Replacing Pretreatment Verification With In Vivo EPID Dosimetry for Prostate IMRT

Purpose: To investigate the feasibility of replacing pretreatment verification with in vivo electronic portal imaging device (EPID) dosimetry for prostate intensity-modulated radiotherapy (IMRT). Methods and Materials: Dose distributions were reconstructed from EPID images, inside a phantom (pretreatment) or the patient (five fractions in vivo ) for 75 IMRT prostate plans. Planned and EPID dose values were compared at the isocenter and in two dimensions using the γ index (3%/3 mm). The number of measured in vivo fractions required to achieve similar levels of agreement with the plan as pretreatment verification was determined. The time required to perform both methods was compared. Results: Planned and EPID isocenter dose values agreed, on average, within ±1% (1 SD) of the total plan for both pretreatment and in vivo verification. For two-dimensional field-by-field verification, an alert was raised for 10 pretreatment checks with clear but clinically irrelevant discrepancies. Multiple in vivo fractions were combined by assessing γ images consisting of median, minimum and low (intermediate) pixel values of one to five fractions. The “low” γ values of three fractions rendered similar results as pretreatment verification. Additional time for verification was ∼2.5 h per plan for pretreatment verification, and 15 min ± 10 min/fraction using in vivo dosimetry. Conclusions: In vivo EPID dosimetry is a viable alternative to pretreatment verification for prostate IMRT. For our patients, combining information from three fractions in vivo is the best way to distinguish systematic errors from non-clinically relevant discrepancies, save hours of quality assurance time per patient plan, and enable verification of the actual patient treatment.