Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

Purpose  The dose changes in the buildup region and beam attenuation by a carbon fiber tabletop were investigated for 6-and 18-MV photon beams. Materials and methods  Measurements were performed for 2 × 2 cm to 40 × 40 cm field sizes. The surface dose and percentage depth doses (PDD) were measured by a Markus parallel plate chamber. Attenuation measurements were made at the cylindrical phantom for 180° rotation of the beam. Results  A carbon fiber tabletop increases the surface dose from 7.5% to 63.0% and from 4% to 43% for small fields at 6 and 18 MV, respectively. The increase was nearly fivefold for the 10 × 10 cm field and nearly twofold for the 40 × 40 cm field. Beam attenuation of the tabletop varies from 3.0% to 5.6% for 180° and 120° gantry angles for 6 MV. Conclusion  The carbon fiber tabletop significantly decreases the skin-sparing effect. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage.     

Monte Carlo study on a flattening filter-free 18-MV photon beam of a medical linear accelerator

PURPOSE: Several studies on the dosimetric properties of unflattened photon beams have shown some advantages for radiotherapy. In this study, the effect of removing the flattening filter from an 18-MV photon beam was investigated using the Monte Carlo method. MATERIALS AND METHODS: The 18-MV photon beam of an Elekta SL25 linear accelerator was simulated using the MCNP4C Monte Carlo (MC) code. Beam dosimetric features, including central axis absorbed doses, beam profiles, and photon energy spectra, were calculated for flattened and unflattened 18-MV photon beams. RESULTS: A 4.24-fold increase in the dose rate was seen for the unflattened beam with a field size of 10 x 10 cm(2). A decrease in the out-of-field dose up to 30% was seen for the unflattened beam. For the unflattened beam, photon energy spectra were softer, and the mean energies of the spectra were higher for a smaller field size. CONCLUSION: Our study showed that the increase in dose rate and lower out-of-field dose can be possible advantages for an unflattened 18-MV beam.     

A Monte Carlo study on neutron and electron contamination of an unflattened 18-MV photon beam

Recent studies on flattening filter (FF) free beams have shown increased dose rate and less out-of-field dose for unflattened photon beams. On the other hand, changes in contamination electrons and neutron spectra produced through photon (E>10 MV) interactions with linac components have not been completely studied for FF free beams. The objective of this study was to investigate the effect of removing FF on contamination electron and neutron spectra for an 18-MV photon beam using Monte Carlo (MC) method. The 18-MV photon beam of Elekta SL-25 linac was simulated using MCNPX MC code. The photon, electron and neutron spectra at a distance of 100 cm from target and on the central axis of beam were scored for 10×10 and 30×30 cm² fields. Our results showed increase in contamination electron fluence (normalized to photon fluence) up to 1.6 times for FF free beam, which causes more skin dose for patients. Neuron fluence reduction of 54% was observed for unflattened beams. Our study confirmed the previous measurement results, which showed neutron dose reduction for unflattened beams. This feature can lead to less neutron dose for patients treated with unflattened high-energy photon beams.     

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

PURPOSE: To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. MATERIAL AND METHODS: Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. RESULTS: Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). CONCLUSION: Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.     

The effect of titanium stabilization rods on spinal cord radiation dose.

The purpose of this study was to investigate the dosimetric effect of a titanium-rod spinal stabilization system on surrounding tissue, especially the spinal cord. Ion chamber dosimetry was performed for 6- and 18-MV photon beams in a water phantom containing a titanium-rod spinal stabilization system. Isodose curves were obtained in the phantom with and without rods. To assess the ability of a treatment planning system to reproduce the effects of the stabilization system on the radiation dose delivered to surrounding tissue, dose distributions were calculated after appropriate modifications were made in the computed tomography number-to-density conversion table to account for the increased density of the titanium rods. The resultant heterogeneity-corrected plans were compared with uncorrected plans. At a 7-cm depth in the water phantom, corresponding to the depth of the spinal cord, the beam was attenuated by 4% under the rods alone and by 13% rods under the rods with screws for the 6-MV photon beam as compared with curves generated in the absence of rods. The beam was attenuated by 3% and 11%, respectively, for the 18-MV beam. Using anteroposterior (18-MV) and posteroanterior (6-MV) photon beams, with and without heterogeneity correction for the rods, the corrected isodose plan showed an approximately 2% beam attenuation 4 cm anterior to the rods as compared with the uncorrected plan. No significant difference in the spinal cord dose was observed between the 2 plans, however. The titanium-rod spinal stabilization system tested in this study caused a decrease in the dose delivered distal to the rods but did not significantly affect the dose delivered to the spinal cord.     

