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A Comparison of Inter-Fractional Dose in Prostate Volume Modulated Arc Therapy

M Nilsen*, S Park , Y Feng , W Jiang , East Carolina University, Greenville, NC


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To compare the inter-fractional volume of organ at risk(OAR) and dose variation using Volumetric Modulated Arc Therapy(VMAT) in prostate cancer treatments.

Methods: The CT(pCT) used for original treatment planning was acquired on the simulation day for 4 prostate cancer patients. On-board kV Cone Beam CT(CBCT) was acquired before each fraction to assist patient setup. The CBCT calibration stability was monitored. The CBCT data acquired on first fraction, middle fraction, and last fraction were imported into the Eclipse TPS. A 3D patient model was generated from each CBCT and fused to the pCT model using rigid registration(RR) and deformable registration(DR) techniques respectively for mapping the contours, which were further edited under the guidance of a senior dosimetrist to adapt any change in the patient’s anatomy. Varian’s RapidArc was employed for pCT-based and CBCT-based dose calculations and subsequent comparisons on volume and dose.

Results: The averaged percentage difference(APD) of volume for rectum, bladder and target are 15.39%±38.20%, -18.98%±36.22% and 15.05%±16.77%, respectively. The first fraction and middle fraction present the largest APD of volume on rectum(25.97%±58.71%) and bladder(32.47%±26.36%), respectively. In dosimetric comparison, the APD of V50 and V75 for rectum are 3.54±38.6% and -14.44±42.50%, respectively. The APD of V65 and V80 for bladder are -16.27±51.66% and 6.61%±50.16%, respectively. The APD for Dmean, D95%, and V95% of the target in DR dataset are -4.684%±4.08%, -18.44%±11.23%, and -30.04%±24.23%, respectively;The APD for Dmean, D95% and V95% of the target calculated in RR are -5.94%±6.95%, -19.07%±14.79%, and -34.44%±28.75%, respectively.

Conclusion: The target dose calculated in DR dataset compared with the RR is closer to the original plan. Significant discrepancy on inter-fractional OARs and dose between the original treatment plan and the CBCT-based calculation suggests the importance of inter-fractional organ movements in VMAT treatments and the need for adaptive therapy.


Treatment Planning, Cone-beam CT


TH- RT Interfraction motion management : X-ray projection/CBCT-based

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