Simultaneous measurement of in vivo and transit mid-plane doses with ionization chambers in gynecological malignancy patients undergoing three-dimensional conformal radiotherapy

Abstract

Purpose: The aim of this study is to estimate delivered radiation doses inside planning tumor volume (PTV) using the in vivo (mid-plane dose) measurement and transit measurement methods in gynecological malignancy patients undergoing three-dimensional conformal radiotherapy (3DCRT) using calibrated ionization chambers. Materials and Methods: Six patients with histopathologically proven carcinoma of the cervix or endometrium were planned with four-field 3DCRT to the pelvic site. Isocenter was at the geometric mid-plane of PTV with a dose prescription of 50 Gy in 25 fractions. Clinical mid-plane dose (Diso,Transit) estimates were done in one method (transit) using the FC-65 positioned at electronic portal imaging device level. In another method, a repeat computerized tomography scan was performed (at the 11thfraction) using CC-13 having a protective cap in the vaginal cavity for in vivo measurements (Din vivo). Simultaneous measurements were performed with the two chambers from the 11thfraction onward at least 3-4 times during the remaining course of treatment. Results: The agreement of mean doses from these two described methods and treatment planning system reference doses was in the range of -4.4 ± 1.1% (minimum) to -0.3 ± 2.0% (maximum) and -4.0 ± 1.7% (minimum) to 1.9 ± 2.4% for Dinvivoand Diso,Transit, respectively, which are an acceptable range of daily radiation dose delivery. Conclusion: The fundamental importance of this study lies in simultaneous validation of delivered dose in real time with two methods. A study in this small number of patients has given the confidence to apply transit measurements for quality assurance on a routine basis as an accepted clinical dosimetry for the selected patients.