Abstract
PURPOSE: Daily image-guided radiotherapy (IGRT) for prostate cancer reduces treatment-related toxicity. However, positional and anatomical variations can result in patient re-setup, increased treatment time, and additional imaging dose. A simple technique of two distinct imaging modalities was investigated: initially, an anteroposterior kilovoltage (AP-kV) planar image was acquired, followed by cone beam computed tomography (CBCT). To determine whether this dual imaging modality (DIM) can reduce repeat CBCTs with an AP-kV screening image compared to CBCT alone, i.e. a single imaging modality (SIM). METHODS: Patients were enrolled sequentially before and after the new departmental protocol for IGRT. Initially, all patients had a CBCT prior to treatment (SIM group) as usual. For the DIM group, AP-kV imaging was added to the first three fractions, and subsequent AP-kV imaging (on demand) for patients unable to maintain adequate bladder and bowel compliance. Sixty intact prostate or post-prostatectomy patients were included: 30 before the change in protocol (SIM group) and 30 after (DIM group). Bladder volume, rectal filling, and positioning errors were evaluated. RESULTS: In the SIM group, 30 patients underwent a total of 1116 CBCTs. In the DIM group, 30 patients had 156 AP-kV images and 1077 CBCTs. In the SIM group, 166 were repeat CBCTs with a median of 4 repeat CBCTs per patient. In DIM group, 132 were repeat CBCTs with a median of 3 repeat CBCTs per patient. We found a significant difference in incidence of repeat CBCTs due to suboptimal bladder filling (p = 0.028) and rectal gas (p = 0.045), indicating the number of repeat CBCTs was significantly lower in patients imaged with DIM. CONCLUSION: The DIM technique was found to be superior to the SIM technique, as it allowed the desired bladder volume, rectal condition, and patient positioning to be achieved prior to CBCT, reducing the need for repeat CBCTs.