Radioprotection of eye lens using protective material in neuro cone-beam computed tomography: Estimation of dose reduction rate and image quality

PurposeIn cerebral angiography, for diagnosis and interventional neuroradiology, cone-beam computed tomography (CBCT) scan is frequently performed for evaluating brain parenchyma, cerebral hemorrhage, and cerebral infarction. However, the patient’s eye lens is more frequently exposed to excessive doses in these scans than in the previous angiography and interventional neuroradiology (INR) procedures. Hence, radioprotection for the lenses is needed. This study selects the most suitable eye lens protection material for CBCT from among nine materials by evaluating the dose reduction rate and image quality.MethodsTo determine the dose reduction rate, the lens doses were measured using an anthropomorphic head phantom and a real-time dosimeter. For image quality assessment, the artifact index was calculated based on the pixel value and image noise within various regions of interest in a water phantom.ResultsThe protective materials exhibited dose reduction; however, streak artifacts were observed near the materials. The dose reduction rate and the degree of the artifact varied significantly depending on the protective material. The dose reduction rates were 14.6%, 14.2%, and 26.0% when bismuth shield: normal (bismuth shield in the shape of an eye mask), bismuth shield: separate (two separate bismuth shields), and lead goggles were used, respectively. The “separate” bismuth shield was found to be effective in dose reduction without lowering the image quality.ConclusionWe found that bismuth shields and lead goggles are suitable protective devices for the optimal reduction of lens doses.     

A comparison of electronic portal dosimetry verification methods for use in stereotactic radiotherapy

Three methods of transit dosimetry using Electronic Portal Imaging Devices (EPIDs) were investigated for use in routine in-vivo dosimetry for cranial stereotactic radiosurgery and radiotherapy. The approaches examined were (a) A full Monte Carlo (MC) simulation of radiation transport through the linear accelerator and patient; (b) Calculation of the expected fluence by a treatment planning system (TPS); (c) Point doses calculated along the central axis compared to doses calculated using parameters acquired using the EPID. A dosimetric comparison of each of the three methods predicted doses at the imager plane to within ±5% and a gamma comparison for the MC and TPS based approaches showed good agreement for a range of dose and distance to agreement criteria. The MC technique was most time consuming, followed by the TPS calculation with the point dose calculation significantly quicker than the other methods.     

Monte Carlo as quality control tool of stereotactic body radiation therapy treatment plans

Purpose/objectiveThe objective of this study was to verify the accuracy of treatment plans of stereotactic body radiation therapy (SBRT) and to verify the feasibility of the use of Monte Carlo (MC) as quality control (QC) on a daily basis.Material/methodsUsing EGSnrc, a MC model of Agility™ linear accelerator was created. Various measurements (Percentage depth dose (PDD), Profiles and Output factors) were done for different fields sizes from 1x1 up to 40x40 (cm²). An iterative model optimization was performed to achieve adequate parameters of MC simulation. 40 SBRT patient’s dosimetry plans were calculated by Monaco™ 3.1.1. CT images, RT-STRUCT and RT-PLAN files from Monaco™ being used as input for Moderato MC code. Finally, dose volume histogram (DVH) and paired t-tests for each contour were used for dosimetry comparison of the Monaco™ and MC.ResultsValidation of MC model was successful, as <2% difference comparing to measurements for all field’s sizes. The main energy of electron source incident on the target was 5.8 MeV, and the full width at half maximum (FWHM) of Gaussian electron source were 0.09 and 0.2 (cm) in X and Y directions, respectively. For 40 treatment plan comparisons, the minimum absolute difference of mean dose of planning treatment planning (PTV) was 0.1% while the maximum was 6.3%. The minimum absolute difference of Max dose of PTV was 0.2% while the maximum was 8.1%.ConclusionSBRT treatment plans of Monaco agreed with MC results. It possible to use MC for treatment plans verifications as independent QC tool.     

