Treatment plan adaptation for MRI-guided radiotherapy using solely MRI data: a CT-based simulation study.

An integrated MRI-accelerator system provides MRI images before and during irradiation. Our purpose is to investigate the feasibility of treatment plan adaptation using solely MRI data, which lack density information. In this study we used CT data to quantify the tissue density effect. Treatment planning was performed for five prostate cancer patients. We simulated correction of a 3, 5, 7 and 10 mm prostate shift relative to the body contour in the anterior, posterior, superior and inferior directions. We applied the original treatment plan to each corrected prostate shift and recalculated the dose distribution using the same monitor units (MU). We calculated the dose differences with and without density information. The latter mimics geometrically correct MRI data. Physical path lengths, available in MRI data, are used to perform MU rescaling per beam and are shown to be of more importance than tissue densities for treatment plan adaptation in prostate cancer. As the change in the physical path length of the central beam axis is representative of the entire beam, MU rescaling based on central beam axis information works fine. In conclusion, MRI data could be used for treatment plan adaptation in prostate cancer provided that the images are geometrically correct.

Inverse plan optimization accounting for random geometric uncertainties with a multiple instance geometry approximation (MIGA).

Radiotherapy treatment plans that are optimized to be highly conformal based on a static patient geometry can be degraded by setup errors and/or intratreatment motion, particularly for IMRT plans. To achieve improved plans in the face of geometrical uncertainties, direct simulation of multiple instances of the patient anatomy (to account for setup and/or motion uncertainties) is used within the inverse planning process. This multiple instance geometry approximation (MIGA) method uses two or more instances of the patient anatomy and optimizes a single beam arrangement for all instances concurrently. Each anatomical instance can represent expected extremes or a weighted distribution of geometries. The current implementation supports mapping between instances that include distortions, but this report is limited to the use of rigid body translations/ rotations. For inverse planning, the method uses beamlet dose calculations for each instance, with the resulting doses combined using a weighted sum of the results for the multiple instances. Beamlet intensities are then optimized using the inverse planning system based on the cost for the composite dose distribution. MIGA can simulate various types of geometrical uncertainties, including random setup error and intratreatment motion. A limited number of instances are necessary to simulate Gaussian-distributed errors. IMRT plans optimized using MIGA show significantly less degradation in the face of geometrical errors, and are robust to the expected (simulated) motions. Results for a complex head/neck plan involving multiple target volumes and numerous normal structures are significantly improved when the MIGA method of inverse planning is used. Inverse planning using MIGA can lead to significant improvements over the use of simple PTV volume expansions for inclusion of geometrical uncertainties into inverse planning, since it can account for the correlated motions of the entire anatomical representation. The optimized plan results reflect the differing patient geometry situations which can be important near the surface or heterogeneities. For certain clinical situations, the MIGA optimization approach can correct for a significant part of the degradation of the plan caused by the setup uncertainties.

Is uniform target dose possible in IMRT plans in the head and neck?

PURPOSE: Various published reports involving intensity-modulated radiotherapy (IMRT) plans developed using automated optimization (inverse planning) have demonstrated highly conformal plans. These reported conformal IMRT plans involve significant target dose inhomogeneity, including both overdosage and underdosage within the target volume. In this study, we demonstrate the development of optimized beamlet IMRT plans that satisfy rigorous dose homogeneity requirements for all target volumes (e.g., +/-5%), while also sparing the parotids and other normal structures. METHODS AND MATERIALS: The treatment plans of 15 patients with oropharyngeal cancer who were previously treated with forward-planned multisegmental IMRT were planned again using an automated optimization system developed in-house. The optimization system allows for variable sized beamlets computed using a three-dimensional convolution/superposition dose calculation and flexible cost functions derived from combinations of clinically relevant factors (costlets) that can include dose, dose-volume, and biologic model-based costlets. The current study compared optimized IMRT plans designed to treat the various planning target volumes to doses of 66, 60, and 54 Gy with varying target dose homogeneity while using a flexible optimization cost function to minimize the dose to the parotids, spinal cord, oral cavity, brainstem, submandibular nodes, and other structures. RESULTS: In all cases, target dose uniformity was achieved through steeply varying dose-based costs. Differences in clinical plan evaluation metrics were evaluated for individual cases (eight different target homogeneity costlets), and for the entire cohort of plans. Highly conformal plans were achieved, with significant sparing of both the contralateral and ipsilateral parotid glands. As the homogeneity of the target dose distributions was allowed to decrease, increased sparing of the parotids (and other normal tissues) may be achieved. However, it was shown that relatively few patients would benefit from the use of increased target inhomogeneity, because the range of improvement in the parotid dose is relatively limited. Hot spots in the target volumes are shown to be unnecessary and do not assist in normal tissue sparing. CONCLUSION: Sparing of both parotids in patients receiving bilateral neck radiation can be achieved without compromising strict target dose homogeneity criteria. The geometry of the normal tissue and target anatomy are shown to be the major factor necessary to predict the parotid sparing that will be possible for any particular case.

Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change.

A novel technique is reported for detecting permanent and transient light-induced refractive-index changes (photosensitivity) in optical fibers. The index change is detected by irradiating one arm of an unbalanced Mach-Zehnder fiber interferometer with UV light, thereby changing its optical path length. From a measurement of the change in the spectral response of the Mach-Zehnder interferometer, the change in the fiber core index as a function of wavelength can be determined. The equilibrium change in the core index is found to have an almost constant value of approximately 2.3 x 10(-5) over the measured wavelength range of 700 to 1400 nm.

Ultraviolet-light photosensitivity in Er(3+)-Ge-doped optical fiber.

Light-induced refractive-index change in Er(3+)-Ge-doped optical fiber is reported for the first time to the authors' knowledge. Evidence of the change is observed when UV light (lambda = 249 nm) irradiates one arm of an unbalanced Mach-Zehnder interferometer made from Er(3+)-Ge-doped fiber. From a measurement of the change in the spectral response of the interferometer with UV exposure, the change in fiber core index as a function of wavelength is determined. The equilibrium change in core index is found to vary between 2.3 x 10(-5) and 3.7 x 10(-5) over the measured wavelength region of 800 to 1700 nm. Also for the first time to our knowledge, fused couplers made of Er(3+)-doped fiber are reported. These identical fiber couplers are used in a novel all-fiber unbalanced Mach-Zehnder interferometer.