Ⅰ.INTRODUCTION

The use of 3D imaging such as tomotherapy, gamma-knife, intensity-modulated radiotherapy (IMRT), cyber-knife and stereotactic radiosurgery has been markedly increased recently in the field of radiotherapy⁽¹⁾. Tomotherapy among them, unlike IMRT u sing t he f ixed b eams, uses b eam which intensity is controlled during the rotation. The dose distribution of tomotherapy is characterized by a 360° rotation of the gantry, accompanied by the irradiation from 51 directions in a single-session of the rotation. Because these mechanical characteristics of the equipment, treatment plans of tomotherapy presented an excellent result for conformality, homogeneity and the protection of the normal tissue as compared with a conventional type of IMRT treatment plans. In rotational radiotherapy including the tomotherapy, however, there is an overall distribution of a low-dose area around the target sites. If there is a great amount of the prescription dose like radiosurgery, the treatment dose in a low-dose field is also found to be relatively higher. Hence, the calculation and measurement of the treatment dose in these peripheral areas are as important as the accuracy of the target sites^(2,3).

An accurate and systematic quality assurance (QA) is necessary for the successful performance of radiotherapy. It is classified into the QA associated with the treatment equipment and plan and patient delivery quality assurance (DQA). Equipment QA relates to radiotherapy equipment including multileaf collimator and has connection with systematic errors. To minimize these errors, regular QA is needed. Moreover, the patient DQA must proceed obligatorily for the prediction and prevention of errors that might arise from the radiotherapy of patients. Prior to the radiotherapy, patient DQA can be classified into two classes. First, the same beam as the actual treatment is applied to the phantom depending on the methods of the irradiation. Second, individual analysis depending on each irradiation was performed. In addition, there are mainly using absolute dose evaluation of measuring point dose to the target of radiotherapy and the relative dose evaluation of the treatment plans for radiotherapy^(4-7).

Radiographies film was generally used for the 2-dimensional dose analysis because it has advantages which is a higher spatial resolution for the dose analysis of beam and are possible to perform the measurement for large field by one irradiation and such as low cost. But there are also problems that can’t be checked the real-time availability for measurement results and a variety of variations arising in photographic developing⁽⁸⁾. Recently, the studies have been conducted using 2D ionization chamber array and 2D diode array according as the interest is increasing for the simplification and the accurate measurement of QA. Both studies are valuable in promptly performing the measurement and real-time analysis without photographic developing. But the 2D ionization chamber array remains at the weak point for the dose measurement of radiotherapy where there are marked changes in the dose perturbation because it has a lower resolution. The 2D diode array shows limitations such as a data loss in the lateral beam of the target. These equipment are mainly used for the 2-dimensional dose measurement on the plane. They, however, still show limitation on the accurate dose measurement in overall radiological areas^(9,10). Furthermore, the 3D dose measurement has been performed using a polymer gel and MRI until recently for the assessment of dose distribution in the overall areas where irradiations are applied. The methods of dose analysis using gel could be visually confirmed transparent gel such as a white cloud because microscopic particles of gel would be scattered if there are irradiations in tissue equivalent gel. Depending on the procedure of the analysis, however, at least 24 hours are required for the measurement time of MRI and the result of dose measurement. It remains as the major weakness. Moreover, it is also possible that there might be a serious measurement error depending on the accuracy of the image fusion⁽¹¹⁾. To overcome these limitations, various, 3D volume phantoms have been developed, including Delta4 phantom (Scandidos, Uppsala, Sweden), Octavius (PTW, Freiburg, Germery) and ArcCHECK^TM(Sun Nuclear, USA). In particular, the ArcCHECK^TM phantom has been fabricated as a cylinder shape. Thus, it is able to measure the beams to multiple directions. Moreover, it is also useful in 3D assessing dose measurement of arc therapy such as conventional IMRT, VMAT, Rapid Arc and Tomotherapy^(12-14). Currently, these methods for dose measurement in radiotherapy are widely used. In recent years, however, there has been increased work load for the patient DQA according to developed novel methods of radiotherapy. To improve these shortcomings, there has also been an increase interest in the reproduction of the accurate QA technology. In addition, the efficient patient DQA should be possible that simple operation and short time for analysis alongside with an accurate ability dose verification. In this study, we performed an assessment accuracy of dose evaluation and necessity of patient DQA using the evaluation of the point dose using ion chamber and the evaluation of the dose using the 3D volume phantom instead of film in radiosurgery with tomotherapy.

