Characterization of lung tumors motion baseline using cone-beam computed tomography.

PURPOSE: To characterize the interfractional variability in lung tumor volume, position, and tumor boundaries. METHODS: Cone-beam computed tomography (CBCT) scans were acquired weekly during the course of treatment for 34 lung cancer patients (1-20 scans) with large tumors. Spatial registration based on bones was performed between contoured planning CT and CBCT. Gross tumor volume (GTV) on each CBCT was then contoured. Tumor volume, centroid, and boundaries variability were quantified. A commercial deformable registration software was tested and results were compared to manual contours. RESULTS: Mean volume reduction was 41 ± 32% (p < 0.001) after an average time of 51 days. Tumor centroid drifts were 0.03, 0.14, and -0.13 cm in right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions with standard deviations of 0.55, 0.50, and 0.51 cm. GTV boundaries displacements were -0.27, -0.14, and -0.16 cm with standard deviations of 0.64, 0.57, and 0.59 cm in RL, AP, and SI directions. Relative error between deformed and manual contours with the commercial deformable registration software rose up exponentially with the GTV decrease. CONCLUSIONS: GTV size changes for large lung tumors are similar to those for standard tumors. Magnitude absolute values of displacement vector for centroid and boundaries shifts show that there is not a preferred direction for the drifts but shrinkage.

Functional avoidance of lung in plan optimization with an aperture-based inverse planning system.

PURPOSE: To implement SPECT-based optimization in an anatomy-based aperture inverse planning system for the functional avoidance of lung in thoracic irradiation. MATERIAL AND METHODS: SPECT information has been introduced as a voxel-by-voxel modulation of lung importance factors proportionally to the local perfusion count. Fifteen cases of lung cancer have been retrospectively analyzed by generating angle-optimized non-coplanar plans, comparing a purely anatomical approach and our functional approach. Planning target volume coverage and lung sparing have been compared. Statistical significance was assessed by a Wilcoxon matched pairs test. RESULTS: For similar target coverage, perfusion-weighted volume receiving 10 Gy was reduced by a median of 2.2% (p=0.022) and mean perfusion-weighted lung dose, by a median of 0.9 Gy (p=0.001). A separate analysis of patients with localized or non-uniform hypoperfusion could not show which would benefit more from SPECT-based treatment planning. Redirection of dose sometimes created overdosage regions in the target volume. Plans consisted of a similar number of segments and monitor units. CONCLUSIONS: Angle optimization and SPECT-based modulation of importance factors allowed for functional avoidance of the lung while preserving target coverage. The technique could be also applied to implement PET-based modulation inside the target volume, leading to a safer dose escalation.

Optimization of photon beam energy in aperture-based inverse planning.

Optimal choice of beam energy in radiation therapy is easy in many well-documented cases, but less obvious in some others. Low-energy beams may provide better conformity around the target than their high-energy counterparts due to reduced lateral scatter, but they also contribute to overdosage of peripheral normal tissue. Beam energy was added as an optimization parameter in an automatic aperture-based inverse planning system. We have investigated two sites (prostate and lung), representative of deep-seated and moderately deep-seated tumors. For each case and different numbers of beam incidences, four plans were optimized: 6 MV, 23 MV, and mixed energy plans with one or two energies per incidence. Each plan was scored with a dose-volume cost function. Cost function values, number of segments, monitor units, dose-volume parameters and isodose distributions were compared. For the prostate and lung cases, energy mixing improved plans in terms of cost function values, with a more important reduction for a small number of beam incidences. Use of high energy allows better peripheral tissue sparing, while keeping similar target coverage and sensitive structures avoidance. Low energy contribution to monitor units usually increased with the number of beam incidences. Thus, for deep-seated and moderately deep-seated tumors, energy optimization can produce interesting plans with less peripheral dose and monitor units than for low energy alone.

Dose escalation in the radiotherapy of non-small-cell lung cancer with aperture-based intensity modulation and photon beam energy optimization for non-preselected patients.

PURPOSE: To verify the potential of aperture-based intensity-modulated radiotherapy (AB-IMRT) to realize dose escalation plans for non-preselected non-small-cell lung cancer (NSCLC) patients, using photon beam energy optimization. METHODS AND MATERIALS: Seven cases of NSCLC were retrospectively studied. Clinical reference plans were made at 60 Gy by an experienced dosimetrist. Dose escalation was applied to PTV2, a subvolume within the main PTV1. Escalation plans were optimized by considering beam angles (table and gantry), energy (6 and 23 MV) and weights, for an increasing dose to the PTV2, starting from 66 Gy and keeping 30 fractions. RESULTS: In five cases, doses over 78 Gy could be achieved before exceeding organs at risk (OARs) standard tolerance. Peripheral overdosages, as well as lung and spinal cord tolerance doses, limited escalation. Means+/-SD V(95%) parameters were (97.3+/-0.9)% for PTV1s and (96.7+/-2.2)% for PTV2s. Doses to OARs were also maintained at acceptable levels. Optimized plans made use of both low- and high-energy beams and had a similar number of monitor units compared to the 60 Gy clinical plans. CONCLUSIONS: The AB-IMRT system can successfully realize dose escalation for a sizeable number of cases. Plans produced contained few large segments, and are applicable to a wide range of tumor volumes and locations.

Multiobjective optimization with a modified simulated annealing algorithm for external beam radiotherapy treatment planning.

