Room: Exhibit Hall | Forum 9
Purpose: To evaluate the benefit of daily CBCT imaging for prostate radiotherapy based on retrospective target and organ doses based on observed CBCT imaging shifts.
Methods: A random sample of 20 patients receiving prostate radiotherapy were selected to analyze the dosimetric benefit of daily CBCT localization compared to daily KV orthogonal image matching with weekly CBCT. All selected patients were set up with daily 2D KV imaging with CBCT localization 2-5 times per week. The planned dose was used to represent the expected dose delivered with daily CBCT localization. The hypothetical dose without CBCT imaging was determined by spatially perturbing the treatment planning dose based on the recorded CBCT shifts. DVH metrics for target and organ doses were used to evaluate the dosimetric differences when using daily CBCT compared to daily KV localization with only weekly CBCT.
Results: The majority (>60%) of recorded CBCT shifts were within 1 mm (> 90% within 3 mm, mean absolute shift = 1.2 mm Â± 1.8 mm). Daily CBCT imaging showed minimal differences in target dose coverage (Î”Dmin<1% Â± 0.2% and D95<0.6% Â± 0.1% for CTV) or organ sparing (bladder Î”D15% < 1% Â± 0.1%, rectum Î”D1cc<1% Â± 0.3% for rectum) compared with daily KV localization.
Conclusion: There was no dosimetric benefit from daily CBCT, since the localization shifts were well within the CTV-PTV treatment planning margin (8-10 mm). Moreover, with shifts being randomly distributed over all directions, any dosimetric changes over a single fraction are potentially averaged out over all fractions. Considering the added treatment time (4 min Â± 5 min) and imaging dose (~2-3 cGy/fx), frequent CBCT imaging may only be useful to verify organ filling and anatomic changes.