Doo Hyun Lee

Dosimetric comparison of four different external beam partial breast irradiation techniques: Three-dimensional conformal radiotherapy, intensity-modulated radiotherapy, helical tomotherapy, and proton beam therapy

BACKGROUND AND PURPOSE As an alternative to whole breast irradiation in early breast cancer, a variety of accelerated partial breast irradiation (APBI) techniques have been investigated. The purpose of our study is to compare the dosimetry of four different external beam APBI (EB-APBI) plans: three-dimensional conformal radiation therapy (3D-CRT), intensity-modulated radiation therapy (IMRT), helical tomotherapy (TOMO), and proton beam therapy (PBT). METHODS AND MATERIALS Thirty patients were included in the study, and plans for four techniques were developed for each patient. A total dose of 30Gy in 6Gy fractions once daily was prescribed in all treatment plans. RESULTS In the analysis of the non-PTV breast volume that was delivered 50% of the prescribed dose (PD), PBT (mean: 16.5%) was superior to TOMO (mean: 22.8%), IMRT (mean: 33.3%), and 3D-CRT (mean: 40.9%) (p<0.001). The average ipsilateral lung volume percentage receiving 20% of the PD was significantly lower in PBT (0.4%) and IMRT (2.3%) compared with 3D-CRT (6.0%) and TOMO (14.2%) (p<0.001). The average heart volume percentage receiving 20% and 10% of the PD in left-sided breast cancer (N=19) was significantly larger with TOMO (8.0%, 19.4%) compared to 3D-CRT (1.5%, 3.1%), IMRT (1.2%, 4.0%), and PBT (0%, 0%) (p<0.001). CONCLUSIONS All four EB-APBI techniques showed acceptable coverage of the PTV. However, effective non-PTV breast sparing was achieved at the cost of considerable dose exposure to the lung and heart in TOMO.

Simultaneous Integrated Boost Intensity-Modulated Radiotherapy in Patients With High-Grade Gliomas

PURPOSE We analyzed outcomes of simultaneous integrated boost (SIB) intensity-modulated radiotherapy (IMRT) in patients with high-grade gliomas, compared with a literature review. METHODS AND MATERIALS Forty consecutive patients (WHO grade III, 14 patients; grade IV, 26 patients) treated with SIB-IMRT were analyzed. A dose of 2.0 Gy was delivered to the planning target volume with a SIB of 0.4 Gy to the gross tumor volume with a total dose of 60 Gy to the gross tumor volume and 50 Gy to the planning target volume in 25 fractions during 5 weeks. Twenty patients received temozolomide chemotherapy. RESULTS At a median follow-up of 13.4 months (range, 3.7-55.9 months), median survival was 14.8 months. One- and 2-year survival rates were 78% and 65%, respectively, for patients with grade III tumors and 56% and 31%, respectively, for patients with grade IV tumors. Age (≤50 vs. >50), grade (III vs. IV), subtype (astrocytoma vs. oligodendroglioma or mixed), and a Zubrod performance score (0-1 vs. >2) were predictive of survival. Of 25 (63%) patients who had recurrences, 17 patients had local failure, 9 patients had regional failure, and 1 patient had distant metastasis. Toxicities were acceptable. CONCLUSIONS SIB-IMRT with the dose/fractionation used in this study is feasible and safe, with a survival outcome similar to the historical control. The shortening of treatment time by using SIB-IMRT may be of value, although further investigation is warranted to prove its survival advantage.

Performance evaluation of field-in-field technique for tangential breast irradiation.

Conventional hard or dynamic wedge systems are commonly applied to reduce the dose inhomogeneity associated with whole breast irradiation. We evaluated the dosimetric benefits of the field-in-field (FIF) technique by comparing it with the electronic compensator (EC), Varian enhanced dynamic wedge (EW) and conventional hard wedge (HW) techniques. Data were obtained from 12 patients who had undergone breast-conserving surgery (six left-sided and six right-sided). For these patients, the average breast planning target volume (PTV) was 447.4 cm(3) (range, 211.6-711.8 cm(3)). For the experiments, a 6 MV photon beam from a Varian 21 EX was used, the HW and EW angles were applied from 15 to 45 degrees, while 40-50% isodose values were chosen to achieve the best dose distribution for electronic compensation. In applying the FIF technique, we used two or three subfields for each portal. To evaluate the performance for each planning technique, we analysed a dose-volume histogram (DVH) for the PTV and organs-at-risk (OARs). To evaluate the effects of these techniques on dose inhomogeneity, we defined the PTV Dose Improvement (PDI) index, which was derived from a PTV volume between 97-103% of the differential DVHs. In addition, we compared the average monitor units (MUs) for each technique. The average PDI index with FIF is 76.4%, while the PDI indices for other treatments were 65.8, 41.8 and 50.9% for EC, EW and HW, respectively. This study demonstrated an improved performance using the FIF technique compared with the conventional HW/EW system, as well as a new modality for EC. We demonstrated that FIF is a very useful technique for improving PTV conformity, while protecting the OARs from breast tangential irradiation.

