Antrochoanal polyp (Killian's polyp) is a benign polypoid lesion originating from the maxillary sinus antrum mucosa, extending through the ostium into the nasal cavity and posteriorly toward the choana, typically presenting unilaterally. It accounts for approximately 4-6% of all nasal polyps and most commonly occurs in children and young adults. It classically demonstrates a three-component structure: an intramaxillary cystic component (mother cyst) within the maxillary sinus, a narrow pedicle traversing the ostium, and a polypoid mass expanding in the nasal cavity and nasopharynx. Unilateral nasal obstruction and posterior nasal drip are the most common presenting symptoms. Bilateral presentation is exceedingly rare; when bilateral antrochoanal polyps are encountered, cystic fibrosis and sinonasal polyposis should be excluded. Treatment requires endoscopic sinus surgery with complete excision including the cystic component; incomplete resection carries a high recurrence risk.
Age Range
5-40
Peak Age
20
Gender
Equal
Prevalence
Uncommon
Antrochoanal polyp begins with the progressive enlargement of a retention cyst in the maxillary sinus mucosa; the accumulation of mucosal fluid within the cyst generates hydrostatic pressure causing expansion toward the sinus ostium. As the cystic component traverses the ostium and reaches the low-pressure environment of the nasal cavity, it undergoes polypoid degeneration with prominent stromal edema and loose connective tissue accumulation in the intranasal portion. Histopathologically, the intramaxillary cyst component is lined by respiratory epithelium with subepithelial edema, scant inflammatory cells, and sparse vascular structures — this low vascularity translates to minimal enhancement on contrast-enhanced imaging. Unlike sinonasal polyposis, antrochoanal polyps typically demonstrate a neutrophilic or paucicellular inflammatory pattern rather than eosinophilic infiltration, with mechanical obstructive pathophysiology predominating over IgE-mediated allergic mechanisms. The growing polypoid component in the nasal cavity can exert mass effect on surrounding structures causing septal deviation, concha lateralization, and nasopharyngeal obstruction; however, bone destruction is not expected — this feature is a critical finding in differentiation from malignant lesions.
Demonstration of the three-component structure consisting of a cystic component within the maxillary sinus, a narrow pedicle traversing the natural ostium, and a polypoid mass expanding in the nasal cavity is considered pathognomonic for antrochoanal polyp. Demonstration of all three components together on coronal reformatted images confirms the diagnosis. The cystic component is of low density, the pedicle is of soft tissue density, and the intranasal component is typically of homogeneous soft tissue density. Identification of this three-component structure is the most reliable finding in differentiating the lesion from simple mucosal retention cyst, sinonasal polyposis, and other unilateral nasal masses.
Within the maxillary sinus, a well-defined, round or oval cystic component of low density (10-25 HU) is typically seen. This 'mother cyst' component sits with a broad base on the sinus floor or medial wall and shows homogeneous fluid density content. The cyst wall is thin and smooth, and wall calcification is not expected. The intramaxillary cyst typically fills part of the sinus lumen and may cause expansile remodeling of the sinus walls but does not produce bone destruction. The density of cyst contents may vary depending on protein concentration; high-protein cysts may demonstrate values between 25-40 HU.
Report Sentence
A well-defined, homogeneously hypodense (approximately ... HU) cystic lesion is noted in the inferior left/right maxillary sinus, with a broad base on the sinus floor; no destruction of the sinus walls is identified.
At the level of the maxillary sinus ostium (or accessory ostium), a narrow soft tissue density pedicle extending from the intrasinus cystic component into the nasal cavity is visualized. This pedicle is best evaluated on coronal and sagittal reformatted images and traverses through the widened ostium. Smooth expansion (remodeling) of the bony structure at the ostium level due to pressure may be seen, but irregular erosion or destruction is absent. Identification of the pedicle confirms the maxillary sinus origin of the lesion and is critically important for surgical planning. Passage through an accessory ostium is less common but has been reported.
Report Sentence
A narrow soft tissue density pedicle is identified traversing the natural ostium of the maxillary sinus into the nasal cavity, with smooth pressure-related widening at the ostium level; no bone destruction is present.
