Alderson Phantom Research Articles

Optimizing a perfusion CT protocol for head and neck cancer

Abstract Perfusion computed tomography (CTP) images tumor angiogenesis and can assess tumor aggressiveness. However, the CTP examinations are dose intensive. This study aimed to optimize a routinely used CTP protocol for the head and neck region in oncology in order to reduce the effective dose to the patient and simultaneously achieve the same image quality. The Alderson phantom was scanned on a GE Revolution CT scanner. A scan with our standard protocol for head and neck cancer patients was used (100kV, 80mAs, 5mm slice thickness and backprojection algorithm) and in seven predefined regions (ROI) the signal to noise ratio (SNR) was measured. For the dose optimized protocol, the tube voltage was lowered and the mAs adaptation protocol was used. To improve image quality different percentage of an adaptive statistical iterative reconstruction (ASiR) was applied. For a better resolution we set the slice thickness to 2.5 mm. The mAs adaption range and the percentage of the ASiR reconstruction were varied until we found a combination with the same median SNR in the seven defined ROIs as for our old protocol. For the old and the optimized protocol dose measurements were performed using 25 LiF-TLDs. Organ doses were calculated and the effective dose was determined based on the weighting factors of ICRP103. The optimized scanning protocol used a voltage of 80kV, a mAs range between 15 and 80, a noise level of 10%, and 50% ASiR reconstruction. The median SNR ratio was slightly better (14% better SNR) with the new protocol. An effective dose of 8 mSv was measured with the original protocol and 4 mSv with the optimized scanning protocol. For organs in the scanning field the dose was reduced by a factor of 2 and outside the field by a factor of 2.2. Advanced reconstruction algorithms allow a significant dose reduction and an improvement of image resolution, while maintaining the image quality.

Whole-body dose and energy measurements in radiotherapy by a combination of LiF:Mg,Cu,P and LiF:Mg,Ti

PurposeLong-term survivors of cancer who were treated with radiotherapy are at risk of a radiation-induced tumor. Hence, it is important to model the out-of-field dose resulting from a cancer treatment. These models have to be verified with measurements, due to the small size, the high sensitivity to ionizing radiation and the tissue-equivalent composition, LiF thermoluminescence dosimeters (TLD) are well-suited for out-of-field dose measurements. However, the photon energy variation of the stray dose leads to systematic dose errors caused by the variation in response with radiation energy of the TLDs. We present a dosimeter which automatically corrects for the energy variation of the measured photons by combining LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD100H) chips. MethodsThe response with radiation energy of TLD100 and TLD100H compared to 60Co was taken from the literature. For the measurement, a TLD100H was placed on top of a TLD100 chip. The dose ratio between the TLD100 and TLD100H, combined with the ratio of the response curves was used to determine the mean energy. With the energy, the individual correction factors for TLD100 and TLD100H could be found. The accuracy in determining the in- and out-of-field dose for a nominal beam energy of 6MV using the double-TLD unit was evaluated by an end-to-end measurement. Furthermore, published Monte Carlo (M.C.) simulations of the mean photon energy for brachytherapy sources, stray radiation of a treatment machine and cone beam CT (CBCT) were compared to the measured mean energies. Finally, the photon energy distribution in an Alderson phantom was measured for different treatment techniques applied with a linear accelerator. Additionally, a treatment plan was measured with a cobalt machine combined with an MRI. ResultsFor external radiotherapy, the presented double-TLD unit showed a relative type A uncertainty in doses of −1%±2% at the two standard deviation level compared to an ionization chamber. The type A uncertainty in dose was in agreement with the theoretically calculated type B uncertainty. The measured energies for brachytherapy sources, stray radiation of a treatment machine and CBCT imaging were in agreement with M.C. simulations. A shift in energy with increasing distance to the isocenter was noticed for the various treatment plans measured with the Alderson phantom. The calculated type B uncertainties in energy were in line with the experimentally evaluated type A uncertainties. ConclusionThe double-TLD unit is able to predict the photon energy of scatter radiation in external radiotherapy, X-ray imagine and brachytherapy sources. For external radiotherapy, the individual energy correction factors enabled a more accurate dose determination compared to conventional TLD measurements.

