A dynamic compensation strategy to correct patient-positioning errors in conformal prostate radiotherapy

Abstract

Traditionally, pretreatment detected patient-positioning errors have been corrected by repositioning the couch to align the patient to the treatment beam. We investigated an alternative strategy: aligning the beam to the patient by repositioning the dynamic multileaf collimator and adjusting the beam weights, termed dynamic compensation. The purpose of this study was to determine the geometric range of positioning errors for which the dynamic compensation method is valid in prostate cancer patients treated with three-dimensional conformal radiotherapy. Twenty-five previously treated prostate cancer patients were replanned using a four-field technique to deliver 72 Gy to 95% of the planning target volume (PTV). Patient-positioning errors were introduced by shifting the patient reference frame with respect to the treatment isocenter. Thirty-six randomly selected isotropic displacements with magnitudes of 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 cm were sampled for each patient, for a total of 5400 errors. Dynamic compensation was used to correct each of these errors by conforming the beam apertures to the new target position and adjusting the monitor units using inverse-square and off-axis factor corrections. The dynamic compensation plans were then compared with the original treatment plans via dose-volume histogram (DVH) analysis. Changes of more than 5% of the prescription dose, 3.6 Gy, were deemed significant. Compared with the original treatment plans, dynamic compensation produced small discrepancies in isodose distributions and DVH analyses. These differences increased with the magnitudes of the initial patient-positioning errors. Coverage of the PTV was excellent: D95 and Dmean were not increased or decreased by more than 5% of the prescription dose, and D5 was not decreased by more than 5% of the prescription dose for any of the 5400 simulated positioning errors. D5 was increased by more than 5% of the prescription dose in only three of the 5400 positioning errors, all three occurring with a positioning error of 10.0 cm. Dose increases for adjacent organs at risk were more common. D33 of the rectum and the periprostatic rectum was increased by more than 5% of the prescription dose in 235 (4.4%) and 212 (3.9%) of the 5400 positioning errors, respectively. D10 of the right femoral head increased by more than 5% of the prescription dose in 444 (8.2%) positioning errors, and the degree of change was highly related to individual patient anatomy and simulation position. For the bladder D20, there were three increases of more than 5% of the prescription dose. These data demonstrate the robustness of dynamic compensation for correction of patient-positioning errors in four-field conformal prostate radiotherapy, with minimal deviation from the original treatment plans even for errors greatly exceeding those commonly encountered in the clinic. Dynamic compensation can be performed remotely, thus eliminating errors that may result from unnecessary increases in treatment time or from secondary patient motion induced by couch...

A strategy for the use of image-guided radiotherapy (IGRT) on linear accelerators and its impact on treatment margins for prostate cancer patients

“…High resolution images at the time of treatment, automatic registration and calculation of required shifts with respect to a planning CT provide the tools for utmost precision in patient positioning [9,10,17]. However, the process is time consuming and in clinical reality, a compromise between complexity of treatment and extensive workload has to be elaborated [5,12,14,[18][19][20].…”

Determination of action thresholds for electromagnetic tracking system‐guided hypofractionated prostate radiotherapy using volumetric modulated arc therapy

“…Adaptive strategies that track the motion signal, calculate any trend in position, predict the near-term motion trajectory, and make a decision whether to correct the shift, will be more powerful. [29][30][31] Future investigations are needed to incorporate dosimetric-driven criteria into the design of advanced management strategies and evaluating its clinical efficacy.…”

Residual Translational and Rotational Errors after kV X-Ray Image-Guided Correction of Prostate Location Using Implanted Fiducials

“…We likewise did not see secondary patient motion provoking a systematic error. Lauve et al [21] proposed an alternative strategy: aligning the beam to the patient by repositioning the dynamic multileaf collimator and adjusting the beam weights, termed dynamic compensation.…”