An automated Monte Carlo QC system for volumetric modulated arc therapy: Possibilities and challenges

PurposeTo develop and implement an automated Monte Carlo (MC) system for patient specific VMAT quality control in a patient geometry that generates treatment planning system (TPS) compliant DICOM objects and includes a module for 3D analysis of dose deviations. Also, the aims were to recommend diagnose specific tolerance criteria and an evaluation procedure.MethodsThe EGSnrc code package formed the basis for development of the MC system. The workflow consists of a number of modules connected to a TPS by means of manual DICOM exports and imports which were executed sequentially without user interaction. DVH comparison was performed in the TPS. In addition, MC- and TPS dose distributions were analysed by applying the normalized dose difference (NDD) formalism. NDD failure maps and a pass rate for a certain threshold were obtained. 170 clinical plans (prostate, thorax, head-and-neck and gynecological) were selected for analysis.ResultsAgreement within 1.5% was found between clinical- and MC data for the mean dose to the target volumes and within 3% for parameters more sensitive to the shape of the DVH e.g. D_98% PTV. Regarding the NDD analysis, tolerance criteria 2%/3 mm were established for prostate plans and 3%/3 mm for the rest of the cases.ConclusionsAn automated MC system was developed and implemented. Evaluation procedure is recommended with NDD-analysis as a first step. For pass rate < 95%, the evaluation continues with comparison of DVH parameters. For deviations larger than 2%, a visual inspection of the clinical- and MC dose distributions is performed.     

Dosimetric impact of statistical uncertainty on Monte Carlo dose calculation algorithm in volumetric modulated arc therapy using Monaco TPS for three different clinical cases

AimTo study the dosimetric impact of statistical uncertainty (SU) per plan on Monte Carlo (MC) calculation in Monaco? treatment planning system (TPS) during volumetric modulated arc therapy (VMAT) for three different clinical cases.BackgroundDuring MC calculation SU is an important factor to decide dose calculation accuracy and calculation time. It is necessary to evaluate optimal acceptance of SU for quality plan with reduced calculation time.Materials and methodsThree different clinical cases as the lung, larynx, and prostate treated using VMAT technique were chosen. Plans were generated with Monaco? V5.11 TPS with 2% statistical uncertainty. By keeping all other parameters constant, plans were recalculated by varying SU, 0.5%, 1%, 2%, 3%, 4%, and 5%. For plan evaluation, conformity index (CI), homogeneity index (HI), dose coverage to PTV, organ at risk (OAR) dose, normal tissue receiving dose ≥5 Gy and ≥10 Gy, integral dose (NTID), calculation time, gamma pass rate, calculation reproducibility and energy dependency were analyzed.ResultsCI and HI improve as SU increases from 0.5% to 5%. No significant dose difference was observed in dose coverage to PTV, OAR doses, normal tissue receiving dose ≥5 Gy and ≥10 Gy and NTID. Increase of SU showed decrease in calculation time, gamma pass rate and increase in PTV max dose. No dose difference was seen in calculation reproducibility and dependent on energy.ConclusionFor VMAT plans, SU can be accepted from 1% to 3% per plan with reduced calculation time without compromising plan quality and deliverability by accepting variations in point dose within the target.     

Clinical verification of treatment planning dose calculation in lung SBRT with GATE Monte Carlo simulation code

PurposeThis study aims to use GATE/Geant4 simulation code to evaluate the performance of dose calculations with Anisotropic Analytical Algorithm (AAA) in the context of lung SBRT for complex treatments considering images of patients.MethodsFour cases of non-small cell lung cancer treated with SBRT were selected for this study. Irradiation plans were created with AAA and recalculated end to end using Monte Carlo (MC) method maintaining field configurations identical to the original plans. Each treatment plan was evaluated in terms of PTV and organs at risk (OARs) using dose-volume histograms (DVH). Dosimetric parameters obtained from DVHs were used to compare AAA and MC.ResultsThe comparison between the AAA and MC DVH using gamma analysis with the passing criteria of 3%/3% showed an average passing rate of more than 90% for the PTV structure and 97% for the OARs. Tightening the criteria to 2%/2% showed a reduction in the average passing rate of the PTV to 86%. The agreement between the AAA and MC dose calculations for PTV dosimetric parameters (V₁₀₀; V₉₀; Homogeneity index; maximum, minimum and mean dose; CI_Paddick and D_2cm) was within 18.4%. For OARs, the biggest differences were observed in the spinal cord and the great vessels.ConclusionsIn general, we did not find significant differences between AAA and MC. The results indicate that AAA could be used in complex SBRT cases that involve a larger number of small treatment fields in the presence of tissue heterogeneities.     

