Exit dosage for 2-MVP x-rays.

Abstract

In the use of supervoltage x-rays, there are many instances in which accurate knowledge of the exit dose is more important to the radiation therapist than of the surface dose. It has been shown by the present authors (1) that the surface dose through the entrance portal is considerably lower than the peak dosage. In many cases, the dose to the skin covered by the exit port will exceed that delivered to the skin comprising the entrance port. It is therefore of importance to know the deviation of the exit dose from that read from depth-dose curves as measured in infinite phantoms. After traversing various thicknesses of absorbing material, the beam will contain increasing proportions of Compton scattered x-rays. Their contribution to the back-scatter will be considerably greater than that of the primary beam. This can lead to error when one uses a thimble chamber for such measurements. The results reported here are those obtained with a large field extrapolation chamber which has been previously described in this journal (1). To use exit dose measurements as given here, it is necessary to compare these with thimble-chamber measurements made in infinite phantoms. It is therefore of interest to determine the per cent depth dose obtained at various depths in an infinite phantom using each of these two types of ionization chambers. This was done for one field size (10 × 10 cm.). Experimental The extrapolation chamber and measuring technics used were identical with those described in our earlier paper, with a single exception. Instead of the thin film used as one of the electrodes in the studies described there, a polyethylene film having a thickness of 4.67 mg./cm.2 was employed throughout the experiment for all exit and depth dose measurements. Each point was determined by extrapolation to zero volume and therefore represents a minimum of four to five sets of experimental data. The depth-dose curve was obtained with Masonite having a density of 0.988 gm./c.c. Care was taken that no air spaces existed between sheets of Masonite or between the Masonite and the polyethylene film which acted as the chamber electrode. A stack of the same Masonite served as backing for the chamber. This backing was maintained at a height of 30 cm. For the exit dose measurements, a horizontal beam was used. The chamber and absorbing material were supported in such a way that all parts of the supporting structure were well out of the beam. The closest wall which could contribute scattered radiation to the chamber was at a distance of approximately 10 feet. This should be a negligible contribution to the total ionization recorded. The beam was filtered by 1.0 mm. of lead, producing a resultant h.v.l. of 7.6 mm. of lead. The target-surface distance was 100 cm. All beam sizes referred to were measured at the surface of entry into the air cavity.