72 THE NOSE AND PARANASAL SINUSES shadow, often on the floor of the maxillary sinus (Fig. 31), but multiple polypi may be found on the antral walls and roof. It may not be easy radio- graphically to distinguish a polypus from a cyst of the antrum. The cyst tends to be larger and a tooth may be seen within the opacity if the condition is one of dentigerous cyst. The bony walls of the affected sinus must be carefully inspected for erosion which might suggest the presence of a malignant tumour (Fig. 44, p. 91). On the whole, radiography of adult sinuses gives a true picture of the contained pathology. In children the films may be less accurate because the Fig. 31. Radiograph, of paranasal sinuses demonstrating a polypus in the right antrum. sinuses are in a state of growth and mucosal thickening may occur without significant pathology. Also, the child's nose may contain secretions which have not been blown clear so that air entry into the sinuses is impaired with a consequent loss of translucency on radiography. In cases of doubt in adults a radio-opaque oil may be introduced into the maxillary sinus at proof puncture when subsequent radiography will make the pathology more evident. When erosion of a bony wall of one of the sinuses is suspected tomography will clarify the situation (Fig. 45, p. 91). There may be an absence of the frontal sinus in one or both sides. This sinus is usually only seen radiologically after the age of 8 years by which age the cupola of the nasofrontal duct reaches the upper rim of the orbit. Arrest of growth of the sinus may occur in childhood but this does not denote any underlying pathology. Frontal sinuses are not always symmetrical as their development depends upon the absorption of cancellous bone between the outer and inner tables of the frontal bone. All varieties of asymmetry may occur without there being any pathology. A very shallow frontal sinus may simulate an opaque sinus on radiography in the occipitomental view but the lateral projection will generally disclose the fact that this opacity is apparent rather than real.