Ⅱ.MATERIALS AND METHODS

1.Treatment plans

For the assessment of patient DQA using 3D volume phantom, the tomotherapy (Accuray, Hi ART, USA) was used. To establish the treatment plans for the assessment of patient DQA, the RANDO phantom (Radiology Support Devices, USA) and CT (Light Speed^TMRT16,GE,USA) to set 1.25 mm slice thickness were followed by the image acquisition. The volume of interest (VOI) was drawn with a globular shape which diameter is 0.5, 1, 1.5, 2, 2.5 and 3 cm respectively in RANDO phantom. This was followed by the transmission of prescription dose 1,200 cGy at target volume 95%. The reverse direction treatment plans were established using planning station 4.0.4 (Accuray, Hi ART, USA), setting the following conditions: the field width 1.05 cm, MP (modulation factor) 2.5, pitch 0.123 and calculation grid is fine. The volume of target was set at 0.06, 0.51, 1.70, 4.05, 8.01 and 13.64 cc. To perform the patient DQA, treatment plans calculated in the RANDO phantom were mapped onto the cheese phantom and ArcCHECK phantom, respectively. This was followed calculation of dose once again (Fig. 1, Table 1).

2.Equipment for the measurement

To measure the absolute dose of the phantom, ion chamb er (A1SL, S tandard Imaging, USA), micro i on chamber (Exradin A16, Standard Imaging, USA) and tomoelectrometer (Standard Imaging, USA) were used. Gafchromic EBT2 film (International specialty products, Wayne, USA) was attached to a cheese phantom for the 2-dimensional evaluation of dose distribution. The measured film was digitized using VIDER scanner (VXR-16. Vider System. USA). In addition, the ArcCHECK phantom (Sun nuclear, USA) was used for the 3D measurement of dose distribution. CT scan imaging of ArcCHECK phantom and contoured DICOM RT files for imaging of treatment plans was mapped. This was followed by the calculation of dose distribution in the areas where a diode was located. Thus, it was spread with a diode map. Thereafter, the dose evaluation was assessed following a comparative analysis of dose distribution based on the treatment plans depending on the correlation with dose of the measured diode (Fig. 2, 3).

3.The evaluation of the dose calculation accuracy

To assess the accuracy in measuring the absolute dose for Cheese phantom and ArcCHECK phantom, each phantom was compared to the values of dose at the same locations as well as measurements obtained from the irradiated volume in the areas where an ion chamb er w as p laced. T he m easurement u sing two phantoms was repeatedly performed five times for each volume to validate the reproducibility. Measurements of the dose distribution were evaluated using analytical software of the system. The measurement of t he a b solute d ose using an i on chamb er w as c onverted into t he a b sorption dose according to the TG-51 protocol for the dose calibration[2]. The ɤ-index was used to evaluate a quantitative analysis of whether the agreement between the dose distribution of treatment plans and the measured one. There is an agreement between dose distribution of the treatment plans and the measured one at a ɤ-index of < 1 using dual acceptance criteria method for distance-to agreement (DTA) in high-dose gradient region and dose difference in low-dose gradient region. For evaluation of whether the agreement for point dose evaluation and dose distribution for both phantoms, criteria for acceptance of 3%/3 mm and 2%/2 mm were applied. This was followed by the comparison of the gamma passing ratio of the criteria irradiation volume and the mean irradiated volume for the error measurement. Thus, the percentage of pixels that was lower than the gamma-value of 1 (ɤ %<1) was analyzed (Fig.4, 5).

Ⅲ.RESULTS

Using an ion chamber, the point dose for a cheese phantom was measured. The error on the overall target was ob served a t 0.76±0.59%. The error was greatest value with 1.77±0.48% in the target with a diameter of 0.5 cm. In addition, it was the lowest value to 0.28±0.24% at a diameter of 2.5 cm. Following the measurement of the point dose for the ArcCHECK phantom, the overall error of the target was 1.37±0.76%. The error was greatest value with 1.88±0.54% in the target with a diameter of 1.5 cm. In addition, it was the lowest value with 0.63±0.38% at a diameter of 1.0 cm. Thus, it was confirmed that the rate of error of the prescribed dose and the measured dose for the Cheese phantom and ArcCHECK phantom was within the range of the error (3%) (Fig. 6).