New developments in EPID-based 3D dosimetry in The Netherlands Cancer Institute

EPID-based offline 3D in vivo dosimetry is performed routinely in The Netherlands Cancer Institute for almost all RT treatments. The 3D dose distribution is reconstructed using the EPID primary dose in combination with a back-projection algorithm and compared with the planned dose distribution. Recently the method was adapted for real-time dose verification, performing 3D dose verification in less than 300 ms, which is faster than the current portal frame acquisition rate. In this way a possibility is created for halting the linac in case of large delivery errors. Furthermore, a new method for pre-treatment QA was developed in which the EPID primary dose behind a phantom or patient is predicted using the CT data of that phantom or patient in combination with in-air EPID measurements. This virtual EPID primary transit dose is then used to reconstruct the 3D dose distribution within the phantom or patient geometry using the same dose engine as applied offline. In order to assess the relevance of our clinically applied alert criteria, we investigated the sensitivity of our EPID-based 3D dose verification system to detect delivery errors in VMAT treatments. This was done through simulation by modifying patient treatment plans, as well as experimentally by performing EPID measurements during the irradiation of an Alderson phantom, both after deliberately introducing errors during VMAT delivery. In this presentation these new developments will be elucidated.

COMPARING MEASURED AND CALCULATED DOSES IN INTERVENTIONAL CARDIOLOGY PROCEDURES.

Interventional cardiology requires complex procedures and can result in high doses and dose rates to the patient and medical staff. The many variables that influence the dose to the patient and staff include the beam position and angle, beam size, kVp, filtration, kerma-area product and focus-skin distance. A number of studies using the Monte Carlo method have been undertaken to obtain prospective dose assessments. In this paper, detailed irradiation scenarios were simulated mathematically and the resulting dose estimates were compared with real measurements made previously under very similar irradiation conditions and geometries. The real measurements and the calculated doses were carried out using or simulating an interventional cardiology system with a flat monoplane detector installed in a dedicated room with an Alderson phantom placed on the procedure table. The X-ray spectra, beam angles, focus-skin distance, measured kerma-area product and filtration were simulated, and the real dose measurements and calculated doses were compared. It was shown that the Monte Carlo method was capable of reproducing the real dose measurements within acceptable levels of uncertainty.

Measurements of doses from photon beam irradiation and scattered neutrons in an anthropomorphic phantom model of prostate cancer: a comparison between 3DCRT, IMRT and tomotherapy

Abstract Introduction. The rapid development of new radiotherapy technologies, such as intensity modulated radiotherapy (IMRT) or tomotherapy, has resulted in the capacity to deliver a more homogenous dose in the target. However, the higher doses associated with these techniques are a reason for concern because they may increase the dose outside the target. In the present study, we compared 3DCRT, IMRT and tomotherapy to assess the doses to organs at risk (OARs) resulting from photon beam irradiation and scattered neutrons. Material and methods. The doses to OARs outside the target were measured in an anthropomorphic Alderson phantom using thermoluminescence detectors (TLD 100) 6Li (7.5%) and 7Li (92.5%). The neutron fluence rate [cm−2·s−1] at chosen points inside the phantom was measured with gold foils (0.5 cm diameter, mean surface density of 0.108 g/cm3). Results. The doses [Gy] delivered to the OARs for 3DCRT, IMRT and tomotherapy respectively, were as follows: thyroid gland (0.62 ± 0.001 vs. 2.88 ± 0.004 vs. 0.58 ± 0.003); lung (0.99 ± 0.003 vs. 4.78 ± 0.006 vs. 0.67 ± 0.003); bladder (80.61 ± 0.054 vs. 53.75 ± 0.070 vs. 34.71 ± 0.059); and testes (4.38 ± 0.017 vs. 6.48 ± 0.013 vs. 4.39 ± 0.020). The neutron dose from 20 MV X-ray beam accounted for 0.5% of the therapeutic dose prescribed in the PTV. The further from the field edge the higher the contribution of this secondary radiation dose (from 8% to ~45%). Conclusion. For tomotherapy, all OARs outside the therapeutic field are well-spared. In contrast, IMRT achieved better sparing than 3DCRT only in the bladder. The photoneutron dose from the use of high-energy X-ray beam constituted a notable portion (0.5%) of the therapeutic dose prescribed to the PTV.

