PHYSICAL ASPECTS OF TOTAL BODY IRRADIATION TECHNIQUE

Daniel Venencia, MS; Silvia Bustos, MS; Silvia Zunino, MD
Instituto de Radioterapia - Fundación Marie Curie. CEPROCOR

Purpose

The development of a total body irradiation (TBI) technique in order to provide a simple, repeatable, quick and comfortable set-up for the patient and to deliver the dose homogeneously in a short time per fraction.

Methods and Materials

Irradiation was bilateral with 10 MV photon horizontal beams using a Philips SL-15 linear accelerator, at a source-surface distance of 3.50m. The patient lay in a supine position on a double inclined-plan board, with his knees pulled up and his arms crossed on the abdomen. An acrylic screen 16 mm thick was placed at 15 cm from the patient?s surface to obtain the maximum dose. Compensators were used to make the patient lateral dimensions uniform. The stability, output and flatness of the linear accelerator were previously adjusted at the isocenter, with a prf of 200 MU/min and 400 MU/min. Dosimetric measurements were performed using Farmer NE2571, PTW 31003 and PTW 34001 ionization chambers to determine the performance variation with irradiation time, dose increase at the entrance surface with screen, percentage depth dose (DDP), dose profiles in air, dose profile at a depth of 10 cm, dose loss at exit surface, and absolute dosimetry. Thermoluminescence dosimeters (TLD) chips and rods, were used for in vivo dosimetry. Dosimetric evaluation with TLD was made measuring DDP, profiles in air and dose loss at the exit surface.

Results

No variations in the output with irradiation time were found. The acrylic screen produced the maximum dose on the surface. The quality index of the beam corrected by distance was 0.782. DDP values measured with different chambers agreed within 1%. DDP scan produced variations greater that 5 %, at a depth of 20 cm, as compared to the values measured by integration. Dose homogeneity was within ±10% and ±15%. No dose loss was found at the exit surface. The average dose rate selected at the patient median plane was 12 cGy/min (prf =200MU/min). Measurements using TLD agreed with the values obtained with the ionization chambers within 2%.

Conclusions

This TBI technique proved to be simple, repeatable and comfortable. Dose homogeneity was within the expected values.

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