CyberKnife stereotactic radiosurgery for extramedullary plasmacytoma in the external auditory canal: illustrative case

Abstract

BACKGROUND

Plasmacytoma, a rare plasma cell disorder, often presents as a solitary or multiple tumors within the bone marrow or soft tissues, typically associated with multiple myeloma. Extramedullary plasmacytomas (EMPs), particularly those located in the external auditory canal (EAC), are exceedingly rare and pose significant treatment challenges given their location, anatomical complexity, and high risk of recurrence.

OBSERVATIONS

The authors report the case of a 72-year-old male with a history of multiple myeloma, presenting with recurrent left EAC plasmacytoma. After initial conventional radiotherapy for the lesion, a recurrence was documented in 7 years. The patient subsequently underwent stereotactic radiosurgery, which proved successful, leading to complete resolution of the lesion without any long-term adverse effects or radiation-related complications over a 45-month period.

LESSONS

This case is a unique instance of utilizing stereotactic radiosurgery for recurrent EMP in the EAC, highlighting its potential as an effective approach in managing complex plasmacytomas.

Plasmacytoma, a relatively rare plasma cell disorder, is characterized by the growth of a solitary or multiple masses in either bone marrow or soft tissues.¹ Although plasmacytomas are commonly associated with multiple myeloma (MM), the most prevalent plasma cell neoplasm, there are limited instances in which plasmacytomas can manifest as solitary lesions, known as “solitary plasmacytomas.” These are classified into two types: solitary plasmacytoma of bone and extramedullary plasmacytoma (EMP). Although 80% of EMPs occur within the head and neck regions, they constitute less than 1% of all head and neck tumors, emphasizing their rarity.^2,3 Plasmacytomas presenting in the external auditory canal (EAC) are exceptionally rare, estimated to account for approximately 1% of all plasmacytomas.²

The management of EMP, particular in cases where the lesion presents in the EAC, requires careful consideration of treatment modalities. Conventional approaches predominantly involve radiotherapy and surgery.^4,5 Stereotactic radiosurgery (SRS) offers targeted radiation with higher precision, which can be advantageous for lesion treatments in complex regions or previously irradiated fields.⁶ Here, we report a case with a history of MM and recurrent EMP in the EAC.

Illustrative Case

A 72-year-old male had an extensive history of MM treated with radiotherapy, including a sacral lesion (early 1990s), an external auditory canal lesion (21 years later), as well as surgical debulking and CyberKnife (Accuray Inc.) treatment (12 Gy in 1 fraction, 21 years after sacral lesion treatment) for a recurrent, previously irradiated, right occipital plasmacytoma. Furthermore, an intradural extramedullary contrast-enhancing mass posterior to the spinal cord at the T7 level was identified and was currently under observation. He then presented to the ear, nose, and throat clinic with recurring left-ear plugging. Otoscopic examination revealed a soft, broad-based pink mass arising from the posterior bony ear canal. This was confirmed through resection of a bony lesion and subsequent pathological analysis. Magnetic resonance imaging (MRI) demonstrated a left EAC with a defined enhancing lesion (Fig. 1). Histological analysis indicated morphological and immunophenotypic features most consistent with involvement by recurrent plasmacytoma (CD138+, MUM1+, PAX5−, CD79A−, CK5/6−).

FIG. 1.

CyberKnife radiosurgery plan for the EAC lesion from axial (left) and coronal (right) views. The target volume was 0.20 mL. A marginal dose of 18 Gy was prescribed to the 78% isodose line in 3 fractions. The maximum dose administered was 23.08 Gy. The contour of the target is represented by the red line, whereas the green line delineates the prescribed 78% isodose line at 18 Gy. The conformity index (CI) was 1.34. The biological effective dose (BED) was determined to be 28.8 Gy, with a single fraction equivalent dose (SFED) of 12.69 Gy. The left cochlea is contoured by a pink line, and the 5-Gy isodose line is marked by a blue line, which shows that the cochlea is located at a distance from the 5-Gy isodose line and receives less than 5 Gy.

A preoperative audiology assessment, including audiometry, was conducted, revealing symmetric sensorineural hearing loss in the patient’s ears. Notably, the patient reported no associated symptoms.

Treatment options were discussed, and the patient opted for CyberKnife radiosurgery. Based on the Radiation Therapy Oncology Group protocol 90–05 criteria, the patient received a prescribed dosage of 18 Gy in three fractions due to proximity to risk of organs, the tumor’s size and metastatic nature, and prior radiotherapy (Fig. 1).⁷ Dosimetrically, the right cochlea was exposed to a mean dose of 2.39 Gy and maximum dose of 4.38 Gy based on the general dose-volume constraints guideline for organs at risk.⁸ The volume of the right cochlea receiving more than 17.1 Gy, based on the three-fraction dose-constraints guideline, was 0 mL.⁹

Subsequent follow-up MRI at the 3-, 6-, 12-, and 24-month intervals showed a progressive reduction in the size of the lesion, and the latest follow-up at 45 months after treatment demonstrated complete resolution of the lesion. The lesion is no longer visible on MRI (Fig. 2). Importantly, during this period, the patient did not report any secondary effects such as progressive hearing loss, and there were no discernible adverse radiation effects observed on the MRI scans. At the last follow-up, approximately 4 years after treatment, the patient’s hearing was stable.

