Fig. 12.1
Leksell Gamma Knife Perfexion (Elekta AB, Norcross, GA)
Developed by Lars Leksell in the mid-twentieth century, the Gamma Knife radiosurgical system utilizes a headframe apparatus (Fig. 12.2 ) which helps immobilize the head during treatment and also allows for fiducial markers during imaging to provide submillimeter stereotactic accuracy. Collimated beams of radiation from either 201 (models U, B, and C) or 192 (model Perfexion) cobalt-60 sources are used to produce highly conformal single fraction radiation dose plans. Automated coordinate adjustments allow for rapid stereotactic dose delivery. Advantages of the Gamma Knife system include the ability to utilize multiple isocenters of radiation (ranging from a single focus to more than 20) to conform the delivery field (Fig. 12.3). This is especially useful in JPs, which grow in a more infiltrative, irregular pattern through the jugular foramen or within the temporal bone. With the latest iteration of the Gamma Knife system, tumors with extension to the C2 vertebra can be treated. Modifications including the use of a relocatable frame or frameless techniques have been used for lower cervical lesions.
Fig. 12.2
Fiducial box utilized for pretreatment imaging (CT and MRI) prior to stereotactic radiosurgery
Fig. 12.3
Headframe assembly used for cranial stabilization during Gamma Knife treatment
Linear accelerator-based SRT systems utilize mini-multileaf collimators to focus the delivery of megavoltage X-rays. While systems in use during the 1990s required a headframe for stereotaxis, the CyberKnife system utilizes a linear accelerator attached to a robotic arm to deliver beams without the need for rigid skull fixation. Movements of the patient are detected by the system, and adjustments are made in subsequent incremental dose administrations, resulting in a negligible overall error. Continuous registration of the linear accelerator position and the patient allow for treatment of lesions below the cranial base without compromising stereotactic accuracy [13]. The CyberKnife utilizes fewer beams (roughly 100 versus 192 beams with the Gamma Knife Perfexion system), resulting in a less steep dose drop-off from the target volume [14]. The total cumulative dose is most commonly delivered in a hypofractionated manner (three to seven sessions) but can be performed in a single session. Results with each system are discussed later in this chapter.
Prior to the publication of series demonstrating successful tumor control, the typical candidates for radiotherapy were elderly, medically infirm, or those with very large tumors. With the discovery that long-term tumor control is possible with SRS or hypofractionated SRT, all patients are increasingly being offered these options as primary treatment. Certain methodological limitations of present series should be noted to interpret these data accurately. Accurate tumor size estimation and determining strict tumor control on serial posttreatment imaging is challenging in JP. Volumetric tumor measurements performed with formulae utilizing linear or radial measurements are of questionable value given the non-ellipsoid shape of most JPs. Though rarely used, tracing the area of tumor on each MRI slice and integrating the result likely achieve the greatest accuracy in estimating tumor volume and detecting changes within the resolution of the imaging device [15]. Even with this approach, variation in scanner type and slice thickness can introduce measurement error. Relative to the slow-growing nature of most JPs, the duration of follow-up in most large series may be too short to truly detect progression. Subsequent analyses of the same patients will be valuable to validate these estimates. Finally, akin to the vestibular schwannoma literature, understanding the natural history of untreated JP will more clearly delineate tumor control rates with stereotactic radiation. Since most centers employ upfront treatment of JP, it largely remains unknown what percentage of tumors do not grow or result in progressive cranial neuropathy if left untreated. Currently, very few studies from highly selected patient cohorts have reported the clinical course of observed JP [16, 17].
In this section, the treatment of JP with SRS and hypofractionated SRT is discussed, with an emphasis on the key clinical situations where this approach may be useful: primary treatment, treatment of residual tumor after subtotal resection, treatment of tumor recurrence, and treatment of catecholamine secreting tumors. In the interest of clarity and consistency with currently accepted radiosurgical practices, the studies presented met the following criteria: (1) radiation was delivered using contemporary dose and delivery parameters, (2) appropriate reporting of serial tumor volume assessment was available, and (3) results for patients treated with primary SRS or SRT were separable from those who underwent prior treatment.
Primary Treatment
The majority of patients with JP are candidates for radiosurgery or hypofractionated SRT. Patients with tumors causing brainstem compression or obstructive hydrocephalus should be treated with surgery, reserving SRS for treatment of residual or recurrent tumor. Very large tumors may not be appropriate candidates for radiosurgery as the dose required may place nearby normal structures at greater risk. Inherent limitations of the delivery device (i.e., caudal extension below the C2 vertebra in the case of Gamma Knife) must also be taken into consideration.
Consensus regarding treatment parameters has been reached over the past two decades. The recommended marginal tumor dose (prescribed most commonly to the 50% isodose line when the gamma unit is utilized) is 15–18 Gy, resulting in a maximum dose of 30–36 Gy. In one series of 44 patients followed for a median of 118 months, the likelihood of tumor regression was higher with a minimum marginal dose of 15 Gy [18]. In the Gamma Knife system, the number of isocenters does not correlate to tumor volume and instead depends on the shape and extent of the tumor. For example, tumors may insinuate into the mastoid or petrous apex while also extending intraluminally within the sigmoid sinus or into musculature near the jugular foramen. Single fraction therapy has not been found to be significantly different than multiple fraction therapy (assuming similar total dose) in terms of tumor control or acute toxicity [19]. Table 12.1 summarizes available data for SRS and SRT in the primary treatment of JP. Two meta-analyses published in 2011 provide pooled estimates of tumor control rates with SRS or hypofractionated SRT. While primary and salvage treatment was combined in some cases, the overall tumor control rates reported by each study group were 95 [20] and 97% [21] for 335 and 339 patients, respectively, with a mean follow-up of 71 months in the latter group.
