Evaluating plan robustness for organ deformation and set-up uncertainties in head and neck cancer
Robbins, Jennifer Van Herk, Marcel Green, Andrew Eiben, B. McPartlin, Andrew J Vasquez Osorio, Eliana
Robbins, Jennifer
Van Herk, Marcel
Green, Andrew
Eiben, B.
McPartlin, Andrew J
Vasquez Osorio, Eliana
Citations
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Abstract
Purpose or Objective
In radiotherapy, a planning CT (pCT) is used to plan
treatment, however the pCT only shows a snapshot of the
patient anatomy at a particular point in time. As the
treatment is delivered in multiple fractions several days
after the pCT is acquired, it is expected that the patient’s
internal anatomy will change throughout the treatment
course as organs may deform. Additionally, patient
positioning on the couch may vary for each fraction, e.g.
due to set-up uncertainty. These two factors cause
uncertainties in the dose absorbed by the patient. In this
study we model these uncertainties and simulate the joint
effect they have on treatments.
Material and Methods
Data from 20 patients were used. Organ deformations were
modelled using 6 independent weekly principal component
analysis (PCA) models based on the pCT and 6 weekly conebeam
CT scans from 13 patients. To account for time
trends in organ deformations, 5 daily vector fields were
simulated from each of the 6 weekly PCA models.
Systematic and random set-up uncertainties were
modelled as Gaussian distributions with standard
deviations of Σ = 0, 1, 2 mm and σ = 1, 2 mm respectively.
We simulated 100 30-fraction treatments using the clinical
plan for each patient, accounting for: 1) only organ
deformations, 2) only set-up errors, and 3) both organ
deformations and set-up errors. The difference in the
maximum brainstem dose and mean dose to each parotid
from the planned dose to the simulated doses was
evaluated for the training and validation (n = 7) datasets
separately.
Results
Figure 1 shows the difference in the resulting DVH
parameters from the planning value for all simulated scenarios. As expected, the DVH parameter differences
with both set-up uncertainties and organ deformations
were always larger than where each uncertainty was
considered separately. Considering only organ
deformations resulted in the smallest ranges, except for
(Σ, σ) = (0, 1) mm, and similar distributions for
(Σ, σ) = (1, 1) mm considering only set-up errors. For
simulations considering set-up uncertainties, the range of
the DVH parameter differences increases most as Σ
increases and to a lesser extent as σ increases for both the
training and validation data sets. The overall distributions
are similar for both the 13 training patients and the 7
validation patients.
Conclusion We evaluated the effect of time-varying organ
deformation and set-up uncertainties on clinical head and
neck cancer plans. Our data suggests that the effect of
organ deformation may be modelled by considering
(Σ, σ) = (1, 1) mm for this dataset. We found that for small
set-up uncertainties, (Σ, σ) = (0, 1) mm, organ
deformations are the dominant contribution to DVH
parameter differences for the parotids and the brainstem.
This highlights the need to correct for organ deformations
in modern image-guided radiotherapy workflows, where
patient set-up errors are being considerably reduced.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Robbins J, Van Herk M, Green A, Eiben B, McPartlin A, Vásquez Osorio E. PO-1650: Evaluating plan robustness for organ deformation and set-up uncertainties in head and neck cancer. Radiotherapy and Oncology . 2020 Nov;152:S905–6.