Purpose: To investigate whether Cone-Beam Computed Tomography (CBCT)-based adaptive planning improved dosimetry for patients receiving postoperative prostate stereotactic body radiotherapy (SBRT) over 5 fractions.
Methods: A retrospective cohort of 5 patients with prostate cancer receiving adjuvant or salvage radiotherapy to the prostatic fossa were included. With the Ethos emulator environment, initial treatment plans were created using baseline CT simulation images and prescribed to 32.50Gy in 5 fractions with a 3mm planned target volume (PTV) expansion (initial plan). Subsequently, 25 fractions were delivered by two separate
methods: (1) adaptation to the CBCT of the day (adapted plan) and (2) dose recalculation using the initial plan on the same CBCT (scheduled plan). Dosimetry for targets and organs at risk were recorded. Independent fractions were compared between the initial, adapted, and scheduled plans with paired-samples t-tests.
Results: Rectal dosimetry improved with CBCT-based adaptation (V25Gy 10.7 + 3.4 cc), when compared to both the initial (16.4 + 7.2 cc, p=0.001) and scheduled (16.4 + 8.3 cc, p<0.001) fractions, which appeared primarily secondary to changes in rectal distension from gas or stool. Bladder wall dosimetry slightly improved with adaptation (V25Gy 32.7 + 6.4 cc) when compared to the initial (35.5 + 33.6 cc, p=0.002) and scheduled (33.6 + 6.4 cc, p=0.024) fractions. PTV coverage was acceptable for all fractions: adapted (D95% = 100%), initial (100%), and scheduled (98.5% + 1%). Further, this adaptive planning process on Ethos is largely automated, taking an average of 12.9 + 1.7 minutes to generate a new adaptive plan from the patient’s daily CBCT, of which only 5 to 6.5 minutes were spent manually editing contours.
Conclusion: Overall, CBCT-based adaptation with Ethos provided superior plans with better rectal sparing whilst maintaining excellent target coverage. Ethos offers a fast and effective method to generate adaptive plans for postoperative prostate SBRT.