SU-E-J-191: A Multivariate Framework for N-Tissue Classification in Treatment Assessment of Glioblastomas

PURPOSE

Glioblastoma is the most common primary brain tumor in adults and is rapidly fatal. Treatment monitoring of these patients has increased awareness that many patients have new areas of contrast enhancement without progressive clinical signs and symptoms. Although the enhancing areas mimic tumor progression, the lesions result from treatment effects and subsequently stabilize or improve without further treatment and are not correlated with poorer outcomes. This phenomenon has been termed pseudoprogression and is hypothesized to occur secondarily to edema and vessel permeability in the tumor area as a result of the combined effects of radiation and chemotherapy. Since the new enhancing lesions of pseudoprogression are indistinguishable from true disease progression, there is a need for a predictive model to distinguish the two phenomena.

METHOD

We developed a classification algorithm that combines perfusion and diffusion MRI imaging to effectively partition the cases as one exhibiting true or pseudo progression based on a vector of features containing T1, rCBV and ADC imaging. The multi-sequence classification algorithm uses an expectation maximization (EM) algorithm that learns from training cases with known clinical outcome to assigns each voxel to a type of tissue.

RESULTS

A training set of 20 where the clinical outcome is known from biopsy or from long-term follow-up was used by EM algorithm to model typical imaging values within tissue of pseudo, tumor, edema, necrosis, vessels or brain anatomy to construct a database of expected values for each tissue type. When presented with a new case, the algorithm automatically classifies voxels by their geographical proximities and Mahalanobis distance to the pre-sampled values.

CONCLUSION

Usage of advanced classification techniques allows automated labeling of voxels into normal, pseudoprogression or tumoral tissue types. The technique allows for early detection of pseudo progression to spare patients from unnecessary surgery or toxic chemotherapy.

Comparing central nervous system (CNS) and extra-CNS hemangiopericytomas in the Surveillance, Epidemiology, and End Results program: analysis of 655 patients and review of current literature

BACKGROUND

Hemangiopericytomas (HPCs) are rare tumors in the central nervous system (CNS) and in extra-CNS sites. The authors of this report used the Surveillance, Epidemiology, and End Results (SEER) Program to study prognostic factors in patients with HPC.

METHODS

The SEER database was analyzed for patients who were diagnosed with HPC tumors from 1973 to 2007. Patients were stratified into CNS and extra-CNS groups. Univariate and multivariate analyses were performed for the overall survival (OS) endpoint using major demographic factors (age, race, and sex) and disease factors (tumor site).

RESULTS

In total, 655 patients with HPC were stratified into a CNS group (n = 199) and an extra-CNS group (n = 456). The patients with extra-CNS HPC were statistically older (mean age, 53 years vs 49 years; P = .008) and were more likely to have larger tumors (median greatest dimension, 7.0 cm vs 5.2 cm; P < .001). Patients who had CNS tumors had better OS and cause-specific survival (CSS) compared with patients who had extra-CNS tumors (P < .001 for both). Negative predictors of OS on multivariate analysis included extra-CNS tumor site (hazard ratio [HR], 1.6; P = .005) and older age (ages 40-59 years: HR, 2.08; P = .032; ages 60-79 years: HR, 3.9; P < .001; aged ≥80 years: HR, 7.7; P < .001).

CONCLUSIONS

The current analysis demonstrated that patients with extra-CNS HPCs had worse OS and CSS than patients with CNS HPCs.

Quantitative analysis of pre- and post-treatment PET-CT scans using deformable image registration methods

BACKGROUND AND PURPOSE

To investigate the utility of quantitative PET analysis for early prediction of local control following stereotactic body radiation therapy (SBRT).

MATERIAL AND METHODS

An initial test cohort of fourteen cases and a validation cohort of twenty-three cases were analyzed. All patients had metastatic or recurrent cancer and underwent PET-CTs pre- and post- SBRT to a variety of sites. Local failure was defined as biopsy proven persistent/recurrent disease or progressive disease on radiologic imaging. Using deformable registration, radiation dose was transferred to the PET-CTs. Using the prescription isodose as the volume of interest (VOI), response was assessed by generating metabolic volume histograms (MVH). MVH curves examine metabolic heterogeneity in the VOI. Exploratory analyses of the test cohort evaluated the viability of multiple iso-SUV and iso-volumetric points selected from the MVH curves to serve as novel markers of response. Standard PET response markers (maximum/mean SUV and qualitative analysis) were also assessed.