FIG. 2.

Axial (left) and coronal (right) views of the left EAC plasmacytoma after radiosurgery treatment at the 45-month follow-up. White arrows indicate the treated area, and the lesion is no longer visible on MRI.

Patient Informed Consent

The necessary patient informed consent was obtained in this study.

Discussion

Observations

We present the case of an extramedullary left EAC plasmacytoma treated with CyberKnife radiosurgery. To our knowledge, this is the first reported case of a recurrent plasmacytoma lesion treated with SRS. Follow-up examinations demonstrated complete resolution of the tumor and no secondary effects, such as progressive hearing loss.

EAC involvement by EMPs is an exceedingly uncommon occurrence, with just a few cases reported in the medical literature. Given conventional methods of resection or radiotherapy, the likelihood of recurrence or systemic progression through localized therapy is about 10% in individuals with a solitary plasmacytoma.¹⁰ SRS offers potentially improved outcomes via a higher precision of radiation delivery, possibly reducing recurrence and minimizing radiation exposure.¹¹ This precision may translate to fewer side effects and improved function preservation. SRS has also demonstrated higher efficacy in local tumor control in several cancer subtypes, which might suggest a similar benefit in EMPs, although direct EMP comparisons are currently limited.¹² Our patient had a substantial history of plasmacytomas dating back almost 3 decades prior to the diagnosis of this lesion. The tumor we managed was a recurrence of a plasmacytoma that had been previously addressed with conventional radiotherapy 7 years prior. Comprehensive monitoring protocols and preventative strategies are paramount to the treatment of recurrent EMPs. Regular imaging and physical examinations can help to detect recurrences early, along with patient education on symptom awareness to improve early intervention.

This case provides further support that SRS is an effective therapeutic option after failed radiotherapy for plasmacytomas. Theiler et al. previously reported a patient with locally relapsed plasmacytoma involving the third thoracic vertebra following initial radiotherapy. Further examination revealed no significant neurotoxicity.⁸ Our case adds to this conclusion, indicating that SRS can treat recurrent EMPs. Moreover, this case also adds to the growing body of literature for CyberKnife’s therapeutic potential in the plasmacytoma domain broadly.^13–15

In the management of this case, a comprehensive multidisciplinary approach, including oncology, radiology, and otolaryngology, was critical for patient care. The oncology team played a crucial role in overall medical management and coordination, determining suitable therapeutic strategies based on the patient’s previous medical history and current health status. Radiology experts were instrumental in precise diagnostic imaging and guiding targeted therapies. Otolaryngology specialists provided critical input in evaluating cancerous effects on the auditory system. This collaborative approach ensured holistic treatment to maximize the efficacy of treatment and minimize complications.

Given the complete radiological resolution of the lesion, posttreatment clinical outcomes, and the absence of complications, CyberKnife radiosurgery appears to be an effective therapeutic option for the treatment of EAC and skull-based plasmacytomas. Future investigations of CyberKnife radiosurgery with larger cohort studies for EMP are needed to establish more literature for CyberKnife’s treatment potential.

Lessons

This case demonstrates the safety and efficacy of CyberKnife radiosurgery in treating recurrent EMPs, particularly in complex anatomical regions like the EAC. SRS emerges as a promising option for recurrent plasmacytomas and warrants further exploration. Additionally, this case highlights the necessity of long-term monitoring for potential recurrences in patients with MM.

Author Contributions

Conception and design: Park, Shah, Marianayagam, Veeravagu, Chang. Acquisition of data: Park, Yuan, Shah, Marianayagam. Analysis and interpretation of data: Patil, Shaghaghian, Yuan, Shah, Marianayagam, Veeravagu. Drafting the article: Patil, Yuan, Shah, Marianayagam, Veeravagu. Critically revising the article: Park, Shah, Marianayagam, Soltys, Veeravagu, Gibbs, Li, Chang. Reviewed submitted version of manuscript: Park, Shah, Marianayagam, Soltys, Veeravagu, Gibbs, Li, Chang. Approved the final version of the manuscript on behalf of all authors: Park. Statistical analysis: Patil, Yuan. Administrative/technical/material support: Park, Shah, Veeravagu. Study supervision: Park, Veeravagu, Li, Chang. Patient care and treatment: Li.