Quality assurance and dosimetric evaluation for an endocavitary unit

Abstract

The use of endocavitary contact therapy for selected rectal carcinomas continues to be an effective treatment option. Very small volumes are treated with an extremely high dose rate associated with rapid fall-off in depth and an overall high dose. The clinical benefits of the high dose rate leads to dosimetric and quality-assurance challenges. In addition, the operating room environment creates concerns in terms of dosimetry and radiation safety due to varying room line voltages and uncontrolled environments. The unit at our facility delivers 50 kVp X-rays using an SSD of 35 mm with lead-lined procotscopes of 24 and 29 mm. The dose rate is approximately 10 Gy per minute. Establishment of daily, monthly, and annual quality assurance reviews have been made and assessed. Parallel-plate ionization chambers measure outputs and relative depth dose with different phantoms. Silver bromide film is used to evaluate beam profiles. An electron diode system was chosen for day-of-treatment output checks. An ion chamber survey meter measures scatter and leakage exposure rates. Day-of-treatment output checks have assured output stability in various operating rooms. Trends in output have been confirmed by monthly ion chamber checks. Percent depth dose measurements carried out in liquid water compared well with accepted published data as did corrected polystyrene measurements. Radiation survey measurements detected acceptable exposure rate levels. The established comprehensive quality assurance program incorporates cross checking with multiple dosimetry systems. The confidence level of dose delivery has increased with the introduction of a day-of-treatment output checks. Polystyrene for depth dose data has been confirmed to be an accurate medium. Radiation exposure rates were found to be within safe limits for the uncontrolled environment of the operating room.