created a statistical shape model (SSM) [1] for the calculation of dose coverage probabilities to use as a treatment plan evaluation tool. Materials & Methods.
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18 International Conference on the use of Computers in Radiation Therapy
Dose coverage calculation based on a statistical shape model for cervical cancer radiation therapy David Tilly†,1,2, Agustinus J.A.J. van de Schoot3, Arjan Bel3 and Anders Ahnesjö1 1) Medical Radiation Physics, Uppsala University, Uppsala, Sweden. 2) Elekta Instruments AB, Stockholm, Sweden 3) Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands Introduction Cervical cancer radiotherapy is challenging due to the large inter-fraction organ motion requiring large margins to ensure sufficient target dose. Hence, the PTV extends into organs at risks causing them significant dose burden. It is therefore of high interest to model the motion of involved organs as to enable more realistic estimates of the resulting dose. We created a statistical shape model (SSM) [1] for the calculation of dose coverage probabilities to use as a treatment plan evaluation tool Materials & Methods Five cervical cancer patients with 3-6 repeat CT, 31 image series in total, formed the basis for the creation of the SSM. Each image series were delineated according to a protocol for consistency. The CTV to ITV and ITV to PTV margins were 10 mm and 8 mm respectively. Margins are often designed to ensure at least 95% of the prescribed dose in 90% of the treatments to the entire CTV [2]. All patients were treated in the prone position to 46 Gy (23fx) with IMRT or VMAT. We define a treatment scenario as a possible realisation of a treatment with fraction specific sampled uncertainties. Organ motion is sampled through the SSM and setup-errors from a normal distribution with standard deviation (std) 4 mm to be consistent with the used margins. A 1000 scenarios were simulated per patient (2 mm dose grid) and the execution time was recorded. A dose volume coverage map was calculated based on all treatment scenario DVHs, from where probability isolines were extracted for evaluation [2], [3]. The probabilistic evaluation of 𝐷"#% (CTV) and 𝑉&'() (bladder) and 𝑉*+() (rectum) were compared to the prescribed dose and planning DVHs. It was implemented in Python with PyCUDA [4] for GPU (NVidia Tesla C2075) calculation. The input to the SSM was the intra-patient deformations extracted via deformable image registration (ADMIRE, v1.11, Elekta AB) of each series to a chosen patient reference image series. The deformations from all patients were collected in an average patient, a common frame of reference, by mapping the deformations through the registration between the average patient and respective reference image. All registrations were guided by images as well as contours to handle the large deformations. The registration quality was evaluated with the Dice similarity coefficient (DSC), the mean surface distance (MSD) and the inverse consistency error (ICE). The dominating eigenmodes representing 90% of the total deformation variance were extracted through principal component analysis (PCA). Results The average (std) DSC was 0.95 (0.027), 0.94 (0.021) and 0.93 (0.039) and MSD (mm) was 0.81 (0.21), 1.00 (0.35) and 1.39 (1.24) for the bladder, corpus-cervix-vagina (CCV) union
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18 International Conference on the use of Computers in Radiation Therapy
and rectum respectively. The ICE was ≤ 1 mm for 98% of the entire volume for all patients. Fourteen eigenmodes from the PCA were needed to include 90% of the deformation variance. CCV
Bladder
Rectum Figure 1: First eigenmode (left) - as the bladder filling increases the uterus is pushed upwards. Second eigenmode (right) - the variation in rectal filling pushes the cervix and lower part of the uterus.
Figure 2: DVHs from simulation of 1000 treatment scenarios for patient #3 together with the planned dose. The probability isoline gives the relative volume of the CTV (OAR) which will at least (most) have 90% of all scenarios.
Figure 3: The average value for all patients of 𝐷"#% and the average 𝑉&'()/*+() for the bladder and rectum.
The treatment simulation DVHs show a large spread, see example in figure 2. The average target dose coverage at 90% probability was 90% (std 2.0%), i.e.