RESULTS

In the initial cohort, ten of fourteen patients achieved local control at last follow-up, a median of 225 days following post-SBRT PET. Three out of four local failures had an increase in max SUV, while all patients who achieved local control had a reduction in max SUV (p=0.01). Exploratory analyses using multiple iso-SUV and iso-volumetric points did not yield any factors associated with local control (p>0.05). In the validation cohort, lower post- treatment max SUV (p=.03) and reduction in max SUV (p<0.05) were significantly associated with local control.

CONCLUSIONS

Reduction in max SUV following SBRT is associated with local control.

Stereotactic body radiosurgery for spinal metastatic disease: an evidence-based review

Spinal metastasis is a problem that afflicts many cancer patients. Traditionally, conventional fractionated radiation therapy and/or surgery have been the most common approaches for managing such patients. Through technical advances in radiotherapy, high dose radiation with extremely steep drop off can now be delivered to a limited target volume along the spine under image-guidance with very high precision. This procedure, known as stereotactic body radiosurgery, provides a technique to rapidly treat selected spinal metastasis patients with single- or limited-fraction treatments that have similar to superior efficacies compared with more established approaches. This review describes current treatment systems in use to deliver stereotactic body radiosurgery as well as results of some of the larger case series from a number of institutions that report outcomes of patients treated for spinal metastatic disease. These series include nearly 1400 patients and report a cumulative local control rate of 90% with myelopathy risk that is significantly less than 1%. Based on this comprehensive review of the literature, we believe that stereotactic body radiosurgery is an established treatment modality for patients with spinal metastatic disease that is both safe and highly effective.

Dosimetric effects of manual cone-beam CT (CBCT) matching for spinal radiosurgery: our experience

Radiosurgical treatment of cranial or extracranial targets demands accurate positioning of the isocenter at the beam and table isocenter, and immobilization of the target during treatment. For spinal radiosurgery, the standard approach involves matching of cone‐beam CT (CBCT) in‐room images with the planning CT (pCT) to determine translation and yaw corrections. The purpose of this study was to assess the accuracy of these techniques compared to advanced automatching using mutual information metrics, with consideration given to volume of interest (VOI) and optimizing translations and rotations in all axes. The dosimetric consequences of our current standard matching techniques were also evaluated. Ten consecutive spinal radiosurgery patients treated in the last year were subjected to analysis. For purposes of this analysis, the automatch using mutual information and a VOI was considered to create “the true isocenter” for positioning the patients. Review of the imaging from this automatch confirmed perfect superimposition of the two datasets within the VOI. Matching the CBCT to the pCT using the automatch allowed assessment of the rotations which had been previously ignored. Recalculation of the dose volume histogram was undertaken for each patient, assuming displacement of the true isocenter to the treated isocenter. Comparisons between the delivered doses and the intended doses were made. The mean absolute lateral/vertical/longitudinal translations and vector displacement between the manual CBCT‐pCT matching isocenter and the true isocenter were 0.13, −0.05, and −0.39 mm, with a minimum and maximum individual pixel vector shift of 3.2 and 8.94 mm. The mean pitch, yaw, and roll correction for automatch was −0.30°, 0.25°, and 0.97° with a maximum of 1.65°, 2.92°, and 1.43°. Four of ten patients had a significant change in the coverage of the tumor due to lack of correction of translational and rotational errors. The largest errors were observed in patients with small and irregular target volumes. Our initial results show that precise positioning for spinal radiosurgery cannot be accomplished with manual pCT‐CBCT matching without a clinical strategy to compensate for rotations. In the absence of this, significant underdosing of the tumor may occur.