Total body irradiation-an attachment free sweeping beam technique.

IntroductionA sweeping beam technique for total body irradiation in standard treatment rooms and for standard linear accelerators (linacs) is introduced, which does not require any accessory attached to the linac. Lung shielding is facilitated to reduce the risk of pulmonary toxicity. Additionally, the applicability of a commercial radiotherapy planning system (RTPS) is examined.Material and MethodsThe patient is positioned on a low couch on the floor, the longitudinal axis of the body in the rotational plane of the linac. Eight arc fields and five additional fixed beams are applied to the patient in supine and prone position respectively. The dose distributions were measured in a solid water phantom and in an Alderson phantom. Diode detectors were calibrated for in-vivo dosimetry. The RTPS Oncentra was employed for calculations of the dose distribution.ResultsFor the cranial 120 cm the longitudinal dose profile in a slab phantom measured with ionization chamber varies between 94 and 107 % of the prescription dose. These values were confirmed by film measurements and RTPS calculations. The transmittance of the lung shields has been determined as a function of the thickness of the absorber material. Measurements in an Alderson phantom and in-vivo dosimetry of the first patients match the calculated dose.Discussion and conclusionA treatment technique with clinically good dose distributions has been introduced, which can be applied with each standard linac and in standard treatment rooms. Dose calculations were performed with a commercial RTPS and should enable individual dose optimization.

SU‐F‐J‐150: Development of An End‐To‐End Chain Test for the First‐In‐Man MR‐Guided Treatments with the MRI Linear Accelerator by Using the Alderson Phantom

The Alderson phantom is a human shaped quality assurance tool that has been used for over 30 years in radiotherapy. The phantom can provide integrated tests of the entire chain of treatment planning and delivery. The purpose of this research was to investigate if this phantom can be used to chain test a treatment on the MRI linear accelerator (MRL) which is currently being developed at the UMC Utrecht, in collaboration with Elekta and Philips. The latter was demonstrated by chain testing the future First‐in‐Man treatments with this system.An Alderson phantom was used to chain test an entire treatment with the MRL. First, a CT was acquired of the phantom with additional markers that are both visible on MR and CT. A treatment plan for treating bone metastases in the sacrum was made. The phantom was consecutively placed in the MRL. For MRI imaging, an 3D volume was acquired. The initially developed treatment plan was then simulated on the new MRI dataset. For simulation, both the MR and CT data was used by registering them together. Before treatment delivery a MV image was acquired and compared with a DRR that was calculated form the MR/CT registration data. Finally, the treatment was delivered. Figure 1 shows both the T1 weighted MR‐image of the phantom and the CT that was registered to the MR image. Figure 2 shows both the calculated and measured MV image that was acquired by the MV panel. Figure 3 shows the dose distribution that was simulated. The total elapsed time for the entire procedure excluding irradiation was 13:35 minutes.The Alderson Phantom yields sufficient MR contrast and can be used for full MR guided radiotherapy treatment chain testing. As a result, we are able to perform an end‐to‐end chain test of the future First‐in‐Man treatments.

SU‐F‐J‐198: A Cross‐Platform Adaptation of An a Priori Scatter Correction Algorithm for Cone‐Beam Projections to Enable Image‐ and Dose‐Guided Proton Therapy