On contrast-enhanced CT, the antrochoanal polyp demonstrates minimal or no enhancement. The intramaxillary cystic component shows no enhancement and maintains fluid density. The polypoid component in the nasal cavity may show thin peripheral rim enhancement but no enhancement in the central area. This minimal enhancement pattern reflects the low vascularity of the lesion and is an important finding in differentiating from solid tumoral lesions (inverted papilloma, SCC). The homogeneously low density of the polyp internal structure on contrast-enhanced phases is attributed to prominent edema and sparse vascular network in the stroma.
Report Sentence
On contrast-enhanced series, the lesion shows no significant enhancement, with the intramaxillary cystic component maintaining fluid density; no solid enhancement is identified in the intranasal polypoid component except for thin peripheral rim enhancement.
On T2-weighted sequences, the antrochoanal polyp demonstrates markedly hyperintense signal, reflecting the high free water and edema content of the lesion. The intramaxillary cystic component shows homogeneously very bright T2 signal that may approach the intensity of CSF. The polypoid component in the nasal cavity is also hyperintense but may show slightly lower signal depending on the amount of stromal edema. T2 hyperintensity helps distinguish the antrochoanal polyp from avascular or hypovascular lesions but is not specific; mucosal retention cysts and simple cysts also show similar signal. However, solid tumors (inverted papilloma, SCC) typically show intermediate T2 signal, and this feature is used in differential diagnosis.
Report Sentence
On T2-weighted sequences, the lesion demonstrates markedly hyperintense signal, with the intramaxillary cystic component exhibiting high signal isointense to CSF.
On T1-weighted sequences, the antrochoanal polyp typically shows hypointense or intermediate signal. The intramaxillary cystic component demonstrates low T1 signal and is hypointense relative to muscle. In cystic components with high protein content, T1 signal may increase to intermediate levels. The polypoid component in the nasal cavity shows intermediate signal. T1-weighted sequences are useful for evaluating the anatomic relationships of the lesion and particularly the proximity to bony structures. Minimal peripheral enhancement may be seen on contrast-enhanced T1-weighted sequences; this finding parallels the enhancement pattern on CT.
Report Sentence
On T1-weighted sequences, the lesion demonstrates hypointense signal, with no significant solid enhancement identified on contrast-enhanced T1-weighted sequences.
On diffusion-weighted imaging (DWI), the antrochoanal polyp typically does not show restricted diffusion. High ADC (Apparent Diffusion Coefficient) values are observed on ADC maps (>1.5 × 10⁻³ mm²/s), indicating that the lesion content allows free water movement. The lesion may appear hyperintense on DWI due to T2 shine-through effect, but the absence of low signal on ADC maps confirms the lack of true diffusion restriction. This feature is important in differentiating from malignant lesions with high cellularity (SCC, lymphoma) because malignant tumors typically show low ADC values (<1.0 × 10⁻³ mm²/s). Proteinaceous/viscous content lesions such as fungal sinusitis may also show diffusion restriction, which should be considered in differential diagnosis.
Report Sentence
On diffusion-weighted imaging, the lesion does not show restricted diffusion, with high signal observed on the ADC map.
On endoscopic or transnasal ultrasonography, the antrochoanal polyp appears as a homogeneous hypoechoic or anechoic polypoid mass within the nasal cavity. The cystic component may be anechoic due to its fluid content and may show posterior acoustic enhancement. The polypoid component demonstrates a hypoechoic appearance related to its edema content. On Doppler ultrasonography, no significant vascularity is seen within the polyp; this feature is critically important in differentiating from vascular tumors (juvenile nasopharyngeal angiofibroma). Ultrasonography may be preferred for evaluating the lesion without radiation exposure, especially in pediatric patients, but is limited in assessing the deep sinus component.
Report Sentence
A homogeneous hypoechoic polypoid mass is noted in the nasal cavity, with no significant vascularity identified on Doppler examination.
Criteria
Most common form demonstrating typical three-component structure passing through the natural ostium of the maxillary sinus. Usually unilateral, seen in children and young adults.
