This study assessed different cone beam CT (CBCT) imaging protocols for image-guided radiotherapy in prostate cancer patients. 844 CBCT images from 20 patients undergoing radiotherapy were analyzed. Daily online CBCT verification provided the best target coverage and lowest rectal dose compared to weekly online or offline protocols. Daily imaging helped improve target coverage, with one patient having inadequate coverage for 6 fractions with a weekly protocol versus just 1 fraction with daily imaging. The study concludes that daily online CBCT verification improves target coverage and reduces rectal dose for prostate cancer IGRT.

1. Comparison of cone beam CT imaging
protocols in image guided radiotherapy
for prostate cancer
H Ariyaratne1,2, H Chesham2, J Pettingell2, K Sikora2, R Alonzi1,2
1Mount Vernon Cancer Centre, Northwood, United Kingdom
2Cancer Partners UK, Elstree, United Kingdom
W: genesiscare.co.uk E: research@genesiscare.co.uk
Cancer Partners UK became Genesis Care on 8 January 2016.
This research was previously conducted by Cancer Partners UK.

2. 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

3. • 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.
Materials and methods

4. Figures 1 and 2 Axial and sagittal views of contours on CBCT
Purpose/Objective

5. Results
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).

6. Results
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.

7. Results
Daily online imaging was the best verification protocol with a median of 37 fractions (range 34–37)
achieving CTV coverage V95 > 99%, compared to 34 (range 30-37) and 33 (range 25–37)
fractions with weekly online and weekly offline protocols respectively (Fig. 3).
Figure 3 CTV coverage
Comparison of verification
imaging protocols

8. Results
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.

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

10. Thank you
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