Purpose:Cone‐beam CT (CBCT) imaging may enable image‐ and dose‐guided proton therapy, but is challenged by image artefacts. The aim of this study was to demonstrate the general applicability of a previously developed a priori scatter correction algorithm to allow CBCT‐based proton dose calculations.Methods:The a priori scatter correction algorithm used a plan CT (pCT) and raw cone‐beam projections acquired with the Varian On‐Board Imager. The projections were initially corrected for bow‐tie filtering and beam hardening and subsequently reconstructed using the Feldkamp‐Davis‐Kress algorithm (rawCBCT). The rawCBCTs were intensity normalised before a rigid and deformable registration were applied on the pCTs to the rawCBCTs. The resulting images were forward projected onto the same angles as the raw CB projections. The two projections were subtracted from each other, Gaussian and median filtered, and then subtracted from the raw projections and finally reconstructed to the scatter‐corrected CBCTs. For evaluation, water equivalent path length (WEPL) maps (from anterior to posterior) were calculated on different reconstructions of three data sets (CB projections and pCT) of three parts of an Alderson phantom. Finally, single beam spot scanning proton plans (0–360 deg gantry angle in steps of 5 deg; using PyTRiP) treating a 5 cm central spherical target in the pCT were re‐calculated on scatter‐corrected CBCTs with identical targets.Results:The scatter‐corrected CBCTs resulted in sub‐mm mean WEPL differences relative to the rigid registration of the pCT for all three data sets. These differences were considerably smaller than what was achieved with the regular Varian CBCT reconstruction algorithm (1–9 mm mean WEPL differences). Target coverage in the re‐calculated plans was generally improved using the scatter‐corrected CBCTs compared to the Varian CBCT reconstruction.Conclusion:We have demonstrated the general applicability of a priori CBCT scatter correction, potentially opening for CBCT‐based image/dose‐guided proton therapy, including adaptive strategies.Research agreement with Varian Medical Systems, not connected to the present project.

PO-0808: Validation of a clinical peripheral photon dose model: prostate IMRT irradiation of Alderson phantom

PO-0808: Validation of a clinical peripheral photon dose model: prostate IMRT irradiation of Alderson phantom

Assessment of a 2D electronic portal imaging devices-based dosimetry algorithm for pretreatment and in-vivo midplane dose verification.

Background:The use of electronic portal imaging devices (EPIDs) is a method for the dosimetric verification of radiotherapy plans, both pretreatment and in vivo. The aim of this study is to test a 2D EPID-based dosimetry algorithm for dose verification of some plans inside a homogenous and anthropomorphic phantom and in vivo as well.Materials and Methods:Dose distributions were reconstructed from EPID images using a 2D EPID dosimetry algorithm inside a homogenous slab phantom for a simple 10 × 10 cm2 box technique, 3D conformal (prostate, head-and-neck, and lung), and intensity-modulated radiation therapy (IMRT) prostate plans inside an anthropomorphic (Alderson) phantom and in the patients (one fraction in vivo) for 3D conformal plans (prostate, head-and-neck and lung).Results:The planned and EPID dose difference at the isocenter, on an average, was 1.7% for pretreatment verification and less than 3% for all in vivo plans, except for head-and-neck, which was 3.6%. The mean γ values for a seven-field prostate IMRT plan delivered to the Alderson phantom varied from 0.28 to 0.65. For 3D conformal plans applied for the Alderson phantom, all γ1% values were within the tolerance level for all plans and in both anteroposterior and posteroanterior (AP-PA) beams.Conclusion:The 2D EPID-based dosimetry algorithm provides an accurate method to verify the dose of a simple 10 × 10 cm2 field, in two dimensions, inside a homogenous slab phantom and an IMRT prostate plan, as well as in 3D conformal plans (prostate, head-and-neck, and lung plans) applied using an anthropomorphic phantom and in vivo. However, further investigation to improve the 2D EPID dosimetry algorithm for a head-and-neck case, is necessary.

Dose rate constants for the quantity Hp(3) for frequently used radionuclides in nuclear medicine

According to recent studies, the human eye lens is more sensitive to ionising radiation than previously assumed. Therefore, the dose limit for personnel occupationally exposed to ionising radiation will be lowered from currently 150 mSv to 20 mSv per year. Currently, no data base for a reliable estimation of the dose to the lens of the eye is available for nuclear medicine. Furthermore, the dose is usually not monitored. The aim of this work was to determine dose rate constants for the quantity Hp(3), which is supposed to estimate the dose to the lens of the eye. For this, Hp(3)-dosemeters were fixed to an Alderson Phantom at different positions. The dosemeters were exposed to radiation from nuclides typically used in nuclear medicine in their geometries analog to their application in nuclear medicine, e.g. syringe or vial. The results show that the handling of high-energy beta (i.e. electron or positron) emitters may lead to a relevant dose to the lens of the eye. For low-energy beta emitters and gamma emitters, an exceeding of the lowered dose limit seems to be unlikely.

Influence of the phantom shape (slab, cylinder or Alderson) on the performance of an Hp(3) eye dosemeter.