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.     

Study on surface dose generated in prostate intensity-modulated radiation therapy treatment

The surface doses of 6- and 15-MV prostate intensity-modulated radiation therapy (IMRT) irradiations were measured and compared to those from a 15-MV prostate 4-beam box (FBB). IMRT plans (step-and-shoot technique) using 5, 7, and 9 beams with 6- and 15-MV photon beams were generated from a Pinnacle treatment planning system (version 6) using computed tomography (CT) scans from a Rando Phantom (ICRU Report 48). Metal oxide semiconductor field effect transistor detectors were used and placed on a transverse contour line along the Phantom surface at the central beam axis in the measurement. Our objectives were to investigate: (1) the contribution of the dynamic multileaf collimator (MLC) to the surface dose during the IMRT irradiation; (2) the effects of photon beam energy and number of beams used in the IMRT plan on the surface dose. The results showed that with the same number of beams used in the IMRT plan, the 6-MV irradiation gave more surface dose than that of 15 MV to the phantom. However, when the number of beams in the plan was increased, the surface dose difference between the above 2 photon energies became less. The average surface dose of the 15-MV IMRT irradiation increased with the number of beams in the plan, from 0.86% to 1.19%. Conversely, for 6 MV, the surface dose decreased from 1.33% to 1.24% as the beam number increased from 7 to 9. Comparing the 15-MV FBB and 6-MV IMRT plans with 2 Gy/fraction, the IMRT irradiations gave generally more surface dose, from 15% to 30%, depending on the number of beams in the plan. It was found that the increase in surface dose for the IMRT technique compared to the FBB plan was predominantly due to the number of beams and the calculated monitor units required to deliver the same dose at the isocenter in the plans. The head variation due to the dynamic MLC movement changing the surface dose distribution on the patient was reflected by the IMRT dose-intensity map. Although prostate IMRT in this study had an average higher surface dose than that of FBB, the more even distribution of relatively lower surface dose in IMRT field could avoid the big dose peaks at the surface positions directly under the FBB fields. Such an even and low surface dose distribution surrounding the patient in IMRT is believed to give less skin complication than that of FBB with the same prescribed dose.     

Choice of beam energy and dosimetric implications for radiation treatment in a subpopulation of women with large breasts in the United States and Japan.

Radiation complications are often related to the dose inhomogeneity (hot spot) in breast tissue treated with conservative therapy, especially for large patients. The effect of photon energy on radiation dose distribution is analyzed to provide guidelines for the selection of beam energy when tangential fields and limited slices are used to treat women with large breasts. Forty-eight patients with chest wall separation > 22 cm were selected for dosimetric analysis. We compared the maximum dose in the central axis (CAX) plane (2D) using 6-, 10-, and 18-MV photon beams in all patients and 3D data set for 16 patients. Correlation between hot spot dose (HSD), separation, breast cup size, breast volume, and body weight was derived with beam energy. Among the 48 patients in this study, HSD > 10% in the CAX plane was noted in 98%, 46%, and 4% of the population when 2D dosimetry was performed; however, with 3D study, it was in 50%, 19%, and 6% of the patients with 6-MV, 10-MV and 18-MV beams, respectively. The chest wall separation, body weight, and breast volume were correlated with the HSD in both the 2D and 3D plans. Patient's bra size was not correlated with the hot spot. The chest wall separation was found to be the most important parameter to correlate with hot spot in tangential breast treatment. Simple guidelines are provided for dose uniformity in breast with respect to chest wall separation, body weight, bra size, and breast volume with tangential field irradiations.     

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Since the skin dose becomes the limiting factor while deciding the tumorcidal dose, the detailed analysis of dose distribution in the build-up region is necessary for high-energy photon beams. In this study the beam characteriwstics affecting the build-up and skin dose for 6- and 18-MV photons are analyzed.     

