An Investigation of Dosimetric Consequences of the Anatomic Changes at First Fraction in VMAT of Prostate Cancer Cases using Deformable Planning CT and KVCBCT Image Registration.

BACKGROUND

Kilovoltage Cone Beam Computed Tomography (kVCBCT) is used for patient setup, monitoring the delivered dose, and adapting the treatment to changes in the patient's condition. Radiation therapy has recently shifted from image guidance to dose guidance, resulting in accurately calculating the daily dose, calculated by re-simulating CT-based treatment planning, to increase the precision of the actual treatment dosage. The use of kVCBCT instead of re-simulated CT can simplify the patient pathway and reduce potential therapeutic errors by eliminating the need for additional simulation.

OBJECTIVE

The present study aimed to assess the dosimetric effects of anatomical changes on prostate tumors using Deformation Image Registration (DIR) and kVCBCT.

MATERIAL AND METHODS

In this experimental study, eight patients with primary prostate cancer were treated with Volumetric Modulated Arc Therapy (VMAT), and kVCBCT images were obtained for each patient during the first treatment fraction. Both the planning CT (pCT) and kVCBCT images were imported into DIR software. The pCT was then deformed to the kVCBCT image and imported into a Treatment Planning System (TPS). A new contour was created on the deformed Computed Tomography (dCT) using Atlas-based Auto-segmentation (ABAS). Daily dCT plans were individually created based on the same planning principles using the new contours and also denoted dCTp1 through dCTp8. The outcomes of dose calculations were compared using Dose Volume Histograms (DVH), including mean Planning Target Volume (PTV) doses at the prescribed dose and dose volume limitations for the bladder and rectal wall.

RESULTS

The mean doses to the PTV in the eight dCT-based plans were the same as those in the pCT-based plans. However, the mean doses to organs at risk in the dCT plans were different from those in the pCT plans. The mean doses to the bladder were on average 4% lower than those in the pCT plans, while the mean doses to the rectum were on average 8% higher than those in the pCT plans.

CONCLUSION

The use of VMAT based on kilovoltage kVCBCT and Deformtion Image Registration (DIR) can lead to re-decreasing the dose to the bladder while increasing that to the rectum, with the same PTV dose coverage.