[Air bubble effect during alloy cooling in shielding blocks radiotherapy]

Shaped fields are widely used in radiotherapy to protect critical organs and to avoid unnecessary normal tissue irradiation. The most common system for photon beam shaping consists in a low melting point alloy. We studied the air bubbles which can occur during alloy cooling with both new and remelted alloys and when different cooling techniques are chosen.

Forty cone samples (18 of remelted alloy and 22 of new alloy) were prepared to evaluate the frequence of air bubble recurrence, with reproducible geometric sizes (height = 70 mm, major base surface diameter = 60 mm, minor base surface diameter = 40 mm). Air bubble sizes and dose inhomogeneity were evaluated by reproducing 60Co radiograph of each sample (two orthogonal projections: 6 x 7 cm). The samples were cooled at a constant temperature, following three different modalities: high (25 degrees C), medium (5 degrees C), low (-20 degrees C) temperature. Owing to the small geometrical magnification, air bubble sizes were determined by measuring their surface on samples lateral projections, taking into account the sight detectable bubble edges.

Up to 300 mm2 lateral surface bubbles are always present in all castings. Casting inhomogeneities can produce a film-density inhomogeneity ranging from 9% to 40%. The spatial distribution of bubbles seems to be random.

Bubble recurrence is independent of both the metal alloy (repeatedly used castings) and the different block cooling modalities. The effect of air bubbles on the shielded areas dose inhomogeneity is generally of no relevant importance. However, these inhomogeneities can produce hot spots which must be taken into accurate consideration only in the particular treatments where critical small size organ dose sparing represents a basic issue (i.e. the shielding of eye lens).