Evaluation of dose delivery based on a comparison of dosimetry calculations using open beam and wedged beam depth dose data

The purpose of this study is to evaluate the magnitude of the error in dose delivery caused by the use of open beam depth dose data in dosimetry calculations for wedged photon beams. Isodose pians were calculated for treatments given in a 3-field isocentric prostate or rectal setup using an open AP beam with two lateral wedged beams. The dose distributions were first calculated using open beam depth dose data for all three fields. Next, the open beam data was used only for the AP field and true wedged beam depth dose data was substituted for the two lateral wedged fields. The magnitude of the depth dose variations for wedged vs open beams depends on the nominal beam energy, the wedge angle, and the depth of measurement. Consequently, isodose distributions calculated for wedged fields were found to be different when true wedged beam depth dose data was used instead of open beam data as is commonly done. Monitor unit calculations using a field size specific wedge factor show that dose delivery errors up to 4% can result from the use of open beam depth dose data in wedged beam dose distribution calculations for a 6-MV photon beam. Accurate treatment planning for wedged fields requires the use of wedged beam depth dose data specific to each wedge. Simply using open beam depth dose data in dose calculations for wedged beams will result in dose delivery errors, the magnitude of which depends on the combination of wedge angle, field size, and nominal beam energy.     

Effects of neurosurgical titanium mesh on radiation dose

The purpose of this study was to determine the dosimetric impact of a neurosurgical titanium mesh in patients treated with 6- and 18-MV photon beams. The effects of a 0.4-mm-thick titanium mesh on the dose profile at 3 regions within a solid water phantom were measured using extended dose range-2 (EDR2) film for 6- and 18-MV photon beams. All measurements were performed with the titanium mesh placed at a depth of 1.5 cm in the phantom. Films were exposed immediately above the mesh, immediately below the mesh, and at a depth of 5 cm from the surface of the phantom. The films were scanned using a scanning densitometer. In the region directly above the titanium mesh, there was an increase in dose of 7.1% for 6-MV photons and 4.9% for 18-MV photons. Directly below the titanium mesh, there was an average decrease in dose of 1.5% for 6-MV photons and an increase of 1.0% for 18-MV photons. At 5-cm depth, for 6- and 18-MV photons, there was a decrease in dose of 2.2% and 0.6%, respectively. We concluded that for cranial irradiation with high-energy photons, the dosimetric impact of a 0.4-mm titanium mesh is small and does not require modification in treatment parameters.     

Effects of neurosurgical titanium mesh on radiation dose

The purpose of this study was to determine the dosimetric impact of a neurosurgical titanium mesh in patients treated with 6- and 18-MV photon beams. The effects of a 0.4-mm-thick titanium mesh on the dose profile at 3 regions within a solid water phantom were measured using extended dose range-2 (EDR2) film for 6- and 18-MV photon beams. All measurements were performed with the titanium mesh placed at a depth of 1.5 cm in the phantom. Films were exposed immediately above the mesh, immediately below the mesh, and at a depth of 5 cm from the surface of the phantom. The films were scanned using a scanning densitometer. In the region directly above the titanium mesh, there was an increase in dose of 7.1% for 6-MV photons and 4.9% for 18-MV photons. Directly below the titanium mesh, there was an average decrease in dose of 1.5% for 6-MV photons and an increase of 1.0% for 18-MV photons. At 5-cm depth, for 6- and 18-MV photons, there was a decrease in dose of 2.2% and 0.6%, respectively. We concluded that for cranial irradiation with high-energy photons, the dosimetric impact of a 0.4-mm titanium mesh is small and does not require modification in treatment parameters.     

Influence of compensator thickness,field size,and off-axis distance on the effective attenuation coefficient of a cerrobend compensator for intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) can be performed by using compensators. To make a compensator for an IMRT practice, it is required to calculate the effective attenuation coefficient (μ_eff) of its material, which is affected by various factors. We studied the effect of the variation of the most important factors on the calculation of the μ_eff of the cerrobend compensator for 6-MV photon beams, including the field size, compensator thickness, and off-axis distance. Experimental measurements were carried out at 100 cm source-to-surface distance and 10 cm depth for the 6-MV photon beams of an Elekta linac using various field size, compensator thickness, and off-axis settings. The field sizes investigated ranged from 4 × 4 to 25 × 25 cm² and the cerrobend compensator thicknesses from 0.5–6 cm. For a fixed compensator thickness, variation of the μ_eff with the field size ranged from 3.7–6.8%, with the highest value attributed to the largest compensator thickness. At the reference field size of 10 × 10 cm², the μ_eff varied by 16.5% when the compensator thickness was increased from 0.5–6 cm. However, the variation of the μ_eff with the off-axis distance was only 0.99% at this field size, whereas for the largest field size, it was more significant. Our results indicated that the compensator thickness and field size have the most significant effect on the calculation of the compensator μ_eff for the 6-MV photon beam. Therefore, it is recommended to consider these parameters when calculating the compensator thickness for an IMRT practice designed for these beams. The off-axis distance had a significant effect on the calculation of the μ_eff only for the largest field size. Hence, it is recommended to consider the effect of this parameter only for field sizes larger than 25 × 25 cm².     

