SU-E-T-133: Isocenter Measurements with the Winston-Lutz Test: Impact on Treatment Planning

PURPOSE

To assure high accuracy in radiation delivery during linear accelerator based stereotactic radiosurgery procedures, a Winston-Lutz test is typically performed to measure the effective isocenter wander over the wide range of gantry and couch angles employed. Ideally, this deviation is lower than 1-1.5 mm depending on institutional tolerances. Automation of this test with portal imaging allows a large amount of data to be taken and processed. This study explores the effect of various collimation techniques on beam-center stability, and addresses one technique of repositioning in response.

METHODS

Winston-Lutz shots were obtained using a Trilogy linear accelerator, BrainLab Winston-Lutz pointer, and electronic portal imaging from a full range of gantry and couch angles. The test was repeated using MLCs, a 10mm stereotactic cone from BrainLab, and jaws for beam collimation. All images were processed using an in-house MATLAB-based program which calculates the distance between the tungsten ball and aperture center with accuracy within ∼0.1mm. Ideal shifts were calculated by minimizing this variation from gantry rotation alone, and performed manually.

RESULTS

The in-plane gantry sag was found to be on the order of ∼0.8mm, independent of collimation technique or setup. The MLC carriage had a ∼0.3mm cross-plane displacement to the inferior from the cone center, negated by a 90 degree collimator rotation. The total gantry misalignment was greatest near gantry 180 (∼1.5mm), varying by collimation method. Optimizing to minimize gantry error reduced errors to less than 0.7mm but increased the effect of couch rotation.

CONCLUSIONS

Isocenter evaluation is dependent on collimation method used, and should be conducted with the same technique as delivery. It may be useful to consider the varying delivery accuracy from different couch/gantry angles during treatment planning. Repositioning methods after couch rotation may affect ideal laser positions.

SU-E-T-426: Comparison of HDR Brachytherapy for Cervix Cancer Using an Adaptive Simulated Annealing Program and Oncentra- for Simultaneously Integrated Boost

PURPOSE

High dose rate (HDR) volumetric brachytherapy is an effective method of treating advanced cervix carcinoma. Local failure is associated with multiple factors including higher maximal standardized uptake value (SUV) values in fluorodeoxyglucose positron emission tomography (FDG- PET) scans. The purpose of this study is to evaluate the ability to simultaneously boost regions of high SUV values using an in-house adaptive simulated annealing (ASA) algorithm and the Oncentra® (Nucletron V.B., Veenendaal, The Netherlands) treatment planning system, thereby potentially improving local control.

METHODS

Five cervix cancers were evaluated for brachytherapy treatment (tandem/ring and/or interstitial needles). MRI and PET images were obtained post-implant and fused with treatment planning CTs to define a high-risk (HR) CTV (cervix and tumor on MRI) and GTV (volume with >50% of the maximum SUV on PET). The prescribed dose was 5-6 Gy to the HR CTV and 7-9 Gy to the GTV. Treatment plans were first generated in Oncentra® with IPSA followed by manual graphic optimization by the physician. Plans were also independently optimized using the ASA program. The two plans were compared side by side and one was chosen for treatment. Dose-volume parameters including D90, V100 of targets, D2cc to the critical organs, and generalized equivalent uniform dose (gEUD) of all structures were compared between the ASA and the Oncentra plans.

RESULTS

Both ASA and Oncentra plans were considered acceptable by the physician in four of five cases. Two ASA plans were chosen due to better critical organ sparing and tumor coverage. Two Oncentra plans were preferred because of lower doses to critical organs. One ASA plan was not accepted because of a higher bowel dose.

CONCLUSIONS

Both ASA and Oncentra® planning methods produce acceptable treatment plans for optimized brachytherapy of cervix carcinoma. Continued studies are warranted to further determine the relative strength of each method. This study was supported in part by a research grant from Varian Medical System Inc.

Sparing of the penile bulb and proximal penile structures with intensity-modulated radiation therapy for prostate cancer

Quality of life is an important consideration in the treatment of early prostate cancer. Laboratory and clinical data suggest that higher radiation doses delivered to the bulb of penis and proximal penile structures correlates with higher rates of post-radiation impotence. The goal of this investigation was to determine if intensity-modulated radiation therapy (IMRT) spares dose to the penile bulb while maintaining coverage of the prostate. 10 consecutive patients with clinically organ confined prostate cancer were planned with 3D conformal radiation therapy (3D-CRT) or IMRT to give a dose of 74 Gy without specifically constraining the plans to spare the penile bulb. All 10 patients were ultimately treated with IMRT. Dose-volume histograms were evaluated and the doses to prostate, rectum, bladder and penile bulb were compared. IMRT reduced the mean penile bulb doses compared with 3D-CRT (33.2 Gy vs 48.9 Gy, p<0.001), the percentage of penile bulb receiving over 40 Gy (37.7% vs 67.2%, p<0.001) and the dose received by >95% of penile bulb (5.3 Gy vs 11.7 Gy, p=0.003). Maximum penile bulb doses were higher with IMRT (81.2 Gy vs 73.1 Gy, p<0.001) although the volume of this high dose region was small. Both methods resulted in similar coverage of the prostate. The volume of rectum receiving 70 Gy was significantly reduced with IMRT (18.4% vs 21.9%, p=0.003) but the volumes of bladder receiving 70 Gy were similar (p=0.3). IMRT may potentially reduce long term sexual morbidity by reducing the dose to the majority of the penile bulb.

Ex vivo expansion and genetic marking of primitive human and baboon hematopoietic cells

The achievement of positive outcomes in many clinical protocols involving hematopoietic stem cells (HSCs) has been handicapped by the limited numbers of marrow repopulating cells available to actually bring about therapy. This insufficiency has been especially problematic in stem cell transplantation and gene therapy. A number of studies have been initiated to attempt expansion of HSCs, mainly by manipulation of key cytokines in cell suspension cultures. Unfortunately, these expansion methods usually lead to altered properties in the amplified cells, mainly by reducing their self-renewal and multi-lineage differentiative potentials. Here we discuss our ongoing work, utilizing a unique endothelial cell line that supports primitive hematopoiesis, to attempt to generate expansion of primate HSCs that retain their elementary properties. Genetic marking of early hematopoietic cells to facilitate tracking will be mentioned as will the development and employment of assay systems designed to evaluate the long-term functional attributes of the expanded cells.

Ex vivo expansion of human hematopoietic stem cells: implications for the modern blood bank

Pluripotent hematopoietic stem cells (PHSC) are rare cells within the marrow that are capable of self-renewal and differentiation into multiple hematopoietic lineages. Following myeloablative chemotherapy and radiation therapy and marrow transplantation, hematological reconstitution occurs after a period of 2-3 weeks. Recently, a number of laboratories have shown that both early and delayed phases of engraftment are mediated by PHSC within a graft and that engraftment can be accelerated by transplanting greater numbers of PHSC. Increasing efforts have been directed, therefore, towards developing methods to expand PHSC ex vivo. In this report, we describe an endothelial cell-based culture system to which exogenous cytokines are added which appears to permit the ex vivo expansion of PHSC. Refinement of these technologies will potentially have a major impact on the ability of blood banks to improve the quality of hematopoietic stem cell grafts.