In the past, the operational quantity Hp(3) was defined for calibration purposes in a slab phantom. Recently, an additional phantom in the form of a cylinder has been suggested for eye lens dosimetry, as a cylinder much better approximates the shape of a human head. Therefore, this work investigates which of the two phantoms, slab or cylinder, is more suitable for calibrations and type tests of eye dosemeters. For that purpose, a typical Hp(3) eye dosemeter was irradiated on a slab, a cylinder and on a human-like Alderson phantom. It turned out that the response on the three phantoms is nearly equal for angles of radiation incidence up to 45° and deviates only at larger angles of incidence. Thus, calibrations (usually performed at 0° radiation incidence) are practically equivalent on both the slab and the cylinder phantoms. However, type tests (up to 75° or even 90° radiation incidence) should be carried out on a cylinder phantom, as also for large angles of incidence the response on the cylinder and the Alderson phantoms is rather similar, whereas the response on the slab significantly deviates from the one on the Alderson phantom.

WE-D-BRA-04: Online 3D EPID-Based Dose Verification for Optimum Patient Safety

PURPOSE: To develop an online 3D dose verification tool based on EPID transit dosimetry to ensure optimum patient safety in radiotherapy treatments. METHODS: A new software package was developed which processes EPID portal images online using a back-projection algorithm for the 3D dose reconstruction. The package processes portal images faster than the acquisition rate of the portal imager (? 2.5 fps). After a portal image is acquired, the software seeks for in the reconstructed 3D dose distribution. A hot spot is in this study defined as a 4 cm(3) cube where the average cumulative reconstructed dose exceeds the average total planned dose by at least 20% and 50 cGy. If a hot spot is detected, an alert is generated resulting in a linac halt. The software has been tested by irradiating an Alderson phantom after introducing various types of serious delivery errors. RESULTS: In our first experiment the Alderson phantom was irradiated with two arcs from a 6 MV VMAT H&N treatment having a large leaf position error or a large monitor unit error. For both arcs and both errors the linac was halted before dose delivery was completed. When no error was introduced, the linac was not halted. The complete processing of a single portal frame, including hot spot detection, takes about 220 ms on a dual hexacore Intel Xeon 25 X5650 CPU at 2.66 GHz. CONCLUSION: A prototype online 3D dose verification tool using portal imaging has been developed and successfully tested for various kinds of gross delivery errors. The detection of hot spots was proven to be effective for the timely detection of these errors. Current work is focused on hot spot detection criteria for various treatment sites and the introduction of a clinical pilot program with online verification of hypo-fractionated (lung) treatments.

A novel, fully automated, observer-independent program for semiquantifying striatal 123I-FP-CIT uptake

ObjectiveTo describe and validate a novel, fully automated program specifically designed for the semiquantification of striatal 123I-FP-CIT uptake using volumes of interest (VOI) analysis. Material and methodsThe proposed algorithm is based on a template that mimics the striatal 123I-FP-CIT uptake in a healthy subjects, derived from defined anatomical VOIs available from WFU PickAtlas. Four SPECT studies of the anthropomorphic Alderson phantom filled with variable radioactive concentrations were acquired for the experimental validation. Experimental SPECT images were spatially normalized with respect to the previously created template. The binary VOIs corresponding to left caudate and putamen and right caudate and putamen, which were used to construct the template, were projected onto the experimental images to obtain the counts for these regions. To minimize the partial volume effect, a percentage of the voxels in these regions (threshold), rather than all of them, was used. A binary occipital VOI was used to quantify the non-specific uptake. Experimental binding potentials (BPs) were calculated from the counts in these regions. True BPs were calculated from aliquots taken from the solutions used to fill the phantom. ResultsThere were statistically significant differences in the experimental BP values (p<0.002) according to the percentage of voxels used. A highly significant correlation was achieved between true and experimental BP values, regardless of the percentage of voxels included for quantification. ConclusionsOur novel, observer-independent program automatically performs the semiquantification of striatal 123I-FP-CIT uptake in experimental studies.

Fetus absorbed dose evaluation in head and neck radiotherapy procedures of pregnant patients

In this work the head and neck cancer treatment of a pregnant patient was experimentally simulated. A female anthropomorphic Alderson phantom was used and the absorbed dose to the fetus was evaluated protecting the patient's abdomen with a 7cm lead layer and using no abdomen shielding. The target volume dose was 50Gy. The fetus doses evaluated with and without the lead shielding were, respectively, 0.52±0.039 and 0.88±0.052cGy.