PROTON RANGE UNCERTAINTY DUE TO BONE CEMENT INJECTED INTO THE VERTEBRA IN RADIATION THERAPY PLANNING

We wanted to evaluate the influence of bone cement on the proton range and to derive a conversion factor predicting the range shift by correcting distorted computed tomography (CT) data as a reference to determine whether the correction is needed. Two CT datasets were obtained with and without a bone cement disk placed in a water phantom. Treatment planning was performed on a set of uncorrected CT images with the bone cement disk, and the verification plan was applied to the same set of CT images with an effective CT number for the bone cement disk. The effective CT number was determined by measuring the actual proton range with the bone cement disk. The effects of CT number, thicknesses, and position of bone cement on the proton range were evaluated in the treatment planning system (TPS) to draw a conversion factor predicting the range shift by correcting the CT number of bone cement. The effective CT number of bone cement was 260 Hounsfield units (HU). The calculated proton range for native CT data was significantly shorter than the measured proton range. However, the calculated range for the corrected CT data with the effective CT number coincided exactly with the measured range. The conversion factor was 209.6 [HU · cm/mm] for bone cement and predicted the range shift by approximately correcting the CT number. We found that the heterogeneity of bone cement could cause incorrect proton ranges in treatment plans using CT images. With an effective CT number of bone cement derived from the proton range and relative stopping power, a more actual proton range could be calculated in the TPS. The conversion factor could predict the necessity for CT data correction with sufficient accuracy.

Effect of radiation scattering on dose uniformity in open and closed cell culture vessels

Purpose: Dose uniformity in cell culture vessels such as Petri dishes and anoxic irradiation chambers is very important in radiobiological work as dose uniformity affects cell survival probabilities. In this study, we investigated X-ray dose inhomogeneity, caused by scattering, in typical culture vessels. Materials and methods: Three different cubic cell culture vessels, with side lengths of 10 cm, 15 cm and 20 cm, were designed and irradiated by X-rays of 6 MV and 15 MV at a source-to-surface distance (SSD) of 100 cm using a Varian 2100CD linear accelerator. Results: The relative X-ray dose distribution in a cell culture vessel depended strongly on whether the vessel had a lid. The percentage of the cell culture surface with the dose differing by more than 10% from the mean value of the dose was 43.4% in lidless vessels and 9.7% in lidded vessels. Conclusions: In radiobiological work, X-ray dose inhomogeneity within a cell culture vessel is not negligible and the placement of cells in the vessel should be carefully considered.

Dosimetric Verification of Dynamic Conformal Arc Radiotherapy using the Optimization Algorithm

The purpose of this study is to develop the optimization method for adjusting the shift of film isocenter and to suggest the quantitative criteria for film dosimetry after optimization in the dynamic conformal arc radiation therapy (DCAR). The DCAR planning was performed with 10 patients of brain metastasis. Both absolute dosimetry with ion chamber and relative film dosimetry were performed throughout the. An optimization method for obtaining the global minimum was used to adjust the error due to the shift of film isocenter, which consists of the largest part of systematic errors. The average difference of point dose between measured value using ion chamber and calculated value acquired from planning system was 0.56±0.36% and maximum reached 1.14% with absolute dosimetry. The mean of dose difference before and after optimization was 1.79±0.34% and 1.44±0.33%, respectively and the average of percentage showing over 5% dose difference were 5.61±3.69% and 0.32±0.51%, respectively. After optimization, the dose differences decreased dramatically and the percentage showing over 5% dose difference and average dose difference was less than 2%. Our results show that this optimization method is effective in adjusting the error in isocenter and the quantitative criteria is accurate and useful in clinical application of dosimetric verification using film dosimetry.

X-ray scattering effect on the heterogeneous dose distribution in a cell box

The dose distribution in a cell box is very important factor for the radiobiology experiment since it affects the cell survival probabilities. When the x-ray is irradiated to the closed (or sealed) cell box, which is frequently used for the experiment of oxygen enhancement ratio (OER) research, it is possible that x-ray scattering effect by cell boxes may cause the inhomogeneous dose distribution within a cell box. In this study, we have investigated the inhomogeneity of the dose distribution with three different sizes of cubical cell boxes, whose side lengths are 10, 15 and 20 cm. The difference of the doses at the center of 10 cm box and 20 cm box was measured to be less than 2 % for both 6 MV and 15 MV x-ray irradiation. That indicates the absolute dose at the center of cell box doesn’t much depend on the size of cell box. Unlike the absolute dose value at the center of box, the relative dose distribution in a cell box has the strong dependency upon the condition of the cell box. The results show that the percentage showing more than 10 % dose difference to the average dose value goes up to ∼ 40 %. Our experimental evidence suggests that when the closed box is used for the radiobiological experiment, the dose inhomogeneity within the cell box is not negligible and the location of the cells within a box should be carefully designed.