Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2018)

Intensity-modulated radiotherapy treatment planning is an inverse problem that typically includes numerous parameters that have to be manually tuned by expert planners. This process can take hours or even days and can often lead to suboptimal plans. In this study, we developed a technique for fully automated radiotherapy treatment planning with the guidance of dose predictions using high quality or evolving knowledge bases.

Knowledge-based planning (KBP) dose prediction provides patient-specific estimations for the capabilities and limitations of a plan. Statistical voxel dose learning (SVDL) was developed to predict the voxel dose of new patients. The method was compared to supervised machine learning methods, spectral regression (SR) and support vector regression (SVR), to evaluate the prediction accuracy and robustness of using small training sets. SVDL was found to have higher prediction accuracy than the more sophisticated machine learning methods and effective even with small training sets.

To remove any dependence on hyperparameters that require manual tuning, voxel-based non-coplanar 4π radiotherapy and coplanar volumetric modulated arc therapy (VMAT) optimization problems were modified to include the KBP predicted doses. The new cost functions encourage the plans to meet or improve on the predicted doses. Because of this, the resulting plan quality is heavily reliant on the plan quality of the KBP training set. To ensure high quality plans, non-coplanar and coplanar IMRT plans were manually created using all available beams. The resulting automated plans were of superior quality compared to manually-created plans.

In the case of no existing high quality training set, evolving-knowledge-base (EKB) planning was developed. An initial, low quality training set was used for the first epoch of automated planning. In subsequent epochs, the superior plans from the previous epoch were taken as the training set. Overall plan quality was observed to improve through epochs, plateauing after 3 and 6 epochs for lung and head & neck planning, respectively. The final EKB plans were significantly higher quality than manually-created VMAT plans and equivalent to manually-created 4π plans.

Through the course of this work, we established a robust and accurate KBP dose prediction technique, which we then utilized in our automated planning protocol. Both the use of high quality training sets and EKB planning created high quality plans in a more efficient and consistent manner than hyperparameter tuning.

    Cover page: Fully Automated Radiation Therapy Treatment Planning Through Knowledge-Based Dose Predictions
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2019)

    Purpose

    Non-coplanar 4π radiotherapy generalizes intensity modulated radiation therapy (IMRT) to automate beam geometry selection but requires complicated hyperparameter tuning to attain superior plan quality, which can be tedious and inconsistent. In this study, a fully automated 4π treatment planning was developed using evolving knowledge-base (EKB) planning guided by dose prediction.

    Methods

    Twenty 4π lung and twenty 4π head and neck (HN) cases were included. A statistical voxel dose learning model was initially trained on low-quality plans created using generic hyperparameter templates without manual tuning. To improve the automated plan quality without being limited by the training data quality, a new 4π optimization problem was formulated to include a one-sided penalty on the organ-at-risk (OAR) dose deviation from the predicted dose. This directional OAR penalty encourages superior OAR sparing. The fast iterative shrinkage-thresholding algorithm (FISTA) was used to solve the large-scale beam orientation optimization problem. With the improved plans, new predictions were created to guide the next loop of EKB planning for a total of 10 loops. Plan quality was evaluated using a plan quality metric (PQM) points system based on clinical dose constraints and compared with automated planning approaches guided by manual high-quality plans using all non-coplanar beams, automated plans using individually evolved targeted dose, and manually created 4π plans.

    Results

    For the lung cases, the final EKB plans had significantly higher PQM than manually created 4π (+2.60%). The improvements plateaued after the third loop. The final HN EKB plans and manually created 4π plans had comparable PQMs, but had lower PQM compared to automated plans using a high-quality training set (-3.00% and -4.44%, respectively). The PQM consistently increased up to the sixth loop. Individually evolved plans were able to improve the plan quality from initial condition due to the one-sided cost function but the 60% of them were trapped in undesired local minima that were substantially worse than their corresponding EKB plans.

