Whole-body dose from tomotherapy delivery.

PURPOSE

To measure whole-body dose in tomotherapy of the head and neck region resulting from internal patient scatter and linear accelerator leakage.

METHODS AND MATERIALS

Treatments are performed using a commercial computer-controlled intensity modulated radiation therapy planning and delivery system (Peacock, NOMOS Corp.) and a 6-MV linear accelerator (Clinac 6/100, Varian Corp.). The patient dose outside the treatment field is measured in a water-equivalent phantom using thermoluminescent dosimetry. The whole-body dose components from internal scatter and leakage are separately determined. The use of fixed-portal leakage and scattered radiation measurements to estimate the whole-body dose from tomotherapy is evaluated.

RESULTS

The internally scattered dose is significant near the target, but becomes negligible relative to the leakage dose beyond 15 cm from the target. Dose at 10 cm from the target volume, due to internal scatter and leakage, is approximately 2.5% of the total target dose, reducing to 0.5% at 30 cm. The measured dose is relatively uniform throughout the phantom.

CONCLUSION

The whole-body dose equivalent from a tomotherapy treatment is greater than that from conventional radiation therapy. Further studies are required to assess the trade-off between improved dose distribution conformality and a possible slight increase in radiation-induced fatal malignancies. The accuracy of using fixed-portal leakage and scattered dose measurements to estimate the whole-body dose from tomotherapy treatments is adequate, if the appropriate fixed-portal field size equivalent is used.