PACS number: 87.55.Qr, 87.57.uq, 87.55.km

Dosimetric performance of the new high-definition multileaf collimator for intracranial stereotactic radiosurgery

The objective was to evaluate the performance of a high-definition multileaf collimator (MLC) of 2.5 mm leaf width (MLC2.5) and compare to standard 5 mm leaf width MLC (MLC5) for the treatment of intracranial lesions using dynamic conformal arcs (DCA) technique with a dedicated radiosurgery linear accelerator. Simulated cases of spherical targets were created to study solely the effect of target volume size on the performance of the two MLC systems independent of target shape complexity. In addition, 43 patients previously treated for intracranial lesions in our institution were retrospectively planned using DCA technique with MLC2.5 and MLC5 systems. The gross tumor volume ranged from 0.07 to 40.57 cm3 with an average volume of 5.9 cm3. All treatment parameters were kept the same for both MLC-based plans. The plan evaluation was performed using figures of merits (FOM) for a rapid and objective assessment on the quality of the two treatment plans for MLC2.5 and MLC5. The prescription isodose surface was selected as the greatest isodose surface covering >or= 95% of the target volume and delivering 95% of the prescription dose to 99% of target volume. A Conformity Index (CI) and conformity distance index (CDI) were used to quantifying the dose conformity to a target volume. To assess normal tissue sparing, a normal tissue difference (NTD) was defined as the difference between the volume of normal tissue receiving a certain dose utilizing MLC5 and the volume receiving the same dose using MLC2.5. The CI and normal tissue sparing for the simulated spherical targets were better with the MLC2.5 as compared to MLC5. For the clinical patients, the CI and CDI results indicated that the MLC2.5 provides better treatment conformity than MLC5 even at large target volumes. The CI's range was 1.15 to 2.44 with a median of 1.59 for MLC2.5 compared to 1.60-2.85 with a median of 1.71 for MLC5. Improved normal tissue sparing was also observed for MLC2.5 over MLC5, with the NTD always positive, indicating improvement, and ranging from 0.1 to 8.3 for normal tissue receiving 50% (NTV50), 70% (NTV70) and 90% (NTV90) of the prescription dose. The MLC2.5 has a dosimetric advantage over the MLC5 in Linac-based radiosurgery using DCA method for intracranial lesions, both in treatment conformity and normal tissue sparing when target shape complexity increases.

Early clinical experience with kilovoltage image-guided radiation therapy for interfraction motion management

Interest in image-guided radiation therapy (IGRT) reflects the desire to minimize interfraction positioning variability. Using a kilovoltage (kV) imaging unit mounted to a traditional LINAC allows daily matching of kV images to planning digitally reconstructed radiographs (DRRs). We quantify and evaluate the significance of calculated deviation from the intended isocenter. Since September 2004, 117 patients with various malignancies were treated using the On-Board Imaging (OBI) system, with 2088 treatment sessions. Patients were positioned by the treating therapist; orthogonal images were then obtained with the OBI unit. Couch shifts were made, aligning bony anatomy to the initial simulation image. Routine port films were performed weekly (after that day's OBI session). Ninety percent of all lateral, longitudinal, and vertical shifts were less than 0.8 cm, 0.6 cm, and 0.7 cm, respectively. The median vector shift for each anatomic site was: 0.42 cm for head and neck, 0.40 cm for CNS, 0.59 cm for GU/prostate, and 0.73 cm for breast; shift magnitude did not change with successive OBI sessions. The use of OBI effectively corrects setup variability. These shifts are typically small and random. The use of OBI likely can replace weekly port films for isocenter verification; however, OBI does not provide field shape verification.

IFPA meeting 2008 workshops report

Workshops are an important part of the IFPA annual meeting. At the IFPA meeting 2008 diverse topics were discussed in 12 themed workshops. Topics covered included: immunology of placentation; galectins and trophoblast invasion; signaling in implantation and invasion; markers to identify trophoblast subpopulations; placental pathology; placental toxicology; stereology; placental transport of fatty acids; placental mesenchymal stem cells; comparative placentation; trophoblast and neoplasia; trophoblast differentiation. This report is a summary of the various topics covered.