### Introduction

^{−1}by reducing the beam efficiency. The second reason is to allow a uniform dose distribution along the beam axis. The final reason is to be able to cover the entire body of the patient with large field sizes (e.g. 40×40 cm

^{2}) [7–10].

*In vivo*dosimetry should be executed with a dosimeter to verify the dose calculation and compensator design [14]. The entrance and exit doses are measured by

*in vivo*dosimetry during treatment and are used to estimate the delivered midline doses [10]. However, it is difficult to predict the midline dose from entrance and exit dose measurements since these surface doses change depending upon the thickness and beam energy [15]. Satory developed an empirical formula to calculate the midline dose from MOSFET (metal-oxide-semiconductor field-effect transistor) measurements of the entrance and exit doses. The dependency of the surface dose on the air gap between the spoiler and the surface was investigated with various phantom thicknesses. The entrance and exit doses were combined using an exponential attenuation formula to give an estimate of the midline dose and this was compared to the midline ion chamber measurement for a range of phantom thicknesses [10]. Although there are many advantages of using MOSFETs, such as its portability, reproducibility, and direct readout capability, it is a quite cumbersome to install and attach the device to the surface of the patient since the MOSFET device requires a cable (to apply a voltage to the detector) and a dedicated reader device.

*in vivo*measurements due to their reproducibility, low energy dependence, small size, and ease of use [16]. OSLDs have high sensitivity and can measure a surface dose of patient with a non-destructive readout (i.e. without removal the cover or packing) and reanalysis using a simple readout system. Moreover, measurement accuracy can be improved using OSLDs with fully filled deep electron/hole traps and optimal bleaching conditions [17, 18].

### Materials and Methods

### 1. OSLD preparation

^{60}Co gamma ray source. The OSLDs were bleached to fall below the residual signal level over 4 h. In our institution, the clinical dosimetry using OSLDs with fully filled deep electron/hole traps under appropriate bleaching conditions have been shown to be highly stable and accurate, with no change in either the dose sensitivity or linearity. The uncertainty of OSLDs prepared by the process was less than 3%. The R square of calibration curve was 0.99 within calibration range (20–500 cGy).

### 2. TBI measurement

_{m}_

_{mid}), the dose conversion factor was derived. When one hundred MU (monitor units) was delivered for the reference setup (SSD: 100 cm, field size: 10 cm×10 cm), the integrated coulomb measurement at the depth of the maximum dose (d

_{max}) was measured using a Farmer-type chamber (TN30013, PTW, Freiburg, Germany) and an electrometer (Uniods E, PTW, Freiburg, Germany) in a solid water phantom (SWP, Virtual Watert™, Radiation Products Design Inc., Albertville, MN) for 6 MV and 15 MV. The dose conversion factor was defined as the measured coulomb (C) at d

_{max}when one hundred monitoring unit was delivered. The unit of dose conversion factor was C·Gy

^{−1}.

_{m}_

_{mid}was measured using the Farmer-type chamber that was inserted into a hole in a SWP slab of 2 cm thickness. The chamber was placed at the midline along the central axis of the beam. The coulomb measurements were converted to dose units (Gy) using the dose conversion factor. The solid water phantom slabs were added in order to achieve the desired thickness. In this study, the measurement was performed for a range of phantom thicknesses (6 cm, 10 cm, 16 cm, 20 cm, 22 cm, 26 cm, 30 cm, 36 cm, 40 cm, 46 cm, and 50 cm). To evaluate the dose homogeneity due to the tissue lateral effect for the patient, the dose at a depth of 1 cm (D

_{peak}) was measured using the Farmer-type chamber. The ratio of D

_{peak}to D

_{m}_

_{mid}was calculated for each thickness.

_{en}) and exit (D

_{ex}) doses (Figure 1B). Three OSLDs were attached on each side to reduce measurement uncertainty. Solid water slabs of different thicknesses were joined in various combinations and attached to the front and back sides of the phantom for each energy. Considering dose homogeneity due to the tissue lateral effect, a 6 MV photon beam was used for phantom thicknesses of 6 cm, 10 cm, 16 cm, 20 cm, 22 cm, 26 cm, 30 cm, and 36 cm, and a 15 MV photon beam was used for phantom thicknesses of 10 cm, 16 cm, 20 cm, 22 cm, 26 cm, 30 cm, 36 cm, 40 cm, 46 cm and 50 cm. In consideration of the fading effect, the OSLDs were read using the InLight MicroStar reader (Landauer, Inc., Glenwood, IL) 15 minutes after irradiation [18]. For a reliable reading of the nanoDot OSLDs, a readout was performed three times for each chip, and the predicted midline dose (D

_{c}_

_{mid}) was estimated as the sum of D

_{en}and D

_{ex}.

### Results and Discussion

_{peak}to D

_{m}_

_{mid}as a function of the phantom thickness for 6 MV and 15 MV. As the thickness of the phantom was increased, D

_{peak}also increased rapidly and the lower the energy, the faster the increase. At 6 MV, the ratio increased to 1.32 at a SWP thickness of 36 cm. When the SWP thickness was greater than 30 cm, D

_{peak}was greater than 10% of D

_{m}_

_{mid}at 6 MV. When 15 MV was used at a SWP thickness of 50 cm, D

_{peak}increased to 125% of the midline dose. At a SWP thickness of 30 cm, the ratio was 1.12 for both energies. Therefore, if the thickness of the patient is greater than 30 cm, 15 MV should be used for treatment in order to achieve dose homogeneity [11].

_{en}, D

_{ex}, D

_{c}_

_{mid}, D

_{m}_

_{mid}, and the ratio of D

_{c}_

_{mid}to D

_{m}_

_{mid}for 6 MV. Den was increased and D

_{ex}was decreased as the thickness of the SWP was increased. The ratio of D

_{c}_

_{mid}to D

_{m}_

_{mid}also increased with increasing SWP thickness. However, when the thickness of the SWP was increased from 10 cm to 16 cm, the ratio decreased slightly because the increase of the entrance dose was smaller than the decrease of the exit dose. This is because the entrance dose depends on the distance between the spoiler and the surface of the entrance. For OSLDs measurements, the measurement point was included in the build-up region. Even though the SSD of the measurement point of the entrance dose was decreased, the dose did not increase considerably [10].

_{c}_

_{mid}to be less than D

_{m}_

_{mid}.

_{c}_

_{mid}was approximately 85% of D

_{m}_

_{mid}. For most adult patients, the thickness of the hips with both hands placed adjacent to the body is at least 40 cm. Therefore, the 15 MV beam was selected for treatment in consideration of the need for dose homogeneity. The thickness of the head and neck is slightly less than 25 cm. Therefore, D

_{c}_

_{mid}at these areas of the body can fall below D

_{m}_

_{mid}. When the SWP thickness was greater than 20 cm, the ratio increased in keeping with the SWP thickness. If the SWP thickness exceeded 46 cm, the ratio was greater than 1. Most obese patients have organ thicknesses of greater than 50 cm thickness at the shoulder, chest, umbilicus, and hip. D

_{c}_

_{mid}of these points can exceed over 10% of the prescription dose. D

_{c}_

_{mid}at 15 MV was estimated to be lower than that of 6 MV for same thickness. For 15 MV, a beam spoiler plate of 1 cm thickness is insufficient to produce the initial dose buildup region [10].