A novel, fully automated, observer-independent program for semiquantifying striatal 123I-FP-CIT uptake

To describe and validate a novel, fully automated program specifically designed for the semiquantification of striatal (123)I-FP-CIT uptake using volumes of interest (VOI) analysis. The proposed algorithm is based on a template that mimics the striatal (123)I-FP-CIT uptake in a healthy subjects, derived from defined anatomical VOIs available from WFU PickAtlas. Four SPECT studies of the anthropomorphic Alderson phantom filled with variable radioactive concentrations were acquired for the experimental validation. Experimental SPECT images were spatially normalized with respect to the previously created template. The binary VOIs corresponding to left caudate and putamen and right caudate and putamen, which were used to construct the template, were projected onto the experimental images to obtain the counts for these regions. To minimize the partial volume effect, a percentage of the voxels in these regions (threshold), rather than all of them, was used. A binary occipital VOI was used to quantify the non-specific uptake. Experimental binding potentials (BPs) were calculated from the counts in these regions. True BPs were calculated from aliquots taken from the solutions used to fill the phantom. There were statistically significant differences in the experimental BP values (p<0.002) according to the percentage of voxels used. A highly significant correlation was achieved between true and experimental BP values, regardless of the percentage of voxels included for quantification. Our novel, observer-independent program automatically performs the semiquantification of striatal (123)I-FP-CIT uptake in experimental studies.

The impact of CBCT reconstruction and calibration for radiotherapy planning in the head and neck region – a phantom study

Background. The applicability of cone-beam computed tomography (CBCT) image sets for dose calculation purposes relies on high image quality and CT number accuracy. In this study we have investigated the use of stoichiometric calibration for transforming CT numbers into physical parameters, in combination with a new CBCT scatter correction algorithm, focusing on head and neck geometries.Methods. CBCT projections were acquired using an On-Board-Imager (OBI v1.4; Varian Medical Systems) using both low- and high-dose clinical image acquisition protocols. The CBCT projections were reconstructed twice, using both the standard method (OBI) as well as an experimental pre-clinical reconstruction algorithm (Full Fan Experimental - FFE). Stoichiometric calibration was performed using both a phantom from CIRS with nine tissue equivalent inserts (ranging from lung to dense bone) as well as with a custom made cylindrical PMMA head and neck phantom with variable ‘head’ diameter and with cavities designed to fit the inserts from a Gammex RMI phantom. To benchmark the CBCT performance, the same calibration procedures were performed using two conventional CT scanners. For assessment of influence on dose-volume parameters, the head part of the anthropomorphic Alderson phantom was scanned, reconstructed with both CT and CBCT using the stoichiometric calibration curves, and finally used to compare IMRT dose calculations.Results. The stoichiometric CBCT calibrations with the CIRS phantom resulted in an excellent fit between calculated and measured CT numbers (R = 1.000 for all combinations tested), equivalent to the results for the conventional scanners. Using the PMMA phantom, the stoichiometric calibration curves again showed excellent agreement, although the OBI reconstruction showed marginally increasing deviation from the unity line as the phantom size decreased. For the dose-volume parameters, deviations well within 1% were seen between the different reconstruction methods and acquisition modes.Conclusion. This study showed that the combination of an improved reconstruction method and stoichiometric calibration improved the CT number accuracy of CBCT scans acquired for head and neck phantoms. In particular, a radial size dependence of the scanned object similar to that in conventional CT could be achieved. Although high density inhomogeneities still are challenging for the reconstruction process, clinically acceptable agreement in key dose-volume parameters between CT-based and CBCT-based IMRT planning calculations on a humanoid phantom was found.

