Christos Kanellitsas

Effect of edema, relative biological effectiveness, and dose heterogeneity on prostate brachytherapy

Many factors influence response in low-dose-rate (LDR) brachytherapy of prostate cancer. Among them, edema, relative biological effectiveness (RBE), and dose heterogeneity have not been fully modeled previously. In this work, the generalized linear-quadratic (LQ) model, extended to account for the effects of edema, RBE, and dose heterogeneity, was used to assess these factors and their combination effect. Published clinical data have shown that prostate edema after seed implant has a magnitude (ratio of post- to preimplant volume) of 1.3-2.0 and resolves exponentially with a half-life of 4-25 days over the duration of the implant dose delivery. Based on these parameters and a representative dose-volume histogram (DVH), we investigated the influence of edema on the implant dose distribution. The LQ parameters (alpha=0.15 Gy(-1) and alpha/beta=3.1 Gy) determined in earlier studies were used to calculate the equivalent uniform dose in 2 Gy fractions (EUD2) with respect to three effects: edema, RBE, and dose heterogeneity for 125I and 103Pd implants. The EUD2 analysis shows a negative effect of edema and dose heterogeneity on tumor cell killing because the prostate edema degrades the dose coverage to tumor target. For the representative DVH, the V100 (volume covered by 100% of prescription dose) decreases from 93% to 91% and 86%, and the D90 (dose covering 90% of target volume) decrease from 107% to 102% and 94% of prescription dose for 125I and 103Pd implants, respectively. Conversely, the RBE effect of LDR brachytherapy [versus external-beam radiotherapy (EBRT) and high-dose-rate (HDR) brachytherapy] enhances dose effect on tumor cell kill. In order to balance the negative effects of edema and dose heterogeneity, the RBE of prostate brachytherapy was determined to be approximately 1.2-1.4 for 125I and 1.3-1.6 for 103Pd implants. These RBE values are consistent with the RBE data published in the literature. These results may explain why in earlier modeling studies, when the effects of edema, dose heterogeneity, and RBE were all ignored simultaneously, prostate LDR brachytherapy was reported to show an overall similar dose effect as EBRT and HDR brachytherapy, which are independent of edema and RBE effects and have a better dose coverage.

Monte Carlo Shielding Analysis for an Accelerator Epithermal Neutron Irradiation Facility (AENIF) in a Conventional X-Ray Irradiation Room

An Accelerator Epithermal Neutron Irradiation Facility (AENIF) was proposed for BNCT[1]. The proposed AENIF consists of a low-energy high-current proton accelerator, a high energy beam transport system, a target assembly, and a moderator assembly. The accelerator generates a 30 mA-2.5 MeV proton beam. The protons strike the 7Li target and create neutrons via the 7Li(p,n)7Be reaction. The source neutrons, having an average energy of about 400 keV, are too energetic for BNCT. Therefore a moderator assembly is used to degrade the energy of the neutrons to epithermal energies (1 eV- 1 keV). It is the purpose of this study to investigate the dose equivalent to radiation therapy personnel, if the moderator assembly is placed inside a conventional x-ray irradiation room.

Theoretical Basis and Clinical Methodology for Stereotactic Interstitial Brain Tumor Irradiation Using Iododeoxyuridine as a Radiation Sensitizer and 145Sm as a Brachytherapy Source

A technique to produce radiation enhancement during interstitial brain tumor irradiation by using a radiation sensitizer (iododeoxyuridine-IdUrd) and by stimulation of Auger electron cascades through absorption of low-energy photons in iodine is described. Clinical studies using iododeoxyuridine, 192Ir as a brachytherapy source, and external radiation have produced promising results. Substituting 145Sm for 192Ir in this protocol is planned to evaluate the enhanced dose resulting from photon activation therapy.

