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Comparison of cone beam CT imaging protocols in image guided radiotherapy for prostate cancer H Ariyaratne 1,2, H Chesham 2, J Pettingell 2, K Sikora2, R Alonzi 1,2 1 Mount Vernon Cancer Centre, Northwood, United Kingdom 2 Cancer Partners UK, Elstree, United Kingdom Email: research@cancerpartnersuk.org

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To assess the effect of frequency of verification imaging on the dose delivered to target volume and organs at risk, during a course of image-guided radiotherapy (IGRT) for prostate cancer. Objective

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844 cone-beam CT (CBCT) verification images from 20 patients undergoing radical prostate radiotherapy were analysed. Patients received a dose of 74 Gy in 37 fractions using 7-field intensity-modulated radiotherapy (IMRT). During treatment, patients had daily online CBCT soft tissue match verification using the Elekta XVI system. CBCT images were imported into the Philips Pinnacle treatment planning system. Target volume and organs at risk were contoured manually on each CBCT image (Fig 1,2). A 7 mm margin in all directions was used around the clinical target volume (CTV) to obtain the planning target volume (PTV). Soft tissue match shifts were applied to each CBCT image. CBCT contours were superimposed on the planning CT scan. The same contours were used for comparison of a daily online schedule with a protocol of days 1-3 followed by weekly imaging. Online and offline weekly verification schedules were also compared. Methods

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Figures 1 and 2. Axial and sagittal views of contours on CBCT

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90% of patients had improvement in target volume coverage with daily online imaging in comparison to weekly online imaging, with statistically significant benefits in PTV and CTV dosimetric parameters (Table 1). Results

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80% of patients had a reduction in rectal dose with the daily imaging protocol. There was a 1.12 Gy reduction in mean rectal dose (Table 2), which would mitigate the concomitant dose to the pelvis from imaging. Results

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Daily online imaging was the best verification protocol with a median of 37 fractions (range 3437) achieving CTV coverage V95 > 99%, compared to 34 (range 30-37) and 33 (range 2537) fractions with weekly online and weekly offline protocols respectively (Fig. 3). Results p Figure 3. CTV coverage - Comparison of verification imaging protocols

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Parameter Pearson correlation coefficient r p Repeat imaging carried out in a daily verification protocol helps improve target coverage (Figs. 4-7). Patient illustrated below had 6 fractions with inadequate coverage using the day 1-3 then weekly online protocol, compared with 1 fraction for the daily online protocol. Figure 4,5. AP and lateral views of CTV (red) coverage by 95% isodose (blue) for fraction with simulated weekly online protocol Figure 6,7. AP and lateral views of CTV (red) coverage by 95% isodose (blue) for same fraction with daily online treatment protocol The magnitude of benefit of daily imaging for a patient could not be predicted by characteristics on the planning CT scan.

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Daily online CBCT verification improves CTV coverage and reduces dose to rectum during IGRT for prostate cancer. Conclusions

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