Individualized chest wall compensating bolus for electron irradiation following mastectomy: an ultrasound approach

A-mode ultrasound is used in a procedure to construct individualized tissue compensating bolus for electron beam irradiation of the chest wall, where the thickness of tissues over the lung may vary by as much as 3 cm. Electron energies corresponding to the thickest tissues in the field would normally cause lung tissues beneath the thinner regions to receive the full tumor dose. The problem is made more serious by the fact that electron ranges in lung are 2-3 times greater than in muscle. We feel that some form of individualized compensation is necessary for patients with large variations in chest wall thickness within a given electron treatment field. The A-Scan procedure is particularly suited to deliniation of the pleura-lung interface because of the strong identifiable reflection from this discontinuity. In the first approach, a moldable gelatanous bolus material, mixed to transmit ultrasound at 5 MHz with a velocity equal to the speed of sound in muscle, is placed on the chest wall covering the entire field. The thickness of the compensating material is then reduced at each point in the field so that the total thickness (muscle plus compensator) indicated by the A-scan is everywhere the same as the chosen maximum treatment depth. Because the compensator has nearly the same electron stopping power as muscle, the compensated chest wall is now uniform in thickness over the entire field. In the second approach, we sacrifice the one-step advantages of using sonically transparent compensator material in order to obtain a more rugged and rapid setting compensator. Four patients have been treated with no evidence of pneumonitis. The more elegant combination of these two approaches awaits the development of rugged materials which are both quick setting and sonically transparent