Establishment of postal audit system in intensity-modulated radiotherapy by radiophotoluminescent glass dosimeters and a radiochromic film

We developed an efficient postal audit system to independently assess the delivered dose using radiophotoluminescent glass dosimeters (RPLDs) and the positional differences of fields using EBT3 film at the axial plane for intensity-modulated radiotherapy (IMRT). The audit phantom had a C-shaped target structure as a planning target volume (PTV) with four measurement points for the RPLDs and a cylindrical structure as the organ at risk (OAR) for one measurement point. The phantoms were sent to 24 institutions. Point dose measurements with a 0.6 cm³ PTW farmer chamber were also performed to justify glass dosimetry in IMRT. The measured dose with the RPLDs was compared to the calculated dose in the institution’s treatment planning system (TPS). The mean ± 1.96σ of the ratio of the measured dose with the RPLDs to the farmer chamber was 0.997 ± 0.024 with no significant difference (p = .175). The investigations demonstrated that glass dosimetry was reliable with a high measurement accuracy comparable to the chamber. The mean ± 1.96σ for the dose differences with a reference of the TPS dose for the PTV and the OAR was 0.1 ± 2.5% and −2.1 ± 17.8%, respectively. The mean ± 1.96σ for the right-left and the anterior–posterior direction was −0.9 ± 2.8 and 0.5 ± 1.4 mm, respectively. This study is the first report to justify glass dosimetry for implementation in IMRT audit in Japan. We demonstrate that our postal audit system has high accuracy with a high-level criterion of 3%/3 mm.     

Correlation between eye lens doses and over apron doses in interventional procedures

Measurements of eye lens dose using over apron dosimeters with a geometric correction factor is an international accepted practice. However, further knowledge regarding geometric correction factors in different contexts is required. The authors studied the correlation between eye lens dose and over apron dosimetry for different medical specialties in eleven hospitals, using a standardized protocol, two independent over apron dosimeters (worn at chest and at neck levels) and a dedicated calibration procedure. The results show good correlation between subjects working on the same medical specialty for 5 specialties: Interventional Radiology, Vascular Surgery, Vascular Radiology, Hemodynamics and Neuroradiology. The geometric correction factors resulting from this study could be used to estimate eye lens dose using over apron dosimeters, which are more comfortable than eye lens dosimeters, as reported by the study subjects, as long as the increased uncertainty of the over apron dosimetry compared to the dedicated eye lens dosimetry is acceptable.     

Simplified patient-specific renal dosimetry in 177Lu therapy: a proof of concept

PurposeThe aim of this proof-of-concept study is to propose a simplified personalized kidney dosimetry procedure in ¹⁷⁷Lu peptide receptor radionuclide therapy (PRRT) for neuroendocrine tumors and metastatic prostate cancer. It relies on a single quantitative SPECT/CT acquisition and multiple radiometric measurements executed with a collimated external probe, properly directed on kidneys.MethodsWe conducted a phantom study involving external count-rate measurements in an abdominal phantom setup filled with activity concentrations of ^99mTc, reproducing patient-relevant organ effective half-lives occurring in ¹⁷⁷Lu PRRT. GATE Monte Carlo (MC) simulations of the experiment, using ^99mTc and ¹⁷⁷Lu as sources, were performed. Furthermore, we tested this method via MC on a clinical case of ¹⁷⁷Lu-DOTATATE PRRT with SPECT/CT images at three time points (2, 20 and 70 hrs), comparing a simplified kidney dosimetry, employing a single SPECT/CT and probe measurements at three time points, with the complete MC dosimetry.ResultsThe experimentally estimated kidney half-life with background subtraction applied was compatible within 3% with the expected value. The MC simulations of the phantom study, both with ^99mTc and ¹⁷⁷Lu, confirmed a similar level of accuracy. Concerning the clinical case, the simplified dosimetric method led to a kidney dose estimation compatible with the complete MC dosimetry within 6%, 12% and 2%, using respectively the SPECT/CT at 2, 20 and 70 hrs.ConclusionsThe proposed simplified procedure provided a satisfactory accuracy and would reduce the imaging required to derive the kidney absorbed dose to a unique quantitative SPECT/CT, with consequent benefits in terms of clinic workflows and patient comfort.     