High-energy electron and photon therapy to the parotid bed: radiation dose perturbations with a titanium mandibular implant

Adjuvant radiation therapy to the parotid bed is commonly administered following surgical resection using either a pair of angled wedged photon beams or an ipsilateral mixed-beam portal of electrons and photons. The present study seeks to determine the optimal parotid bed treatment technique in the presence of a titanium mandibular implant by investigating perturbations in the dose distribution deep to this implant for a 15-MeV electron beam and a 6-MV photon beam. A titanium mandibular plate was embedded in a tissue-equivalent phantom, and irradiated with 15-MeV electrons, and 6 MV photons. Radiation doses behind the plate were measured with both thermoluminescent dosimeters and radiographic film. With 15-MeV electrons, there is a clinically significant decrease in the dose beyond the titanium plate, which is most important at 5-mm and 10-mm depths (18-27%). With 6-MV photons the dose at the deep interface of titanium and tissue is reduced by between 15 and 18%, but rapidly drops to < 5% at a depth of 5 mm. In adjuvant treatment to the parotid bed, when the clinical target volume includes tissue positioned deep to a titanium implant, significant underdosage occurs with ipsilateral beam arrangements, especially when electrons are used.     

Surface and superficial dose dosimetric verification for postmastectomy radiotherapy

In patients given postmastectomy radiotherapy (PMRT), the chest wall is a very thin layer of soft tissue with a low-density lung tissue behind. Chest wall treated in this situation with a high-energy photon beam presents a high dosimetric uncertainty region for both calculation and measurement. The purpose of this study was to measure and to evaluate the surface and superficial doses for patients requiring PMRT with different treatment techniques. An elliptic cylinder cork and superflab boluses were used to simulate the lung and the chest wall, respectively. Sets of computed tomography (CT) images with different chest wall thicknesses were acquired for the study phantom. Hypothetical clinical target volumes (CTVs) were outlined and modified to fit a margin of 1–3 mm, depending on the chest wall thickness, away from the surface for the sets of CT images. The planning target volume (PTV) was initially created by expanding an isotropic 3-mm margin from the CTV, and then a margin of 3 mm was shrunk from the phantom surface to avoid artifact-driven results in the beam-let intensity. Treatment techniques using a pair of tangential wedged fields (TWFs) and 4-field intensity-modulated radiation therapy (IMRT) were designed with a prescribed fraction dose (D_p) of 180 cGy. Superficial dose profiles around the phantom circumference at depths of 0, 1, 2, 3, and 5 mm were obtained for each treatment technique using radiochromic external beam therapy (EBT) films. EBT film exhibits good characteristics for dose measurements in the buildup region. Underdoses at the median and lateral regions of the TWF plans were shown. The dose profiles at shallow depths for the TWF plans show a dose buildup about 3 mm at the median and lateral tangential incident regions with a surface dose of about 52% of D_p. The dose was gradually increased toward the most obliquely tangential angle with a maximum dose of about 118% of D_p. Dose profiles were more uniform in the PTV region for the 4-F IMRT plans. Most of the PTV region had doses >94% of D_p at depths >1 mm. The mean surface dose was about 65% of D_p for the 4-F IMRT plans. The maximum dose for the 4-F IMRT plans was <118.4% of D_p. The application of added bolus has to consider the treatment technique, tumor coverage, and possible skin reactions. For PMRT, if the chest surface and wall are treated adequately, at least 3 mm bolus should be added to the chest wall when tangential beams and 6-MV photon energy are arranged. However, when the surface and superficial regions are not high-risk areas, an IMRT plan with tangential beams and 6-MV photon energy can provide uniform dose distributions within the PTV, spare the skin reaction, and deliver sufficient doses to the chest wall at depths >1 mm.     

