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CBCT-guided online adaptive radiotherapy has been introduced as a standard treatment for all our prostate cancer patients and is supervised by either a radiation-oncologist (first four fractions) or an advanced adapter. To this end, a group of RTT’s was trained to contour male pelvic organs on planning-CTs. The objective of the study was to evaluate our training methodology by analysing differences in contours defined by the advanced adapter compared to the approved contours by the radiation-oncologist. 

In this retrospective study, planning CTs of thirty prostate cancer patients were included. All approved contours relevant for adaptive radiotherapy (prostate, seminal vesicles, rectum and bladder) were transferred to Eclipse (Varian Medical Systems, Palo Alto, CA) and subsequently masked. Six advanced adapters re-contoured these organs. Resulting contours were compared with the approved contours. In addition, Dice similarity coefficient (DSC), volume changes and Center of Mass (CoM) were calculated with respect to the approved contours. 

The DSC was 0.96 ± 0.01 (bladder), 0.87 ± 0.06 (rectum), 0.83 ± 0.06 (prostate) and 0.73 ± 0.10 (seminal vesicles). For rectum, the main difference was seen in the most cranial and caudal part of the rectum. When only the part of the rectum adjacent to the PTV was measured, the DSC improved (0.92 ±0.03) and the difference in volume diminished (Table 1). For the seminal vesicles DSC was the lowest of all structures. Since the seminal vesicles often had a small volume (10.07 ± 4.78 cm³), a small deviation resulted in a lower DSC.

The organs contoured by the advanced adapters and the radiation oncologists were shown to be in excellent (bladder and rectum) or good (prostate and seminal vesicles) agreement. This study demonstrates that transition of this work from radiation oncologist to RTT is qualitatively feasible. Furthermore it facilitates a more efficient distribution of the workload in the department.