Assessment of out-of-field doses in radiotherapy treatments of paediatric patients using Monte Carlo methods and measurements

PurposeTo assess out-of-field doses in radiotherapy treatments of paediatric patients, using Monte Carlo methods to implement a new model of the linear accelerator validated against measurements and developing a voxelized anthropomorphic paediatric phantom.MethodsCT images of a physical anthropomorphic paediatric phantom were acquired and a dosimetric planning using a TPS was obtained. The CT images were used to perform the voxelization of the physical phantom using the ImageJ software and later implemented in MCNP. In order to validate the Monte Carlo model, dose measurements of the 6 MV beam and Linac with 120 MLC were made in a clinical setting, using ionization chambers and a water phantom. Afterwards TLD measurements in the physical anthropomorphic phantom were performed in order to assess the out-of-field doses in the eyes, thyroid, c-spine, heart and lungs.ResultsThe Monte Carlo model was validated for in-field and out-of-field doses with average relative differences below 3%. The average relative differences between TLD measurements and Monte Carlo is 14,3% whilst the average relative differences between TLD and TPS is 55,8%. Moreover, organs up to 22.5 cm from PTV center show TLD and MCNP6 relative differences and TLD and TPS relative differences up to 21.2% and 92.0%, respectively.ConclusionsOur study provides a novel model that could be used in clinical research, namely in dose evaluation outside the treatment fields. This is particularly relevant, especially in pediatric patients, for studying new radiotherapy treatment techniques, since it can be used to estimate the development of secondary tumours.     

Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements

PurposeIn radiotherapy, accurate calculation of patient radiation dose is very important for good clinical outcome. In the presence of metallic implants, the dose calculation accuracy could be compromised by metal artefacts generated in computed tomography (CT) images of patients. This study investigates the influence of metal-induced CT artefacts on MC dose calculations in a pelvic prosthesis phantom.MethodsA pelvic phantom containing unilateral Ti prosthesis was CT-scanned and accurate Hounsfield unit (HU) values were assigned to known materials of the phantom as opposed to HU values produced through the artefact CT images of the phantom. Using the DOSXYZnrc MC code, dose calculations were computed in the phantom model constructed from the original CT images containing the artefacts and artefact-free images made from the exact geometry of the phantom with known materials. The dose calculations were benchmarked against Gafchromic EBT3 film measurements using 15 MeV electron and 10 MV photon beams.ResultsThe average deviations between film and MC dose data decreased from 3 ± 2% to 1 ± 1% and from about 6 ± 2% to 3 ± 1% for the artefact and artefact-free phantom models against film data for the electron and photon fields, respectively.ConclusionsFor the Ti prosthesis phantom, the presence of metal-induced CT artefacts could cause dose inaccuracies of about 3%. Construction of an artefact-free phantom model made from the exact geometry of the phantom with known materials to overcome the effect of artefacts is advantageous compared to using CT data directly of which the exact tissue composition is not well-known.     

Addressing the dosimetric impact of bone cement and vertebroplasty in stereotactic body radiation therapy

PurposeBone cement used for vertebroplasty can affect the accuracy on the dose calculation of the radiation therapy treatment. In addition the CT values of high density objects themselves can be misrepresented in kVCT images. The aim of our study is then to propose a streamlined approach for estimating the real density of cement implants used in stereotactic body radiation therapy.MethodsSeveral samples of cement were manufactured and irradiated in order to investigate the impact of their composition on the radiation dose. The validity of the CT conversion method for a range of photon energies was investigated, for the studied samples and on six patients. Calculations and measurements were carried out with various overridden densities and dose prediction algorithms (AXB with dose-to-medium reporting or AAA) in order to find the effective density override.ResultsRelative dose differences of several percent were found between the dose measured and calculated downstream of the implant using an ion chamber and TPS or EPID dosimetry. If the correct density is assigned to the implant, calculations can provide clinically acceptable accuracy (gamma criteria of 3%/2 mm). The use of MV imaging significantly favors the attribution of a correct equivalent density to the implants compared to the use of kVCT images.ConclusionThe porosity and relative density of the various studied implants vary significantly. Bone cement density estimations can be characterized using MV imaging or planar in vivo dosimetry, which could help determining whether errors in dose calculations are due to incorrect densities.     

