SU-E-J-139: Feasibility of Using EPID for Real-Time Target Localization during Treatment

PURPOSE

This study aims to investigate the feasibility of using the images of the treatment fields acquired by an electronic portal imaging device (EPID) for real-time target localization.

METHODS

Forty one patients treated with IMRT and RapidArc were recruited in this study including 37 prostate patients and 4 lung patients. These patients were grouped as: prostate IMRT with lymph node (n=14), prostate IMRT without lymph node (n=17), prostate RapidArc (n=6), and lung IMRT (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, and four fiducial markers during the treatment was recorded.

RESULTS

For the prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%-59%), 39% (23%-51%), 24% (7%-38%), and 12% (4%-29%), respectively. For the prostate IMRT patients with lymph nodes, the probability was 41% (24%-51%), 29% (12%-42%), 15% (3%-24%), and 7% (4%-15%), respectively. For prostate RapidArc treatments using two arcs, the average probability of seeing at least one fiducial marker was 81% (58%-90%) for the full arc and 74% (53%-94%) for the partial arc. For the lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%-52%).

CONCLUSIONS

The continuous image acquisition from the EPID during the treatment provides sufficient target movement information for real-time target localization and intrafractional target motion correction for advanced radiotherapy treatments.

WE-G-BRB-01: Per-Frame Analysis of MatriXX Data

PURPOSE

One of the most widely used IMRT QA devices (MatriXX) is an array of ionization chambers which are periodically read during plan delivery. Although the ionization chambers are not expected to exhibit strong angular dependence, the measured dose distribution is often found to significantly differ from the planned dose distribution. We identify the origin of all factors that affect the measurement accuracy of the MatriXX and develop a per-frame post-processing strategy that reduces their impact on the passing rate of IMRT and VMAT plans.

METHODS

We developed software that reads the dose frame sequence recorded by the MatriXX and applies a number of correction factors to each frame. Angular correction factors are computed as ratios of measured dose at the isocenter of the phantom and planned dose at the same location for all clinically used photon energies. For every clinical case, the recorded movie file is read and the dose for every frame is corrected according to the angle of the beam. In addition, the background evolution is tracked in the 'beam-off' frames which are subsequently subtracted from the 'beam-on' frames according to a predictive model. Machine output correction is also implemented, which significantly improves the absolute dose measurements. The IMRT effective plane of measurement of the MatriXX was identified and found not to coincide with the isocentric plane.

RESULTS

The clinical passing rates are significantly improved when the per-frame analysis software was introduced in our IMRT QA procedure. For a group of 800 patients with no corrections the average passing rate was 93.6%, while for the first 300 cases with per frame corrections the average passing rate was 97.3%.

CONCLUSIONS

We identify all factors that impact the measurement accuracy of the MatriXX (angular effects, background evolution, machine output, plane of measurement) and propose a strategy for their elimination.

SU-E-T-441: A Feasibility Study to Replace Electron Cutouts with a Motorized Electron Multileaf Collimator

PURPOSE

Fabrication of electron beam cutouts not only is a time consuming process but also involves the handling of cerrobend which is a toxic material. Hospital workers involved in cutout construction can actually be exposed to toxic fumes that are usually generated during the process. The aim of this work is to study the feasibility of replacing electron cutouts with our prototype motorized electron multileaf collimator (eMLC).

METHODS

Electron beams collimated by an eMLC have very similar penumbra to those collimated by applicators and cutouts as we already demonstrated in a previous study. However undulation of the isodose curves is expected due to the finite size of the eMLC. This may be a problem when the field edge is close to critical structure. Thus ten different breast cases that were previously treated with an electron boost were selected from our database. An inhouse Monte Carlo based treatment planning system were used for dose calculation using the patients CTs. For each patient two plans were generated one with electron beams collimated using the applicator/cutout combination and the other plan with beams collimated only by the eMLC. Treatment plan quality was compared for each patient based on dose distribution and dose volume histogram. In order to determine the optimal position of the leaves, the impact of the different leaf positioning strategies were investigated.

