Faculty Bibliography

PURPOSE/OBJECTIVE:Pelvic nodal irradiation is often used for high-risk prostate adenocarcinoma. A commonly used alternative to low dose rate (LDR) brachytherapy, a 3-fraction SBRT boost with fiducial tracking may allow for better coverage of extracapsular extension and macroscopic seminal vesicle invasion. This study evaluates the practical impact of prior pelvic nodal irradiation on fiducial tracking during a subsequent 3-fraction robotic stereotactic body radiation therapy (SBRT) boost for high-risk prostate cancer and compares these outcomes to a cohort of patients undergoing definitive 5-fraction SBRT. METHODS:In this institutional analysis, we prospectively collected fiducial tracking data for patients receiving a 3-fraction boost to the prostate and seminal vesicles after conventional nodal radiation. We also identified patients treated with 5-fraction SBRT with a low risk of nodal involvement. Monte Carlo estimates of the Fisher's Exact Test assessed fiducial tracking loss. Continuous variables within the 5- and 3-fraction cohorts were compared using the Mann-Whitney Test. Changes in fiducial tracking and their association with pre-treatment factors were analyzed through the Kruskal-Wallis test and Monte Carlo for tracking patterns, and Spearman Correlation Coefficient and Mann-Whitney Test for deviations in tracking over 5 fractions. RESULTS:A total of 405 patients were treated from April 2021 to September 2023 with: (1) 5-fraction SBRT (n = 309, 76%), and (2) 3-fraction boost after nodal irradiation (n = 96, 24%). There was no significant fiducial tracking loss over the three-fraction boost treatment regimen that proceeded nodal treatment (p = 0.63). However, there was a significant (p < 0.001) loss of fiducial tracking fidelity as demonstrated by progressive loss of one tracked fiducial over 5-fractions. There was significantly more volatility observed in the 5-fraction versus 3-fraction boost treatment (median volatility 2.4 vs. 0.0, p < 0.001). There were no significant associations between fiducial tracking, independently for 3- or 5-fractions, using either analysis method or volatility for ADT, time from fiducial placement to SBRT, CTV, and QOD vs. daily SBRT. CONCLUSIONS:Pelvic nodal treatment does not affect the quantity/quality of fiducial tracking in 3-fraction treatments. However, 5-fraction treatments showed a progressive loss and increased volatility in fiducial tracking over time. No pre-treatment factors significantly influenced fiducial tracking changes in either cohort, though ADT use trended towards increased volatility in the 5-fraction group. With a minimum of 4 fiducials placed for treatment, the loss/volatility of a single fiducial had no clinical impact on the tracking system.

Patient-specific quality assurance in external beam radiation therapy is performed to ensure the safe and effective administration of ionizing radiation to patients. This individualized quality assurance process often involves the measurement of a planar radiation dose distribution obtained during irradiation of a surrogate phantom, in order to sample the actual delivered doses to the patient; this measured dose distribution is then compared with that calculated by the radiation therapy treatment planning software. The current work, a continuation of the authors' previous efforts, uses a covariance matrix adaption-evolutionary strategy approach to optimize transform parameters of an affine transformation which includes normalization, translation, dilation (or erosion), roll, pitch, and yaw, such that the application of the transformation warps the measured dose distribution to the planned dose distribution. The deviations, from unity, of the transformation parameters associated with translations and with scaling, and the deviations, from zero, of those associated with rotations, are a means of assessing possible errors in experimental setup, as well as possible errors in the dose calculation algorithm used in the treatment planning system. The optimization selects a Pareto-optimal set of eight transformation parameters that minimizes the difference, in the sense of the sum of least squares, between the doses at distinct points in the calculated and measured dose distributions. The method was tested using four distinct treatment plans, and the results indicate that it may prove useful in the clinic.

The calculation of the radiation absorbed dose to tissue requires the modeling of several parameters to describe the collimated beam. One of these parameters, the off-center ratio (OCR), describes the beam profile, i.e. the shape of the beam along a particular axis orthogonal to the direction in which the beam is aimed. The OCR is defined as the ratio of the dose at a point a specified distance from the center of the beam to the dose at the center of the beam. In the present work, the OCR of the CyberKnife X-band linear accelerator was modeled using error functions, with each individual error function representing a scatter kernel, and the summation describing the shape of the circular beam, with the assumption of azimuthal symmetry about the central axis of the beam. An evolutionary algorithm was used to optimize the amplitudes and standard deviations associated with the error functions, as well as the radius of the profile. Calculated OCR values were plotted against measured OCR values acquired during the commissioning of the CyberKnife. The model correctly predicts the beam profile.

A memetic algorithm is proposed for optimizing beam weights for robotic radiosurgical treatments delivered via the CyberKnife (Accuray, Inc., Sunnyvale, CA) system. The fitness function includes terms representing the tumor as well as terms representing organs-at-risk, and is of a quadratic form. Optimization involves inverse treatment planning, during which a set of beam weights is sought such that the user-specified radiation dose distribution is produced by the optimized ensemble of beam weights; the dose to the tumor is maximized, while the doses to the critical structures are minimized. In the present study, four distinct CT data sets for patients with carcinoma of the prostate were used to generate eight treatment plans, so that for each data set, a hypofractionated treatment plan (5 fractions) was created, as well as a treatment plan for a standard protracted dose fractionation schedule (38 fractions). In all cases, the memetic algorithm produced a treatment plan satisfying all clinical criteria in optimization times of 22-46 minutes.