Table 12.1
Results of primary radiosurgery abstracted from available case series
Authors (date) | Radiation source | Number of tumors | Average marginal dose (Gy) | Tumor control rate (%) | Follow-up duration | Definition of failure |
|---|---|---|---|---|---|---|
Chen et al. (2010) [36] | GK | 11 | 14.4, single fraction | 73 | Median 32 mos. | >15% growth |
Dobberpuhl et al. (2016) [31] | GK | 12 | 15.5, single fraction | 100 | Mean 28 mos. | Growth |
Foote et al. (2002) [46] | GK | 13 | 15, single fractiona | 100 | Median 37 mos.a | Growth |
Genc et al. (2010) [39] | GK | 7 | 15.4, single fraction | 10 | Median 37 mos. | Growth |
Gerosa et al. (2006) [47] | GK | 3 | 16.3, single fraction | 100 | Median 32 mos. | Growth |
Pollock (2004) [4] | GK | 19 | 14.9, single fractiona | 95 | Mean 44 mos.a | Growth |
Sharma et al. (2008) [48] | GK | 6 | 17.3, single fraction | 100 | Median 20 mos. | Not defined, but none progressed |
Sheehan et al. (2012) [30] | GK | 83 | 14.8a, single fraction | 86ª | Median 50.5 mos. | Growth |
Hurmuz et al. (2013) [49] | CyberKnife | 13a (one had previous surgery, unclear extent) | 25, median 5 fractions | 100 (13/13) | Median 39 mos. | Absence of progressive disease; regression as 20% decrease in volume |
Poznanovic et al. (2006) [41] | Novalis | 8 | 15, single fraction | 100 (8/8) | Median 18 mos. | Not defined, but none progressed |
Stereotactic Radiosurgery or Radiotherapy After Subtotal Resection
Subtotal resection of large JPs is often necessary to preserve cranial nerve function and avoid vascular injury. Since up to 70% of patients with JPs exhibit normal lower cranial nerve function at presentation [22], a shift toward less aggressive surgical resection has occurred, with the goal of preserving neurologic function for as long as possible. A recent series evaluating 12 patients, with a mean follow-up of 45 months, demonstrated that the growth of residual tumor is less likely if greater than 80% of the tumor is resected [23]. Further studies with longer follow-up and larger patient numbers will be required to confirm these promising, but preliminary results. Given that the majority of tumors are embolized preoperatively potentially limiting their growth potential, it is reasonable to observe the remaining tumor until symptoms or significant enlargement is observed. In many cases, particularly in older patients, treatment may be deferred indefinitely.
While large series supporting these assertions are unavailable, SRS has been shown to be a useful salvage option with good tumor control. In spite of tumor residua often being located medial to the jugular bulb (as this is considered the limit of safe tumor dissection with preservation of lower cranial nerve function), the incidence of new cranial neuropathy after salvage radiosurgery is low [24, 25]. For this reason, planned SRS after subtotal resection (rather than observation alone) is advisable in younger patients or those with tumors that may present a future risk to cranial nerves. Prior to treatment, allowing postsurgical inflammation to resolve is helpful to better distinguish residual tumor from normal tissue, although postoperative targeting is often challenging regardless of timing.
Radiation Treatment of Recurrent Jugular Paraganglioma
Recurrent JP presents a formidable challenge. Defined as residual disease exhibiting growth, these may represent more aggressive JP variants and may exhibit greater variability in presentation and response to treatment [26]. Merely determining the extent of tumor on MR imaging is challenging; even high-resolution modern techniques are often unable to resolve differences between fibrosis, inflammation, and tumor. Large, rapidly growing, or unusually infiltrative JPs do not necessarily exhibit cytologic evidence of aggressiveness in the way that other neoplasms often do [27]. Therefore, it is difficult to predict both the growth potential of residual disease and the response of a recurrence to radiotherapy. Nevertheless, radiosurgery has been shown to play a key role in these scenarios particularly when the morbidity of repeat surgical resection with the goal of complete tumor extirpation is unacceptable. The first is the treatment of a tumor recurrence typically discovered months to years after the initial surgical resection. The second is in an adjuvant form, where a subtotal resection of recurrent tumor is performed with planned postoperative radiosurgery. The majority of available series do not differentiate between these clinical situations due to the relative rarity of each, but on the whole, salvage radiosurgery offers excellent long-term tumor control. The results of SRS in this group are summarized in Table 12.2.
Table 12.2
Results of salvage radiosurgery abstracted from available case series
Authors (date) | Radiation source | Number of tumors | Average marginal dose (Gy) | Tumor control rate (%) | Follow up duration | Definition of failure |
|---|---|---|---|---|---|---|
Chen et al. (2010) [36] | GK | 4 | 15, single fraction | 100 | Median 70 mos. | >15% growth |
Foote et al. (2002) [46] | GK | 12 | 15, single fractiona | 100 | Median 37 mos.a | Growth |
Genc et al. (2010) [39] | GK | 12 | 15.6, single fraction | 92 | Median 47 mos. | Growth |
Gerosa et al. (2006) | GK
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