The evaluation of dose distribution was assessed using gafchromic EBT2 film and ArcCHECK phantom. The passing ratio at 3%/3 mm was 99.26±0.01% in the ArcCHECK phantom. Moreover, the passing ratio at 3%/3 mm was 97.72±0.02% of the gafchromic EBT2 film. This showed that the passing ratio was higher in the ArcCHECK phantom compared to the gafchromic EBT2 film. The passing ratio at 2%/2 mm was 94.21±0.02% of the gafchromic EBT2 film, which was higher than 93.02±0.01% seen in the ArcCHECK phantom. The standard deviation of the passing ratio was greater in the gafchromic EBT2 film as compared with the ArcCHECK phantom (Fig.7, 8).

Ⅳ.DISCUSSION

The calculation and measurement of dose in the peripheral areas are as important as the accuracy of target sites in rotational radiotherapy such as tomotherapy[15]. Depending on the methods of irradiation, each patient DQA of IMRT is classified into the evaluation of the integrated dose distribution and the evaluation of individual irradiation. However, each patient DQA in tomotherapy is evaluated through an analysis of the integrated dose distribution according to radiotherapy property.

In this study, the absolute and relative dose was evaluated using cheese phantom and ArcCHECK phantom for the assessment of patient DQA in tomotherapy. Following the measurement depending on the size of the target, it confirmed that the error ratio (3%) was within the range of the mean error. Moreover, the assessment of ɤ-index, served as a relative dose evaluation, showed that the mean passing ratio was 94.58% at 3%/3 mm and a target size of 2.0 cm in the gafchromic EBT2 film. Thus, mean passing ratio was lower than 95% and standard deviation was 0.0386, being the greatest value. The mean passing ratio was 96.91 at 2%/2 mm, diameter of 0.5 cm, being the greatest value. At a diameter of 2 cm, it was 90.84%, being the smallest value. Thus, the mean passing ratio was higher than 90%. Following the assessment of ɤ-index using the ArcCHECK phantom, the mean value was 99.24% at 3%/3 mm and 93.02% at 2%/2 mm. The standard deviation in the assessment of ɤ-index was 0.0174 at 3%/3 mm and 0.00207 at 2%/2 mm on the gafchromic EBT2 film. The standard deviation was 0.0069 and 0.0112 respectively in ArcCHECK phantom. This indicates that the gafchormic EBT2 film had a relatively greater standard deviation.

As a result of this assessment, it was found that gafchormic EBT2 film had a higher degree of passing ratio. However, there are limitations in measurement range because dose distribution evaluated on a certain plane where the gafchormic EBT2 film is located instead of confirming the overall dose distribution in rotational radiotherapy. In addition, the standard deviation was relatively greater in the gafchormic EBT2 film as compared with ArcCHECK phantom. In association with this, it is difficult to perform an accurate setting due to the characteristics of dose evaluation of gafchormic EBT2 film. There are also problems with the directionality that may arise during scanning of gafchormic EBT2 film and with a lower degree of reproducibility in setting an equipment for the dose evaluation. Furthermore, the secondary contamination and the photographic density of the gafchormic EBT2 film might also be involved in this phenomenon.

Ⅴ.CONCLUSIONS

In this study, with regards to the measurement using cheese phantom and ArcCHECK phantom in radiosurgery using the characteristics of rotational radiotherapy, the accuracy and the requirement for the absolute and relative dose evaluation were examined. In the patient DQA using tomotherapy, the assessment using a cheese phantom and gafchormic EBT2 film is disadvantageous in accurately confirming 3D measurement of dose distribution in the peripheral and target areas. In the assessment using ArcCHECK phantom, h owever, it i s possib le t o perform an accurate measurement of peripheral areas of the target and real-time assessment. This can promote more accurate, faster radiotherapy for patients. Therefore, it is also presumed that 3D dose evaluation of rotational radiotherapy, leading to the concurrent assessment of b oth target a reas a nd t he o verall irradiation areas, will provide a more accurate dose evaluation.