Managing Inconsistent Bladder Volumes in the Prostate Cancer Patient Using Daily Online Adaptive RT: A Case Report.

Purpose: We report the use of online adaptive radiotherapy (OART) aiming to improve dosimetric parameters in the prostate cancer patient who had lower urinary tract symptoms that caused him not to adhere to the standard bladder filling protocol.

Methods and materials: The reference treatment plan for adaptive radiotherapy plan was generated for the pelvis and the solitary bony lesion using the Ethos treatment planning system. For each treatment session, high-quality iterative reconstructed cone beam CT (CBCT) images were acquired, and the system automatically generated an optimal adaptive plan after verification of contours. Image-guided RT (IGRT) plans were also created using the reference plan recalculated on the CBCT scan and were compared with adaptive plans.

Results: The reference bladder volume in the planning CT scan was 173 cc, and the mean bladder volume difference over the course was 25.4% ± 16.6%. The ART offered superior target coverage for PTV 70 Gy over online IGRT (V95: 90.5 ± 3.2 % Vs 97.3 ± 0.4%; p=0.000) and the bladder was also better spared from the high dose (V65 Gy: 17.9 ± 9.1% vs 14.8 ± 3.6%; p=0.03). However, the mean rectum V65 doses were very similar in both plans.

Conclusion: Managing the inconsistent bladder volume was feasible in the prostate cancer patient using the CBCT-guided OART and our analysis confirmed that adaptive plans offered better target coverage while sparing the bladder from high radiation doses in comparison to online IGRT plans.

Key words: radiotherapy, CBCT, online adaptive radiotherapy, image-guided RT.