Retrospective assessment of delivered proton dose in prostate cancer patients based on daily in-room CT imaging

Purpose:

Retrospective calculation of the delivered proton dose in prostate cancer patients based on a unique dataset of daily CT images.
Methods:
Inter-fractional motion in prostate cancer patients treated at our proton facility is counteracted by water-filled endorectal ballon and bladder filling protocol. Typical plans (XiO, Elekta Instruments AB, Stockholm) for 74 Gy(RBE) sequential boost treatment in 37 fractions include two series of opposing lateral double-scattered proton beams covering the respective iCTV. Stability of fiducial markers and anatomy were checked in 12 patients by daily scheduled in-room control CT (cCT) after immobilization and positioning according to bony anatomy utilizing orthogonal X-ray. In RayStation 4.6 (RaySearch Laboritories AB, Stockholm), all cCTs are delineated retrospectively and the treatment plans were recalculated on the planning CT and the registered cCTs. All fraction doses were accumulated on the planning CT after deformable registration. Parameters of delivered dose to iCTV (D_98%>95%, D_2% <107%), bladder (V_75Gy <15%, V_70Gy <25%, V_65Gy <30%), rectum (V_70Gy <10%, V_50Gy <40%) and femoral heads (V_50Gy <5%) are compared to those in the treatment plan. Intra-therapy variation is represented in DVH bands.
Results:
No alarming differences were observed between planned and retrospectively accumulated dose: iCTV constraints were met, except for one patient (D_98%=94.6% in non-boosted iCTV). Considered bladder and femoral head values were below the limits. Rectum V_70Gy was slightly exceeded (<11.3%) in two patients.
First intra-therapy variability analysis in 4 patients showed no time-dependent parameter drift, revealed strongest variability for bladder dose. In some fractions, iCTV coverage (D_98%) and rectum V_70Gy was missed.
Conclusion:
Double scattered proton plans are accurately delivered to prostate cancer patients due to fractionation effects and the applied precise positioning and immobilization protocols. As a result of rare interventions after daily 3D imaging of the first 12 patients, in-room CT frequency for prostate cancer patients was reduced. The presented study supports this decision.