RESULTS

Results have shown that target coverage and critical structure sparing can be effectively achieved by electron beams collimated by eMLC. Preliminary results have shown that the out-of-field strategy is most conservative and would be the recommended method to define the actual leaf position for the eMLC defined field.

CONCLUSION

The eMLC represents an effective time saving and pollution free device that can completely eliminate the need for patient specific cutouts. This work has been supported by a UICC American Cancer Society Beginning Investigators Fellowship funded by the American Cancer Society.

SU-E-T-457: Multi-Mode Model - A Consistent Approach for Conventionally Fractionated Radiotherapy and Stereotactic Body Radiotherapy

PURPOSE

The accuracy of dosimetric analysis and outcome comparison between conventionally fractionated radiation therapy(CFRT) and stereotactic body radiotherapy(SBRT) requires reliable radiobiological modeling. The aim of this work was to further improve the multi-mode model(MMM) for both CFRT and SBRT.

METHODS

MMM assumes the existence of different modes of cell killing as a Result of radiation damage to different parts of a cell, e.g., a single severe damage to the DNA or two or more small damages to the membrane or DNA. The cell survival probability can then be calculated by s=Product_(i=1,n){1-(1-ê(-d/Di))̂(i)}, where i represents the i-th mode of cell killing that requires i potentially unrepairable damages to the cell as a result of radiation dose d and Di is the dose that gives 63% probability to cause an unrepairable damage for the i-th mode. The dose rate effect is included in MMM assuming 1/D_i=(k_i- r_i/U), where k_i is the radiation damage rate, r_i the repair rate and U the dose rate. The low-dose hypersensitivity is also included in the new model.

RESULTS

A comparison of the goodness-of-fit of the LQ, multitarget, USC and MMM to the survival curve of the H460 non-small-cell lung cancer cell line showed the same agreement between USC and MMM with the survival data, which was significantly better than the fits to the LQ and multitarget models. The parameters used for the LQ, multitarget and USC models were alpha=0.33Gy, alpha/beta=10Gy, D_T =6.2Gy, D_0 =1.25Gy and D_q =1.8Gy. The parameters for MMM(n=4) were D_1=4.0Gy, D_2=4.01Gy, D_3=3.08Gy and D_4=41Gy.

CONCLUSIONS

MMM offers a superior description of the mammalian cell survival curve in both conventional and ablative dose ranges, which can be used for designing new fractionation schemes and predicting and understanding treatment outcomes for both CFRT and SBRT.

SU-E-T-586: Comparison between CT-and FLT-PET-Defined Target Volumes and Dosimetry for Radiotherapy Planning in Esophageal Carcinoma

PURPOSE

To study the advantage of fused 3-deoxy-3-18F-fluorothymidine (FLT) positron emission tomography (PET) and computed tomography (CT) images on radiotherapy planning for esophageal carcinoma patients .

METHODS

Ten patients with esophageal carcinoma were referred for radiotherapy. Each patient underwent CT and FLT PET scan for simulation plan in the same treatment position. FLT PET images were coregistered with CT images. GTVCT was initially performed on CT images, GTVPET-CT were automatically segmented on FLT PET/CT images using a fixed threshold values at 1.4. Two kinds of treatment plans were designed based on GTVCT and GTVPET-CT. The radiation dose was prescribed as 60 Gy in 30 fractions. The dose-volume parameters of target volume and normal tissues, CI and HI of based on the GTVPET- CT and GTVCT were compared.

RESULTS

The mean GTV delineated on CT and PET- CT were 39.28 cm^s and 34.48 cm^s , respectively . The values for mean lung dose, total-lung volume receiving =5, = 10, = 20 and = 30Gy;mean heart dose, heart volume receiving =30Gy and normal tissues of plan based on the GTVPET- CT were significant lower than plan based on the GTV-CT.

CONCLUSIONS

In our study, FLT PET use has an impact on target delineation for radiotherapy planning. GTVPET-CT is significantly smaller than GTVCT with a mean volume reduction of 16%. CT and FLT/PET image fusion appeared to have an advantage on dose reduction of organ at risk in radiotherapy planning of esophageal carcinoma.