Radiation dose reduction in cone-beam computed tomography of extremities: evaluation of a novel radiation shield

Cone-beam computed tomography (CBCT) is a relatively new technique for imaging of extremities. It provides high-resolution images with lower effective dose compared to conventional CT. However following the ALARA principle, CBCT-imaging protocols and practices must also be optimised to minimize the dose absorbed by the patient as well as personnel. The aim of this study is to evaluate the effect of a novel scanner-attached radiation shield on the dose absorbed by the patient and on the amount of scattered radiation around the scanner.An orthopedic CBCT scanner was applied for comparing the doses with and without the shield during an elbow and a knee scan. A homogeneous 8 cm PMMA phantom with either an anthropomorphic Alderson phantom or a 16 cm PMMA phantom simulated the tissues of a patient. Measurements were made for several scan parameters using calibrated dose meters.The results show that the radiation shield significantly decreased the doses measured on the patient during CBCT scans of the elbow and the knee. The usage of the shield decreased the absorbed doses by up to 95.5%. Also scattered radiation around the gantry decreased notably. The use of the shield is highly recommended, especially for pediatric patients.

WE‐D‐BRF‐04: Experimental Investigations On Ion Radiography with Beam Scanning Using a Range Telescope

Purpose:Ion beams exhibit a finite range and an inverted depth‐dose profile, the Bragg peak. These favorable properties allow superior tumordose conformality, but introduce sensitivity to range uncertainties. Hence, imaging techniques play an increasingly important role to support the treatment planning and the in‐vivo monitoring of the actual ion beam treatment.Methods:This work presents the experimental investigations carried out to address the feasibility of ion transmission imaging at the Heidelberg Ion Therapy center using an active raster scanning beam delivery system and a prototype range telescope set‐up based on a stack of 61 parallel‐plate ionization chambers (PPIC) interleaved with 3 mm absorber plates of PMMA.Results:An extensive characterization of the set‐up in terms of beam parameters and settings of the read‐out electronics was performed and results will be presented. A data processing method to increase the range resolution (MIRR) of the PPIC stack was developed. In this approach, the position of the maximum of the Bragg curve is deduced from the ratio of measured signals in adjacent PPIC channels. MIRR evaluation is based on Bragg curves obtained from Monte Carlo simulations and validated with experimental data acquired with the PPIC stack using ion beams. MIRR was applied to the carbon ion radiography of an anthropomorphic Alderson head phantom yielding a resolution of 0.8 mm water equivalent thickness (WET) compared to the nominal value of 3.495 mm WET given by the thickness of the absorber slabs in the PPIC stack. An absolute comparison of the Alderson phantom carbon ion transmitted image with an X‐ray digitally reconstructed radiography, both converted into WET, will also be shown.Conclusion:The obtained results are very promising and motivate further developments of the system towards an eventual clinical use. This work is supported by the German Research Foundation and the German Academic Exchange Service.This work is supported by the German Research Foundation (DFG) and the German Academic Exchange Service (DAAD).

Radiation-induced second malignancies after involved-node radiotherapy with deep-inspiration breath-hold technique for early stage Hodgkin Lymphoma: a dosimetric study

BackgroundTo estimate the risk of radiation induced second cancers after radiotherapy using deep-inspiration breath-hold (DI) technique with three-dimensional conformal (3DCRT) and volumetric arc therapy (VMAT) for patients with Hodgkin’s lymphoma (HL).MethodsEarly-stage HL with mediastinal and supraclavicular involvement was studied using an Alderson phantom. A whole body CT was performed and all tissues were delineated. The clinical target volumes and planning target volumes (PTV) were determined according to the German Hodgkin study group guidelines. Free-breathing (FB) technique and DI technique were simulated by different safety margins for the PTV definition. In both cases, 30 Gy in 15 fractions was prescribed. Second cancer risk was estimated for various tissues with a second cancer model including fractionation.ResultsWhen compared with FB-3DCRT, estimated relative life time attributable risk (LAR) of cancer induction after DI-3DCRT was 0.86, 0.76, 0.94 and 0.92 for breast, lung, esophagus and stomach, respectively. With DI-VMAT, the corresponding values were 2.05, 1.29, 1.01, 0.93, respectively. For breast cancer, the LAR observed with DI-VMAT was not substantially distinguishable from the LAR computed for mantle RT with an administered dose of 40 Gy.ConclusionsThis study suggests that DI may reduce the LAR of secondary cancers of all OARs and may be a valuable technique when using 3DCRT. Conversely, VMAT may increase substantially the LAR and should be cautiously implemented in clinical practice.