Evaluation the Interfractional dose Difference Using Cone-beam CT in Prostate Patients

Purposes: Stereotactic body radiation therapy (SBRT) is a method for the precise and efficient administration of hypofractionated radiotherapy. The aim of this study was to utilize the cone beam computed tomography (CBCT) scans acquired before treatment for dose reconstruction and hereby to assess target volume coverage during prostate SBRT.Materials and Methods: Three patients with early stage prostate cancer were included in this study. All patients had implanted two golden fiducial markers under transrectal ultrasound for image-guided localization. The rectum and bladder in-house preparation protocols were used in this study. Before each treatment, the patient was first set up with a pair of orthogonal kilovoltage (kV) portal images based on fiducial markers and subsequently adjusted to the treatment position with CBCT for soft tissue alignment. A CBCT image set acquired prior to each treatment for patient positioning was employed for the dose reconstruction calculation. The prescription dose was 37.5 Gy in 5 fractions. The criterion of plan is to satisfy at least the 95% of prescription dose to cover 95% of PTV (V95 greater than 95%).Results: All the patients can meet the V95 greater than 95% criterion if we corrected the patient position base on CBCT image. There was only 86% of all fractions can meet the V95 greater than 95% when the patient position was corrected with kV portal images. For bladder, 86.66% and 60% of all fractions could meet the criteria of V50 of bladder less than 50%, and V100 less than 5 c.c respectively. For rectum, 86.66%, 94%, 60%, and 60% of all fractions could meet the criteria of V50 of rectum less than 50%, V80 less than 20%, V90 less than 10%, and V100 less than 5% respectively. Furthermore, there are less than 5cc of rectum receiving 100% of dose for all cases.Conclusion: The correct dose delivered to the correct location is the most important goal of radiation therapy especially for SBRT. The tighter margin than other conventional treatment was applied to this study. Our results show the CBCT registration provides the opportunity to improve the dose delivery accuracy. Although the dose and volume variations of bladder and rectum still cannot be controlled well even if we used in house special protocols in this study. In conclusion, the margin apply to this study is sufficient to achieve the PTV coverage if applying CBCT base on soft tissue matching for patient position registration. We plan to expend our sample size to get more statistically significant results in near future.