Inverse planning in external beam radiotherapy often requires a scalar objective function that incorporates importance factors to mimic the planner's preferences between conflicting objectives. Defining those importance factors is not straightforward, and frequently leads to an iterative process in which the importance factors become variables of the optimization problem. In order to avoid this drawback of inverse planning, optimization using algorithms more suited to multiobjective optimization, such as evolutionary algorithms, has been suggested. However, much inverse planning software, including one based on simulated annealing developed at our institution, does not include multiobjective-oriented algorithms. This work investigates the performance of a modified simulated annealing algorithm used to drive aperture-based intensity-modulated radiotherapy inverse planning software in a multiobjective optimization framework. For a few test cases involving gastric cancer patients, the use of this new algorithm leads to an increase in optimization speed of a little more than a factor of 2 over a conventional simulated annealing algorithm, while giving a close approximation of the solutions produced by a standard simulated annealing. A simple graphical user interface designed to facilitate the decision-making process that follows an optimization is also presented.

Performing daily prostate targeting with a standard V-EPID and an automated radio-opaque marker detection algorithm.

Online prostate positioning using gold markers and a standard video-based electronic portal imaging device is reported. The average systematic (random) errors have been reduced from 2.1 mm (2.7 mm) to 0.5 mm (1.5 mm) in AP direction, 1.1 mm (1.7 mm) to 0.7 mm (1.2 mm) SI and 1.2 mm (1.7 mm) to 0.6 mm (1.3 mm) LR.

Measurements of intrafraction motion and interfraction and intrafraction rotation of prostate by three-dimensional analysis of daily portal imaging with radiopaque markers.

PURPOSE: To measure the interfraction and intrafraction motion of the prostate during the course of external beam radiotherapy using a video electronic portal imaging device and three-dimensional analysis. METHODS AND MATERIALS: Eighteen patients underwent implantation with two or three gold markers in the prostate before five-angle/11-field conformal radiotherapy. Using CT data as the positional reference, multiple daily sets of portal images, and a three-dimensional reconstruction algorithm, intrafraction translations, as well as interfraction and intrafraction rotations, were analyzed along the three principal axes (left-right [LR], superoinferior [SI], and AP). The overall mean values and standard deviations (SDs), along with random and systematic SDs, were computed for these translations and rotations. RESULTS: For 282 intrafraction translational displacements, the random SD was 0.8 mm (systematic SD, 0.2) in the LR, 1.0 mm (systematic SD, 0.4) in the SI, and 1.4 mm (systematic SD, 0.7) in the AP axes. The analysis of 348 interfraction rotations revealed random SDs of 6.1 degrees (systematic SD, 5.6 degrees ) around the LR axis, 2.8 degrees (systematic SD, 2.4 degrees ) around the SI axis, and 2.0 degrees (systematic SD, 2.2 degrees ) around the AP axis. The intrafraction rotational motion observed during 44 fractions had a random SD of 1.8 degrees (systematic SD, 1.0 degrees ) around the LR, 1.1 degrees (systematic SD, 0.8 degrees ) around the SI, and 0.6 degrees (systematic SD, 0.3 degrees ) around the AP axis. CONCLUSION: The interfraction rotations observed were more important than those reported in previous studies. Intrafraction motion was generally smaller in magnitude than interfraction motion. However, the intrafraction rotations and translations of the prostate should be taken into account when designing planning target volume margins because their magnitudes are not negligible.

Automatic generation of anatomy-based MLC fields in aperture-based IMRT.

We have developed an algorithm to automatically generate anatomy-based MLC fields. For each beam, a first field is adjusted to the projection of the target in a beam's eye view, allowing subsequent fields to be derived from this conformal field by removing the overlapping surface of each organ at risk, respectively. The projections are based on a surface sampling of the anatomical structures. On top of the MLC mechanical constraints, verification constraints are imposed on the MLC segments, in order to get reliable dosimetry using a commercial dose calculation engine. Thus, in each direction, the aperture's cross-section must be greater than a specified threshold, in our case 2 cm. Furthermore, junctions are not tolerated in order to avoid underdosage, for instance from the tongue-and-groove effect. The use of such MLC fields simplifies the verification process. The performance of the algorithm is illustrated for head and neck, thorax and prostate cases. Only a fraction of a second of CPU time is required to perform the segmentation for each beam.

Simultaneous optimization of beam orientations, wedge filters and field weights for inverse planning with anatomy-based MLC fields.

As an alternative between manual planning and beamlet-based IMRT, we have developed an optimization system for inverse planning with anatomy-based MLC fields. In this system, named Ballista, the orientation (table and gantry), the wedge filter and the field weights are simultaneously optimized for every beam. An interesting feature is that the system is coupled to Pinnacle3 by means of the PinnComm interface, and uses its convolution dose calculation engine. A fully automatic MLC segmentation algorithm is also included. The plan evaluation is based on a quasi-random sampling and on a quadratic objective function with penalty-like constraints. For efficiency, optimal wedge angles and wedge orientations are determined using the concept of the super-omni wedge. A bound-constrained quasi-Newton algorithm performs field weight optimization, while a fast simulated annealing algorithm selects the optimal beam orientations. Moreover, in order to generate directly deliverable plans, the following practical considerations have been incorporated in the system: collision between the gantry and the table as well as avoidance of the radio-opaque elements of a table top. We illustrate the performance of the new system on two patients. In a rhabdomyosarcoma case, the system generated plans improving both the target coverage and the sparing of the parotide, as compared to a manually designed plan. In the second case presented, the system successfully produced an adequate plan for the treatment of the prostate while avoiding both hip prostheses. For the many cases where full IMRT may not be necessary, the system efficiently generates satisfactory plans meeting the clinical objectives, while keeping the treatment verification much simpler.