Stereotactic radiosurgery for recurrent skull base polymorphous adenocarcinoma invading the cavernous sinus: illustrative case

Abstract

BAKGROUND

Polymorphous adenocarcinomas (PACs) are rare tumors arising from the salivary glands. Radical resection and postoperative radiotherapy are the mainstays of treatment. However, complete tumor resection is not always achievable when the tumor invades the skull base. Stereotactic radiosurgery (SRS) could be a less invasive alternative for treating skull base PACs.

OBSERVATIONS

A 70-year-old male with a history of surgery for a right palatine PAC presented with right visual impairment, diplopia, and ptosis. Imaging studies revealed tumor recurrence invading the right cavernous sinus (CS). SRS using a gamma knife was performed for this recurrence, prescribing a marginal dose of 18 Gy at a 50% isodose line. Five months after SRS, his symptoms were relieved, and the tumor was well-controlled for 55 months without any adverse events.

LESSONS

To the best of the authors’ knowledge, this is the world’s first case of recurrent skull base PAC invading the CS that was successfully treated with salvage SRS. Thus, SRS may be an applicable treatment option for skull base PACs.

Polymorphous adenocarcinomas (PACs) are rare tumors that predominantly arise from the minor salivary glands. Salivary cancers account for 5.9% of the annual incidence of head and neck cancers, and PACs comprise approximately 0.4%–2.5% of those.^1–4 Radical resection with negative margins is the primary treatment goal. Additionally, postoperative conventional radiotherapy is effective for recurrent tumors or those with positive surgical margins.^3,5,6 However, PACs can sometimes extend into or metastasize to the skull base and the surrounding vital neurovascular structures, making the optimal radical treatment more difficult and invasive.^7,8 Because of the rarity of skull base PACs, it is difficult to establish a robust consensus on the treatment strategy.

Stereotactic radiosurgery (SRS) is a confirmed, less invasive treatment that can deliver focused high-dose radiation in a single session. SRS provides 60%–90% local control rates for metastatic brain tumors and skull base tumors.^9–16 To date, no reports on SRS for skull base PACs have been published. Here, we present a case of recurrent PAC invading the cavernous sinus (CS), which was treated with SRS, resulting in excellent local tumor control and improvement of symptoms without any radiation-induced adverse effects (RAEs).

Illustrative Case

A 70-year-old male presented with swelling of the right palate. He underwent a transoral biopsy and was initially diagnosed with adenoid cystic carcinoma arising from the palatine minor salivary gland. The tumor had expanded to his right mandible, sternocleidomastoid muscle, and submandibular gland, and he was referred to our hospital. Otorhinolaryngologists performed a radical resection with a sufficiently wide margin around the tumor. Pathological findings in our institute showed that the uniform tumor cells with pale nuclei constituted multiple architectural patterns of polymorphous appearance (Fig. 1A), and immunostaining was negative for MYB and pan-TRK, leading to the diagnosis of PAC, not adenoid cystic carcinoma.

FIG. 1.

Pathological image (A) of the resected palatine tumor of a 70-year-old male showing uniform tumor cells with pale nuclei constituting multiple architectural patterns of polymorphous appearance, leading to a diagnosis of PAC. Hematoxylin and eosin, original magnification ×200. Gadolinium-enhanced T1-weighted (GdT1) MRI (B) 4 years after curative radical resection revealing a skull base tumor invading the CS with heterogeneous enhancement, which indicates focal recurrence of the PAC. Radiosurgical planning for a recurrent CS PAC. The yellow line indicates the 50% isodose line for the prescribed treatment dose of 18 Gy, and the green lines indicate dose lines of 20, 16, 12, and 10 Gy in the axial (C), coronal (D), and sagittal (E) planes. GdT1 MRI (F) 55 months after the SRS revealing no growth of the treated tumor.

