End-to-end dosimetry audit for three-dimensional image-guided brachytherapy for cervical cancer

Background: End-to-end dosimetry audit for brachytherapy is challenging due to the steep dose gradient. However, it is an efficient method to detect unintended errors in actual clinical practice. Purpose: We aimed to develop an on -site end-to-end test phantom for three-dimensional image-guided brachytherapy (IGBT) for cervical cancer. Methods: The test phantom we developed consisted of a water tank with an applicator/detector holder. The holder was designed to accommodate the applicator and insert an ionization chamber (PinPoint; PTW, Freiburg, Germany) to measure the dose at point A. Imaging and reconstruction were performed in the same way as performed for a patient. The feasibility of our test phantom was assessed in two different hospitals using tandem and ovoid (made of either metal or carbon) applicators that the hospitals provided. Results: The measured and calculated doses at point A were compared for each applicator. We observed that the values obtained using metal applicators were consistently lower, on an average by -2.3%, than the calculated values, while those obtained using carbon applicators were comparable to the calculated values. This difference can be attributed to the attenuation of the dose by the metal applicators, resulting in a lower dose at point A. The majority of treatment planning system, including the one used in this study, do not account for the material of applicator. Conclusions: An end-to-end test phantom for IGBT was developed, tested, and applied in a dosimetry audit in hospitals and showed favorable results for evaluating the point A dose.