Introduction: In the era of dose escalation for localised prostate cancer, the dose-volume histogram (DVH) is integral to the assessment of rectum and bladder dose constraints. However, reliance on a single planning computerised tomography-based (P-CT) dose distribution may not account for variations in delivered dose that results from deformation of the prostate, bladder and rectum. This study uses conebeam CT (CBCT) datasets from five patients to investigate the concordance between the dose prediction from the initial treatment plan and the dose delivered during treatment.Methods: The intensity-modulated radiation therapy distribution used for treatment was superimposed on alternate day CBCT images for each patient. Dose metrics and absolute volumes for the prostate, rectum and bladder were extracted from the CBCT-based DVH. Differences in dose and volumes were compared with the P-CT values, and significance was tested using the Wilcoxon signed-rank test.Results: For all five case studies, the prostate dose coverage on CBCT plans was lower than predicted with an average reduction of 3% in mean dose. Significant differences in rectal volumes and dose were observed in two out of five and four out of five patients, respectively. Reductions in bladder volume and subsequent increases in dose were observed for three out of five patients. Conclusion:The DVH from P-CT was unable to consistently predict the dose delivered to the bladder and rectum. The current bowel and bladder preparation protocols used at our institution did not eliminate variation in bladder and rectum volumes for the five patients included in this study.
322 Background: Stereotactic body radiotherapy (SBRT) has an emerging role for patients with hepatocellular (HCC). The purpose of this study is to describe the efficacy of HCC SBRT at a population level without any tumor size restriction. Methods: A retrospective study of the first 49 HCC patients treated with SBRT between March 2011-July 2015 at the British Columbia Cancer Agency was conducted. All patients were either ineligible for or failed standard local therapies (partial hepatectomy, radiofrequency ablation, percutaneous ethanol injection, transarterial chemoembolization, or radioembolization) and discussed in a multidisciplinary rounds setting. Local control (LC), progression free survival (PFS) (defined as freedom from failure elsewhere in the liver) and overall survival (OS) were analyzed at 1 and 2 years. Changes to Child’s Pugh (CP) score at 3 months post-SBRT were also analyzed. Results: Median follow-up was 14 months with 35 patients (71%) alive at last follow-up. Fifty-two separate HCC lesions were treated with a median size of 4.2 cm (range: 1.3-15.6 cm) and a planning treatment volume (PTV) of 114 cm3 (range: 22-1776 cm3). Over half of the lesions (55%) were ≥ 4cm. Thirty-six patients (74%) had previous local therapies. Prior to SBRT, 57% of patients were CP A5, 33% were CP B6, 8% were CP B7, and 1 patient (2%) was CP B8. At 3 months post-SBRT, 35% of patients had an increased CP score, with a mean increase of 1.8 points. The most common dose and fractionation was 45Gy in 3 or 5 fractions. Median V90 (dose to 90% of the PTV) was 99.8% (range: 74.1-100%). LC for all patients was 96% at 1 and 2 years. LC was comparable between moderate to large tumors ( ≥ 4cm) and those < 4cm (1 and 2 year LC: 96% for ≥ 4cm vs 95% for < 4cm). OS for all patients at 1 year was 67% and 62% at 2 years. PFS was 53% and 38% at 1 and 2 years respectively. The median PFS was 13.7 months with 13 patients (27%) undergoing further local treatment due to regional progression including 4 patients who had liver transplants. Conclusions: SBRT provides high local control for patients with HCC of even moderate to large size. Regional progression is prominent in HCC patients and SBRT does not appear to preclude further local treatments.
Purpose: To improve treatment planning for Total Body Irradiation by using CT‐based phantoms to create a DRR to design lung compensator blocks and to calculate the thickness of these blocks. Methods and Materials: Phantoms were constructed using both the prone and supine CT‐scans of the patient, and a bitmap was created for each of the two field (Anterior and Posterior) by calculating the tissue‐equivalent thickness of the patient along the ray from the source to the virtual film position, which gives the proper scaling and angular divergence of the projection (which commercial TPS systems would not for this geometry). The images could then used by the oncologists to create the outline of the lung compensator blocks. The block thickness was then calculated by computing the average tissue deficit under the block relative to the thickness of the treatment center (prescription point). Thirteen patients were then dual planned with the existing film‐based method as well as the new CT‐Based method, and the DVHs of these treatments were created using a new EGS++ application. Results: A comparison of the calculated thickness showed that the CT‐based method called for slightly thicker block thicknesses compared to the film method (leading to a dose difference of 1–5%), which lead to a more uniform lung‐dose according to the MC simulations. Conclusion: A user‐friendly CT‐based planning method has been clinically implemented which improves the uniformity of the treatment, better achieving the prescribed dose.
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