Four years after the surgery, he demonstrated visual weakness, diplopia, and ptosis in his right eye. Magnetic resonance imaging (MRI) showed a heterogeneously enhanced tumor in his right palatine area, invading the CS and indicating focal recurrence of the PAC (tumor size: 19 × 30 × 30 mm, tumor volume: 10.7 mL; Fig. 1B), along with multiple metastases in his left cervical lymph nodes and his left lung. Conventional radiation therapy was planned for the neck and lung lesions, whereas radical resection via craniotomy was not considered ideal for the CS lesion because of possible tumor invasion into the internal carotid artery (ICA) and cranial nerves. Given the lesion’s possible malignancy, SRS was planned as a palliative treatment to control the tumor with minimal invasiveness and was performed using the Gamma Knife 4C (Elekta AB) with a prescription dose of 18 Gy to a 50% isodose line (Fig. 1C–E). The maximum and mean doses to the right optic nerve could be maintained at 10.8 and 3.9 Gy, respectively. Five months after SRS, visual weakness, ptosis, and diplopia significantly improved due to effective tumor shrinking. Thereafter, the cervical lymph nodes were resected, and the CS lesion had been well-controlled at 55 months after SRS (Fig. 1F).

Discussion

Observations

PACs, classically referred to as polymorphous low-grade adenocarcinomas, have been considered to have a good prognosis with a 10-year disease-specific survival rate of 94%–99%.^5,17 Reports on adverse prognostic factors in PACs, including extrapalatal lesions, hard palate, angiolymphatic metastasis, perineural invasion, bone invasion, necrosis, larger size, and papillary and pseudocribriform components, have been increasing recently.^5,17–19 Our patient was considered to be in the poor prognosis group because of invasion into the CS, ICA, and cranial nerves. Given the potentially poor prognosis, a less invasive palliative treatment with controlling tumor growth was deemed ideal.

Primary radical resection with adequate tumor margins could be a standard treatment to achieve excellent tumor control of PACs.⁵ However, radical resection is not always achievable for tumors invading large vessels and cranial nerves, as in this case.^20,21 Thus, in some cases, the treatment of PACs should not be limited to surgery alone. Other than resection, radiotherapy for PACs as postoperative adjuvant radiotherapy is appropriately used when resection is insufficient or when the tumor recurs after surgery.^2,3,5,6,22 Conventional radiotherapy, including photon radiotherapy, is effective for skull base malignancies but tends to have more RAEs than with SRS, such as brain necrosis.^2,3,5,23

In light of the above, SRS could be a reasonable treatment option for skull base PACs invading critical anatomical structures, providing high-dose focused radiation covering sufficient tumor margins. Previous reports of single-session SRS using various prescription doses of 15–20 Gy for other salivary gland malignant tumors involving the skull base have shown good local tumor control rates.^15,16,24 Moreover, SRS can be delivered with limited radiation effects to the surrounding anatomical structures and lower RAE risks for skull base tumors,^{12–16,24–26} which might be one of the applicable treatment options for skull base PACs, especially in elderly patients or other inoperable cases, as in our case. In the present case, SRS with a prescription dose of 18 Gy achieved excellent midterm tumor control without any RAEs for skull base PAC, although further studies with larger populations and longer follow-ups are needed to evaluate the potential role of SRS in the treatment of skull base PACs.

Lessons

We demonstrate the first reported case of recurrent skull base PAC invading the CS treated with SRS. SRS may be a less invasive alternative treatment when radical resection is unacceptable. Further investigations with a larger number of cases are required to determine the role of SRS in skull base PACs.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Umekawa, Rai, Shinya. Acquisition of data: Umekawa, Rai, Shinya, Katano, Kondo, Shinozaki-Ushiku. Analysis and interpretation of data: Umekawa, Rai, Shinya, Katano, Kondo, Shinozaki-Ushiku. Drafting the article: Umekawa, Rai. Critically revising the article: Umekawa, Rai, Shinya, Hasegawa, Katano, Kondo, Saito. Reviewed submitted version of manuscript: Umekawa, Shinya, Katano, Kondo, Shinozaki-Ushiku, Saito. Approved the final version of the manuscript on behalf of all authors: Umekawa. Administrative/technical/material support: Shinya. Study supervision: Umekawa, Shinya, Hasegawa, Saito.