    Conclusion

    Evolving knowledge-base planning is a novel automated planning technique guided by the predicted three-dimensional dose distribution, which can evolve from low-quality plans. EKB allows new beams to be used in the automated planning workflow for superior plan quality.
      Cover page: Automated 4π radiotherapy treatment planning with evolving knowledge‐base
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2018)

      Purpose

      The accuracy of dose prediction is essential for knowledge-based planning and automated planning techniques. We compare the dose prediction accuracy of 3 prediction methods including statistical voxel dose learning, spectral regression, and support vector regression based on limited patient training data.

      Methods

      Statistical voxel dose learning, spectral regression, and support vector regression were used to predict the dose of noncoplanar intensity-modulated radiation therapy (4π) and volumetric-modulated arc therapy head and neck, 4π lung, and volumetric-modulated arc therapy prostate plans. Twenty cases of each site were used for k-fold cross-validation, with k = 4. Statistical voxel dose learning bins voxels according to their Euclidean distance to the planning target volume and uses the median to predict the dose of new voxels. Distance to the planning target volume, polynomial combinations of the distance components, planning target volume, and organ at risk volume were used as features for spectral regression and support vector regression. A total of 28 features were included. Principal component analysis was performed on the input features to test the effect of dimension reduction. For the coplanar volumetric-modulated arc therapy plans, separate models were trained for voxels within the same axial slice as planning target volume voxels and voxels outside the primary beam. The effect of training separate models for each organ at risk compared to all voxels collectively was also tested. The mean squared error was calculated to evaluate the voxel dose prediction accuracy.

      Results

      Statistical voxel dose learning using separate models for each organ at risk had the lowest root mean squared error for all sites and modalities: 3.91 Gy (head and neck 4π), 3.21 Gy (head and neck volumetric-modulated arc therapy), 2.49 Gy (lung 4π), and 2.35 Gy (prostate volumetric-modulated arc therapy). Compared to using the original features, principal component analysis reduced the 4π prediction error for head and neck spectral regression (-43.9%) and support vector regression (-42.8%) and lung support vector regression (-24.4%) predictions. Principal component analysis was more effective in using all/most of the possible principal components. Separate organ at risk models were more accurate than training on all organ at risk voxels in all cases.

      Conclusion

      Compared with more sophisticated parametric machine learning methods with dimension reduction, statistical voxel dose learning is more robust to patient variability and provides the most accurate dose prediction method.
        Cover page: Performance Comparison of Knowledge-Based Dose Prediction Techniques Based on Limited Patient Data
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2023)

        Purpose

        The purpose of this survey study is to compare the experiences of programs and applicants in the MedPhys Match (MPM) in the 2020-21 match cycle with experiences reported from previous match cycles. The 2020-21 match cycle was unique in that recruitment and interviewing were almost exclusively virtual during the COVID-19 pandemic.

        Methods

        A survey was sent to all applicants and programs registered for the 2020-21 MPM. Survey questions asked about the pre-interview screening, interview, ranking, and post-match stages of the residency match process. Survey data were analyzed using graphical methods and spreadsheet tools.

        Results

        Advantages and disadvantages to the virtual interviewing experience were reported by applicants and program directors (PDs). The advantages included reduced cost and greater scheduling flexibility with fewer scheduling conflicts, allowing applicants to consider more programs. These advantages greatly outweighed the disadvantages such as the inability to meet faculty/staff and current residents in person and gauge the feel of the program. PDs recognized the advantages of minimal costs and time savings for applicants. Programs reported it was difficult to convey workplace culture and the physical environment and to gauge personality and interpersonal skills of the applicants.

        Conclusion

        The virtual interviewing environment for residency recruitment in medical physics is strongly preferred by applicants over required in-person interviews. The advantages identified by applicants outweigh the disadvantages, allowing applicants to feel confident in their ranking decisions and overall satisfied with their match results. PDs acknowledge the greater equity of access to interviews for applicants in the virtual environment, however, they are overall less satisfied with their ability to showcase their program's strengths and to assess the personality of applicants. Caution is urged when considering a hybrid interview model to ensure fair assessments that do not depend on whether an applicant chooses to accept an optional in-person interview or site visit.
          Cover page: Virtual interviewing in the MedPhys match: Experiences of applicants and programs
          Top