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

AIM: Since the skin dose becomes the limiting factor while deciding the tumorcidal dose, the detailed analysis of dose distribution in the build-up region is necessary for high-energy photon beams. In this study the beam characteristics affecting the build-up and skin dose for 6- and 18-MV photons are analyzed. MATERIALS AND METHODS: Measurements were made with 6- and 18-MV photons using a PTW parallel-plate ionization chamber (B 23344-036) and a RDM-1F electrometer. Build-up ionization measurements were made with the chamber fitted into a 25 x 25 x 25 cm polystyrene phantom with a fixed SSD of 100 cm. The entrance and build-up dose measurements were made with a polycarbonate and a mesh type metallic shielding tray and a 45 degrees wedge. Exit dose measurements were carried out for the graphite patient supporting assembly table top, 1.0 cm thick piece of wood and the 1.0 cm thick patient supporting perspex base frame for head and neck treatments. RESULTS: It was observed that the dmax decreased slightly with field size as with other accelerators. For both photon energies the surface dose was observed to increase with increase in field size. It was also noticed that the dose in the build-up region increases slightly when the polycarbonate secondary blocking tray is introduced with the increase in surface dose. The data show that the tray perturbation factor (TPF) at surface decreases steadily with tray-surface distance for both photon beams for all field sizes. It was noted that the TPF was more when the polycarbonate tray was introduced at shorter tray-surface distances for both energies. At tray-surface distances above 60 cm the TPF almost remained close to unity for 6-MV photons for all field sizes, whereas the continuous decrease in TPF could be noted for 18-MV photon beams even after the TPF reached unity. CONCLUSION: The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.     

Quality assurance measurements of a-Si EPID performance.

The performance stability of a Varian aS500 amorphous silicon (a-Si) electronic portal imaging device (EPID) was monitored over an 18-month period using a variety of standard quality assurance (QA) tests. The tests were selected to provide ongoing information about image quality and dose response from the time of EPID acceptance into clinical service. To evaluate imaging performance, we made spatial resolution and contrast measurements using both PortalVision and QC-3V phantoms for 6- and 15-MV photon beams at repetition rates of 100, 300, and 400 MU/min in standard scanning mode. To assess operational stability for dosimetry applications, we measured central axis radiation response and beam pulse variability for the same image acquisition modes. Using the QC-3V phantom, values for the critical frequency of 0.435 +/- 0.005 lp/mm for 6 MV and 0.382 +/- 0.003 lp/mm for 15 MV were obtained. The contrast-to-noise ratio was found to be approximately 20% higher for the lower photon energy. Beam pulse variability remained within the tolerance of 3% set by the manufacturer. The central axis pixel response of the EPID remained constant within +/-1% over a 5-month period for the 6-MV beam, but fell approximately 4% over the same period for the 15-MV beam. The Varian aS500 EPID studied exhibited consistent image quality and a stable radiation response. These characteristics render it suitable for quantitative applications such as clinical dose measurement.     

Estimating dose to implantable cardioverter-defibrillator outside the treatment fields using a skin QED diode,optically stimulated luminescent dosimeters,and LiF thermoluminescent dosimeters

The purpose of this work was to determine the relative sensitivity of skin QED diodes, optically stimulated luminescent dosimeters (OSLDs) (microStar? DOT, Landauer), and LiF thermoluminescent dosimeters (TLDs) as a function of distance from a photon beam field edge when applied to measure dose at out-of-field points. These detectors have been used to estimate radiation dose to patients' implantable cardioverter-defibrillators (ICDs) located outside the treatment field. The ICDs have a thin outer case made of 0.4- to 0.6-mm-thick titanium (～2.4-mm tissue equivalent). A 5-mm bolus, being the equivalent depth of the devices under the patient's skin, was placed over the ICDs. Response per unit absorbed dose-to-water was measured for each of the dosimeters with and without bolus on the beam central axis (CAX) and at a distance up to 20 cm from the CAX. Doses were measured with an ionization chamber at various depths for 6- and 15-MV x-rays on a Varian Clinac-iX linear accelerator. Relative sensitivity of the detectors was determined as the ratio of the sensitivity at each off-axis distance to that at the CAX. The detector sensitivity as a function of the distance from the field edge changed by ± 3% (1–11%) for LiF TLD-700, decreased by 10% (5–21%) for OSLD, and increased by 16% (11–19%) for the skin QED diode (Sun Nuclear Corp.) at the equivalent depth of 5 mm for 6- or 15-MV photon energies. Our results showed that the use of bolus with proper thickness (i.e., ～d_max of the photon energy) on the top of the ICD would reduce the scattered dose to a lower level. Dosimeters should be calibrated out-of-field and preferably with bolus equal in thickness to the depth of interest. This can be readily performed in clinic.