Whole-limb irradiation of the lower calf using a six-field electron technique

The present work demonstrates utilization of electron beam irradiation for the treatment of Kaposi's sarcoma when the full circumference of the lower calf is involved, and when the deep lymphatics are negative for disease. The finite penetration of the electron beam spares deep tissue, preventing the edema associated with photon total limb irradiation. The number of fields with fall-off required to produce a uniform dose to a cylindrical anatomic structure was studied by calculating dose distributions resulting from two-, four-, and six-field techniques for a 5-MeV electron beam and a 9 cm diameter cylinder. The dosimetry and set-up for the six-field technique is demonstrated by a case study. Results show that a six-field electron technique produced a sufficiently uniform dose while remaining relatively easy to set up and use to deliver patient treatment. For the patient case study, dose distributions for the six-field technique showed that (1) the penetration of the 90% dose decreased from 1.5 cm for a single field to approximately 1.0 cm for a 5-MeV beam; (2) the surface dose increased from approximately 70% to 100%, (3) the dose around the circumference of the leg at the depth of 1 cm or less varied from approximately 90% to 120% of the prescribed dose; and (4) the prescribed dose was 2.5 times the maximum central-axis dose from a single field. The six-field treatment was relatively simple to apply and produced an acceptable dose distribution for treatment of Kaposi's sarcoma of the lower calf. This treatment should be applicable to other sites such as the thigh and arms and for other cutaneous diseases such as melanoma.