On the evaluation of edgeless diode detectors for patient-specific QA in high-dose stereotactic radiosurgery

PurposeIn this work, the potential of an innovative “edgeless” silicon diode was evaluated as a response to the still unmet need of a reliable tool for plan dosimetry verification of very high dose, non-coplanar, patient-specific radiosurgery treatments. In order to prove the effectiveness of the proposed technology, we focused on radiosurgical treatments for functional disease like tremor or pain.MethodsThe edgeless diodes response has been validated with respect to clinical practice standard detectors by reproducing the reference dosimetry data adopted for the Treatment Planning System. In order to evaluate the potential for radiosurgery patient-specific treatment plan verification, the anthropomorphic phantom Alderson RANDO has been adopted along with three edgeless sensors, one placed in the centre of the Planning Target Volume, one superiorly and one inferiorly.ResultsThe reference dosimetry data obtained from the edgeless detectors are within 2.6% for output factor, off-axis ratio and well within 2% for tissue phantom ratio when compared to PTW 60,018 diode. The edgeless detectors measure a dose discrepancy of approximately 3.6% from the mean value calculated by the TPS. Larger discrepancies are obtained in very steep gradient dose regions when the sensors are placed outside the PTV.ConclusionsThe angular independent edgeless diode is proposed as an innovative dosimeter for patient quality assurance of brain functional disorders and other radiosurgery treatments. The comparison of the diode measurements with TPS calculations confirms that edgeless diodes are suitable candidates for patient-specific dosimetric verification in very high dose ranges delivered by non-isocentric stereotactic radiosurgery modalities.     

Monte Carlo derivation of AAPM TG-43 dosimetric parameters for GZP6 Co-60 HDR sources

Cobalt 60 source is generally available on high dose rate (HDR) afterloading equipment especially for treatment of gynecological lesions. The GZP6 remote afterloader (Nuclear Power Institute of China) utilizes ⁶⁰Co sources for treatment of intracavitary and intraluminal malignancies. In this study, the AAPM TG-43 dosimetric parameters of three sources in GZP6 system have been studied using MCNP4C Monte Carlo (MC) code; and the results are compared with other available ⁶⁰Co HDR sources. The presented parameters consist of air kerma strength, dose rate constant, radial dose function and anisotropy function. They show less than 1% uncertainty. The TG-43 based dosimetry data can be used not only to validate the dedicated treatment planning software (TPS), but also to introduce new complementary software to enhance the system performance in gynecological treatments.     

Geometric and dosimetric quality assurance using logfiles and a 3D helical diode detector for Dynamic WaveArc

PurposeTo conduct patient-specific geometric and dosimetric quality assurance (QA) for the Dynamic WaveArc (DWA) using logfiles and ArcCHECK (Sun Nuclear Inc., Melbourne, FL, USA).MethodsTwenty DWA plans, 10 for pituitary adenoma and 10 for prostate cancer, were created using RayStation version 4.7 (RaySearch Laboratories, Stockholm, Sweden). Root mean square errors (RMSEs) between the actual and planned values in the logfiles were evaluated. Next, the dose distributions were reconstructed based on the logfiles. The differences between dose-volumetric parameters in the reconstructed plans and those in the original plans were calculated. Finally, dose distributions were assessed using ArcCHECK. In addition, the reconstructed dose distributions were compared with planned ones.ResultsThe means of RMSEs for the gantry, O-ring, MLC position, and MU for all plans were 0.2°, 0.1°, 0.1 mm, and 0.4 MU, respectively. Absolute means of the change in PTV D_99% were 0.4 ± 0.4% and 0.1 ± 0.1% points between the original and reconstructed plans for pituitary adenoma and prostate cancer, respectively. The mean of the gamma passing rate (3%/3 mm) between the measured and planned dose distributions was 97.7%. In addition, that between the reconstructed and planned dose distributions was 99.6%.ConclusionsWe have demonstrated that the geometric accuracy and gamma passing rates were within AAPM 119 and 142 criteria during DWA. Dose differences in the dose-volumetric parameters using the logfile-based dose reconstruction method were also clinically acceptable in DWA.     

