD ratio for the two single-isocenter linac techniques.

Conclusions

Gamma knife methodology remains well validated, with very good isodosimetry, but when hearing preservation is important, the improving linac technologies will compete with the gamma knife for optimal therapy. In these circumstances, the minor differences in isodosimetry between the two techniques will become important.

Introduction

With an annual incidence of approximately 1 per 100,000 of the population, acoustic neuroma accounts for 8% of all new tumors presenting to a neurosurgical referral practice. Benign tumors arising almost invariably from the vestibular branch of the eighth nerve, the tumors present either as extracanalicular, cerebellopontine angle growths, intracanalicular growths, or a mixture. The growth rate may be slow with cross-dimensional measurements increasing by no more than 1 mm per year or even less. Surgical excision was standard treatment, but in recent years, an increasing number of clinical publications concerning focal radiation therapy have established this as a standard alternative. In a recent publication from Pittsburgh, 5-year results were reported on 190 patients treated by stereotactic radiation therapy utilizing the gamma knife (Elekta Instruments AB, Linköping, Sweden) (1). The actuarial 5-year clinical tumor control rate was 97%, and the rates for new facial weakness, facial numbness, and hearing preservation were 1.1%, 2.6%, and 71%, respectively (1). In a previous publication from the same institution, the authors had concluded that, when compared to microsurgery, stereotactic radiation therapy proved to be a very effective alternative that was less morbid in terms of posttherapy complications and less costly in terms of finance and requirement of in-patient hospital time (2). Not surprisingly, there is an increase in the number of patients electing for radiation therapy.

Patients with neurofibromatosis 2 and a bilateral predisposition to acoustic neuromata, or those sporadic cases with hearing impairment in the ear contralateral to the acoustic neuroma, provide a population of patients for whom hearing preservation is particularly important.

A perceived disadvantage of the gamma knife technology is that it is necessarily a single-session therapy, and the overlapping multiple “shots” create a high internal dose gradient; this last feature leads to “double dose hot spots” at various sites (unpredictable) within the target volume; within the internal auditory canal, this can lead to potential morbidity to the “innocent bystander” acoustic nerve 3, 4. However, when it comes to published long-term control of acoustic neuroma by focal radiation therapy, the best documented results come from gamma knife institutions 1, 5.

When consideration of hearing preservation is made, the comparison of the two different photon stereotactic radiation therapy methods becomes relevant. Early publications from our unit (6) and that of others (7) compared 1990s linear accelerator methodology to that of gamma knife and drew attention to the internal dose gradient and fractionation issues. With regard to conformity, it was clear that the gamma knife was superior, and the ability to accentuate further the “dose falloff” at one edge of the treatment volume was a desirable characteristic and an advantage of the gamma knife. However, since that time, the introduction of the micromultileaf collimator systems with fixed or dynamic arc methodology has considerably enhanced the conformity of linac-based stereotactic systems, and the old comparisons have become outdated. In this article, we compare modern linear accelerator isodosimetry for stereotactic radiation therapy of acoustic neuroma with gamma knife technology and discuss the clinical relevance of the differences between the two techniques.

Section snippets

Methods and materials

Seven consecutive acoustic neuroma patients were recruited for this study, to give a total of 8 target volumes (1 patient having bilateral neuromas). By chance, all the neuromas studied were right sided, except the bilateral case. This did not affect the results and subsequent conclusions of this study as the planning of left- or right-sided acoustic neuromas is symmetric about the patient midline. In this study, the bilateral acoustic neuromas were treated as if they were from different

Results

Dose–volume histograms of the target volume and the brainstem of Patient 6 for all three techniques are given in Fig. 2, Fig. 3. The numeric values derived from each patient's DVHs for the comparison parameters are then given in the following tables: Table 3 gives the conformity index, Table 4 gives the minimum dose delivered to the target as defined by the dose level that covers 99.5% of the target volume, Table 5 gives the maximum dose delivered to the brainstem in each case, and Table 6

Discussion and conclusions

The published data on radiosurgery treatment for acoustic neuroma most surely validate the treatment by the gamma knife 1, 2. With the excellent isodosimetric conformity of this technique as applied to the acoustic neuroma and the ablative assuredness of this technique, which “packs” a high dose within the targeted contours (exemplified in this article by a high MD:PD ratio or as a high internal dose gradient), gamma knife technology continues to offer a very good option for treatment. This

References (31)

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