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 dur ing 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 ch amber and tissue maximum ratio, field size factor,SSD factor, and temperatu re dependence studies were performed. Diode system accuracy, linearity, and reproducibility were also tested. Patients' dose data were collected and c omparision was made with respect to treatment-planning dose calculations. T en patients with cervical cancer and endometrial cancer were treated with s urgery 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 cm (2). Results: Diode-measured "tissue maximum ratio" agreed to within 1% with tho se measured with an ion chamber in field from 7 x 7 to 10 x 10 cm(2). The d iode also exhibited a temperature dependence of 0.1%degrees C-1. For 10 pat ients 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 p elvic region, can be performed with n-type diodes accurately. On the other hand, in the vaginal vault boost, external-beam radiotherapy is delivered a ccurately and in vivo dosimetry is really not indicated. (C) 1999 Elsevier Science Inc.