Addressing the efficiency of X-ray protective eyewear: Proposal for the introduction of a new comprehensive parameter,the Eye Protection Effectiveness (EPE)

Radioprotection of the eye lens of medical staff involved in Surgical procedures is a subject of international debates since ICRP recommended, on 2011, a lower equivalent dose limit for the lens of the eye. In this work we address the effectiveness of different models of X-ray protective eyewear by relating actual dosimetry measurements to an ad hoc developed mathematical model, in order to disentangle the contribution of geometrical factors and shield capabilities. Phantom irradiation was carried out in fixed exposure conditions in angiographic room: we found that measured Dose Reduction Factors (DRF) strongly depend on the ergonomics of the investigated eyewear. Actually a very poor DRF was observed in the case of a glass model in spite of its high nominal attenuation, whereas a protective tool with low shielding capabilities such a visor resulted much more effective as a consequence of is shape (i.e. extended geometric protection of the eye lens). Our work highlights the need of the introduction of a specific parameter to quantify the effectiveness of the protection tools and able to predict their DRF by taking into account the geometry of the clinical condition of exposure. Aiming at making steps forward the standardization of the guidelines concerning the features of eye protective tools, we developed a simple mathematical model describing the eye lens irradiation geometry which allows the introduction, for each eyewear, of a comprehensive parameter, the Eye Protection Effectiveness (EPE), that, for any defined clinical irradiation condition and glass shielding capabilities and shape, defines the overall effective X-ray protection of the eyewear.     

Characterization of passive dosimeters in proton pencil beam scanning – A EURADOS intercomparison for mailed dosimetry audits in proton therapy centres

The lack of mailed dosimetry audits of proton therapy centres in Europe has encouraged researchers of EURADOS Working Group 9 (WG9) to compare response of several existing passive detector systems in therapeutic pencil beam scanning.Alanine Electron Paramagnetic Resonance dosimetry systems from 3 different institutes (ISS, Italy; UH, Belgium and IFJ PAN, Poland), ^natLiF:Mg, Ti (MTS-N) and ^natLiF:Mg, Cu, P (MCP-N) thermoluminescent dosimeters (TLDs), GD-352M radiophotoluminescent glass dosimeters (RPLGDs) and Al₂O₃:C optically stimulated dosimeters (OSLDs) were evaluate. Dosimeter repeatability, batch reproducibility and response in therapeutic Pencil Beam Scanning were verified for implementation as mail auditing system.Alanine detectors demonstrated the lowest linear energy transfer (LET) dependence with an agreement between measured and treatment planning system (TPS) dose below 1%. The OSLDs measured on average a 6.3% lower dose compared to TPS calculation, with no significant difference between varying modulations and ranges. Both GD-352M and MCP-N measured a lower dose than the TPS and luminescent response was dependent on the LET of the therapeutic proton beam. Thermoluminescent response of MTS-N was also found to be dependent on the LET and a higher dose than TPS was measured with the most pronounced increase of 11%.As alanine detectors are characterized by the lowest energy dependence for different parameters of therapeutic pencil beam scanning they are suitable candidates for mail auditing in proton therapy. The response of luminescence detector systems have shown promises even though more careful calibration and corrections are needed for its implementation as part of a mailed dosimetry audit system.     

The effects of simulating a realistic eye model on the eye dose of an adult male undergoing head computed tomography

In head computed tomography, radiation upon the eye lens (as an organ with high radiosensitivity) may cause lenticular opacity and cataracts. Therefore, quantitative dose assessment due to exposure of the eye lens and surrounding tissue is a matter of concern. For this purpose, an accurate eye model with realistic geometry and shape, in which different eye substructures are considered, is needed. To calculate the absorbed radiation dose of visual organs during head computed tomography scans, in this study, an existing sophisticated eye model was inserted at the related location in the head of the reference adult male phantom recommended by the International Commission on Radiological Protection (ICRP). Then absorbed doses and distributions of energy deposition in different parts of this eye model were calculated and compared with those based on a previous simple eye model. All calculations were done using the Monte Carlo code MCNP4C for tube voltages of 80, 100, 120 and 140 kVp. In spite of the similarity of total dose to the eye lens for both eye models, the dose delivered to the sensitive zone, which plays an important role in the induction of cataracts, was on average 3% higher for the sophisticated model as compared to the simple model. By increasing the tube voltage, differences between the total dose to the eye lens between the two phantoms decrease to 1%. Due to this level of agreement, use of the sophisticated eye model for patient dosimetry is not necessary. However, it still helps for an estimation of doses received by different eye substructures separately.