External radiation therapy boost to the vaginal vault: feasibility of intracavitary dosimetry using a commercial diode system.

PURPOSE

An overall check of the whole dosimetry procedure by intracavitary in vivo dosimetry, using n-type silicon diode dosimeter, was performed during 6-MV x-ray irradiation of the vaginal vault. The dose delivered to the isocenter by all treatment fields was evaluated.

METHODS AND MATERIALS

The diode dosimeter was calibrated against an ion chamber and tissue maximum ratio, field size factor, SSD factor, and temperature dependence studies were performed. Diode system accuracy, linearity, and reproducibility were also tested. Patients' dose data were collected and comparision was made with respect to treatment-planning dose calculations. Ten patients with cervical cancer and endometrial cancer were treated with surgery and irradiation. During the boost to the vaginal vault, a diode was inserted by an intravaginal device and the vaginal vault was the isocenter of the four fields. The field size generally was not larger than 10 x 10 cm2.

RESULTS

Diode-measured "tissue maximum ratio" agreed to within 1% with those measured with an ion chamber in field from 7 x 7 to 10 x 10 cm2. The diode also exhibited a temperature dependence of 0.1% degrees C(-1). For 10 patients treated with a 6-MV beam, the agreement with treatment-planning dose calculations was shown to be better than +/-4%.

CONCLUSION

The good accuracy and reproducibility of the diode system shows that determination of the dose at isocenter, for patients treated in the pelvic region, can be performed with n-type diodes accurately. On the other hand, in the vaginal vault boost, external-beam radiotherapy is delivered accurately and in vivo dosimetry is really not indicated.