Distinct Features
Passage through the natural ostium, intramaxillary cystic component may fill a large portion of the sinus volume, may extend posteriorly into the nasopharynx in the nasal cavity and expand at the choanal level.
Criteria
Variant passing through the accessory ostium (fontanelle) of the maxillary sinus. The accessory ostium is located posteroinferior to the natural ostium and is generally wider.
Distinct Features
Passage of the pedicle through the accessory ostium is demonstrated posterior to the natural ostium on coronal CT. Accurate identification of the pedicle route is critically important in surgical planning because incomplete excision leads to recurrence.
Criteria
Large antrochoanal polyp completely filling the nasopharynx and extending to the oropharynx. Usually seen in late-diagnosed cases and may cause obstructive sleep apnea or dysphagia.
Distinct Features
Mass completely obliterating the nasopharynx, may prolapse into the oral cavity, may cause bilateral nasal obstruction. On imaging, the maxillary sinus origin of the lesion should be carefully evaluated because it may be confused with a nasopharyngeal mass due to large size.
Criteria
Recurrent antrochoanal polyp after previous surgery. Incomplete excision (especially leaving behind the intramaxillary cystic component) is the most common cause, with recurrence rates reported between 7-25%.
Distinct Features
Development of new cystic component in postoperative sinus anatomy (wide antrostomy, partial uncinectomy). Comparison with preoperative images and review of surgical report is recommended. Recurrent lesions usually appear in the same location and with similar morphology as the primary lesion.
Distinguishing Feature
Retention cyst remains confined within the maxillary sinus and does not extend through the ostium into the nasal cavity. The cyst base sits broadly on the sinus wall with a dome-shaped convex upper surface and contains no pedicle/intranasal component.
Distinguishing Feature
Sinonasal polyposis is characterized by bilateral and multiple polyps; typically starts from the ethmoid sinuses and may involve all sinuses. Infundibular obliteration and bilateral ethmoid opacification are typical. Unlike antrochoanal polyp, it does not contain a unilateral isolated cystic sinus component.
Distinguishing Feature
Inverted papilloma demonstrates a typical 'cerebriform' (brain gyri-like) enhancement pattern on MRI with significant solid enhancement. It shows intermediate signal on T2 (different from the marked hyperintensity of antrochoanal polyp). Additionally, bone destruction/remodeling is more prominent and it typically originates from the lateral nasal wall.
Distinguishing Feature
Juvenile nasopharyngeal angiofibroma is a vascular tumor seen in adolescent males showing intense homogeneous enhancement on contrast-enhanced CT (markedly different from the minimal enhancement of antrochoanal polyp). Pterygopalatine fossa widening and sphenopalatine foramen involvement are characteristic. Requires angiography before biopsy — biopsy carries serious bleeding risk.
Distinguishing Feature
Mucocele is an expansile lesion that completely fills the sinus lumen as a result of chronic obstruction of the sinus ostium, pushing the sinus walls outward and thinning them (balloon effect). Unlike antrochoanal polyp, it does not extend through the ostium into the nasal cavity; conversely, the ostium is obstructed with accumulation within the sinus and prominent expansion/thinning of the sinus walls.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
Endoskopik sinüs cerrahisi sonrası 6-12 ay kontrol BT veya endoskopi ile takip önerilir; rekürrens riski %7-25 olup intramaksiller kistik komponentin tam çıkarılmasına bağlıdır.Antrochoanal polyp is a benign lesion that does not carry malignant transformation risk. However, treatment is required when symptomatic (nasal obstruction, sleep apnea, sinusitis). Complete excision via endoscopic sinus surgery (including the intramaxillary cystic component) is the standard treatment. Preoperative CT is necessary for surgical planning to evaluate the size of the cystic component, the route of the pedicle, and adjacent anatomic structures. Incomplete excision is the most common cause of recurrence. In the presence of bilateral antrochoanal polyps, cystic fibrosis should be screened.
Treatment of antrochoanal polyp is surgical (complete excision via FESS). Recurrence rate is low (10-25%) but increases with incomplete removal. No malignant transformation risk. Should be considered in the differential of unilateral nasal obstruction in young patients.