SU‐FF‐T‐70: An Independent Verification of Dosimetry for the Leksell Gamma Knife® Treatment Planning System

Purpose: To develop an independent and reproducible verification of the dosimetry for the Leksell Gamma Knife® treatment planning system (Gamma Plan). Method and Materials: We have developed a simple method to independently compute point doses for a Gamma Knife® treatment. This was motivated by AAPM Report 46 (Task Group 40), which recommends independent calculation of the dose at one point in a treatment plan. Several authors have suggested methods to independently compute doses for Gamma Knife® treatments. Our method takes a different approach by utilizing measurements of the dose‐rate for an array of points within a polystyrene anthropomorphic phantom while keeping each of the points at the center of the Gamma Knife® focus, and normalizing the measurements to the total source activity. Multiple measurements of the dose‐rates were taken along the Z and Y axes throughout the phantom. We have created a program using the computer code MATLAB which calculates the dose delivered to any point within the patients skull. The program uses patient and treatment specific information from the treatment plan, as well as measured dose rates in the anthropomorphic phantom, to calculate delivered dose at selected points. Results: Validation of the program was initially performed using a CT of our anthropomorphic phantom that was used to create treatment plans. A maximum error of 2% was observed between point doses computed with our program to those by Gamma Plan. Patient treatment plans were also evaluated with the program, with an error no more than 4%. Conclusion: Our independent dose calculation method for the Gamma Knife® can be easily implemented into a PC based application and used to quickly verify treatment plans before treating patients.

SU‐FF‐T‐19: A Study of the Effects of Seed Migration On Prostate Post‐Seed Dose Plan Evaluation

Purpose:Brachytherapy using permanent seed implants has been an effective treatment for prostate cancer. It is a known fact that the seeds will migrate after implant, thus making the evaluation of long‐term (e.g. a few weeks) dose distribution difficult. We have performed a sensitivity analysis to determine the impact of seed migration on post‐seed dose plan evaluation parameters. Method and Materials: The CMS Interplant system and loose Pd‐103 seeds are used for the implant. The prostate is implanted with direct ultrasound visualization. CT scan and radiographs are taken 3 hours after the implant. Dosimetric studies are done using Nucletrons Theraplan Plus treatment planning system based on CTimages. The migrations of the seeds are randomly modeled according to Gaussian distribution. The mean migration is taken to be 0.5 cm and the sigma to be 0.25 cm. These numbers are close to latest clinical observations published. To test the sensitivity, an extreme case with mean migration of 1.0 cm and sigma of 0.5 cm is also modeled and compared. Patients are divided into 3 groups according to the prostate size. For each patient, the seed locations are modeled 10 times for a given mean migration, and the resulting 10 DVHs from the 10 trials are compared. Data are summarized according to prostate size and mean migration. Results: Preliminary results shows that D100 and D90 for prostate change about 3.5% and 2.5% respectively, V100 for prostate changes by about 25%, V100 and V150 for urethra change by about 20% and 50% respectively, and V50 for rectum changes by about 15%. Conclusion:Dose distribution changes as seed migrate, especially for the volume coverage. This effect should be taken into account when evaluating post‐seed plans. Quantitative knowledge of this effect may also factor into the seed distribution in the pre‐plan.

Single-Scan Stereotactic Tumor Biopsy and Brachytherapy

Stereotactic tumor biopsy and brachytherapy catheter implantation can be accomplished with targets derived from computed axial tomography and magnetic resonance scans. Computer manipulation of image data allows both diagnostic and therapeutic procedures to be carried out from a single set of scan slices. This eliminates the need for repeat scanning as part of the surgical procedure. Microcomputer technology is sufficiently advanced to handle the images and graphics necessary for stereotactic neurosurgery. A system based on the IBM PC/AT designed for this purpose uses readily available graphics software and custom-designed imaging programs. Direct loading of computed axial or magnetic resonance scan images from magnetic tape can be accomplished. Determination of points, contours and volumes in three-dimensional space allows intraoperative alignment of image data and patient landmarks within the stereotactic head frame using pattern recognition overlays. Three-axis scaling for magnification correction along with rotational and linear data transformations provide the basis for single-scan stereotaxis. Interactive computer graphics integrate image, patient and frame coordinates for target determination. This method eliminates the need to design and fabricate nonmagnetic or radiolucent scanner-compatible devices.