Lung Dose Calculation Research Articles

MO-E-AUD B-06: A 4D Monte Carlo Dose Calculation Method for Clinical Applications

Purpose: The accumulated dose accuracy depends by both the image registration accuracy and the dose addition strategy. In this study, we introduce a Monte Carlo‐based approach for cumulative dose computation based on energy mapping between various datasets. Method and Materials: EGSnrc/DOSXYZnrc Monte Carlo code is modified such that the energy (rather than the dose) is mapped from a “source” dataset onto a “target” dataset. The dose is subsequently calculated as the ratio of the warped energy deposited and the warped mass. The latter is the sum of all source voxels mass weighted by the ratio of the number of particles from “source” to “target” and the total number of particles scored in the “source” voxel. A lookup table for the source‐to‐target distribution is created for each voxel. The cumulative dose calculation requires displacement vector fields (DVFs) between “source” and “target” images. We use, for exemplification, two different DVFs, one generated by using the ITK “demons”, and the other by using the visco‐fluid model registration method. Results: The algorithm is integrated with Pinnacle planning system and is demonstrated through a 4D treatment plan. The average difference between doses reconstructed using the two DVFs is 15.6% for mean lung dose, 13.6% for heart, and 2.1% for ITV. It appears that lung doses are affected the most by the DVFs used. Conclusion: The forward 4D MC method introduced here uses both mass and energy deposition sampling, is potentially more efficient, as it does not require tracking of the deformed boundary, and is expected to provide more accurate results than interpolation‐based dose accumulation approaches, especially for the lung dose calculation. However, it does not eliminate the bias introduced by image registration errors.

Monte Carlo dose calculations for a 6-MV photon beam in a thorax phantom

In this study we evaluated the accuracy of the Monte Carlo (MC) and effective path length (EPL) methods for dose calculations in the inhomogeneous thorax phantom. The Philips SL 75/5 linear accelerator head was modeled using the MCNP4C Monte Carlo code. An anatomic inhomogeneous thorax phantom was irradiated with a 6-MV photon beam, and the doses along points of the central axis of the beam were measured by a small ionization chamber. The central axis relative dose was calculated by the MCNP4C code and the EPL method in a conventional treatment planning system. The results of calculations and measurements were compared. For all measured points on the thorax phantom the results of the MC method were in agreement with the actual measurement (local difference was less than 2%). For the EPL method, the amount of error was dependent on the field size and the point location in the phantom. The maximum error was +19.5 and +26.8 for field sizes of 10 x 10 and 5 x 5 cm2 for lateral irradiation. Our study showed large, unacceptable errors for EPL calculations in the lung for both field sizes. The accuracy of the MC method was better than the recommended value of 3%. Thus, application of this method is strongly recommended for lung dose calculations, especially for small field sizes.

WE-D-224A-04: Verification of the Lung Dose Calculation of a Commercial IMRT Planning System Using a Realistic Lung Phantom

Purpose: The purpose of this work is to find out the accuracy of a popular IMRT TPS system (NOMOS CORVUS) in lung dose calculation. Method and Materials: Up top now most of lung dose measurements have been done only for slab geometry of inhomogeneity and for a single beam. In this work, we use a realistic lung phantom and deliver all fields of realistic IMRT plans. The phantom is supplied with cylindrical inserts, made of equivalent materials of lung, bone, and tissue, which were used to load the dosimeters. The dose to phantom was calculated, for 16 IMRT plans, with the Corvus system. Verification was done with measurements using TLDs and ionization chambers measurement, as well as Monte Carlo simulation. For each treatment plan, the dose was verified at points located in lung, bone, and tissue. Results: The comparison of the collected data shows that the dose to the lung calculated with Corvus was overestimated by 2% to 10% relative to the Monte Carlo results, and by 2% to 7% relative to the ion chamber measurements. The TLD measurements show better agreement to the Monte Carlo results than to the Corvus results. In bone and tissue the dose results show an agreement, within ±3%, among all the calculations and measurements. Conclusion: The dose calculation accuracy in lung has been estimated for an IMRT planning system. It indicates that the dose algorithms have to be improved in order to have an accuracy of a few percent in lung.

TH‐D‐ValA‐08: Experimental Evaluation of the Accuracy of Contralateral Lung Dose Calculations for IMRT Plans

Purpose: To experimentally determine the accuracy of contralateral lung dose calculated by Eclipse for mesothelioma IMRT plans. Materials and Methods: Two planning techniques were used to create 9 mesothelioma IMRT plans in Eclipse, which were then applied to a Rando torso phantom. Group A consisted of 5 plans with 160° of the contralateral side free from entrance beams while the 4 Group B plans had 80°–90° free. MOSFETs were placed at 10 coplanar points within the contralateral lung of the phantom, and doses measured for each field for each plan. These were compared with doses calculated using the pencil beam algorithm (modified Batho) in Eclipse. Two controls (ipsilateral AP/PA and 40×40cm AP/PA) were evaluated. Results: The 40×40cm AP/PA control plan showed good agreement between Eclipse and experimental results with the difference being 1.04% +/− 3.62% (1 stdev), while the ipsilateral only AP/PA plan showed a disagreement of −33.3% +/− 6.9% (Eclipse was cold compared with measurements) suggesting poor scatter modeling. The overall average difference between measured and calculated daily doses of Group A and B plans was −21.1% +/− 15.7% (−6.98cGy +/− 5.2cGy) and −9.8% +/− 29% (−6.58cGy +/− 19.5cGy), respectively. The differences were dependent on whether the points were in or out of individual fields. The average difference for points in field for Group A and B plans were −5.5% +/−23.4% (−1.169 +/− 4.99cGy) and +3.04% +/− 37.9% (1.57 +/− 19.6cGy), respectively. This increased to −49.2% +/− 10.9% (−5.8 +/− 1.28cGy) and −41% +/− 14.9% (−8.66 +/− 3.12 cGy), respectively, for points outside the field. Conclusions: Eclipse adequately models lung dose in field, but underestimates scatter dose out of field by up to 50%. This should be considered when evaluating contralateral lung DVH's, especially when attempts are made to use IMRT to lower mean lung dose.

Experimental and Monte Carlo evaluation of Eclipse treatment planning system for lung dose calculations

Summary Background In this study the accuracy of a pencil beam based treatment planning system (TPS) was evaluated for lung dose calculations by comparison with measurement and the Monte Carlo (MC) method. Aim In the current study we assessed the performance of the Eclipse treatment planning system in the thorax region by ionization chamber measurements and Monte Carlo calculations. We examined two analytic methods: modified Batho (MB) and equivalent tissue-air ratio “ETAR” methods for thorax region irradiations. For Monte Carlo calculations in the thorax phantom, we modelled a Varian Clinac 2100EX linac. After benchmarking our model with water phantom measurements we used this model for thorax phantom calculations. Materials/Methods 8 and 15 MV photon beams of Varian 21EX linac were used for irradiations. Using MANP4C Monte Carlo code, the geometry of the linac head was simulated. After commissioning “MC” beam models, lung doses were calculated by the Monte Carlo (MC) method. Irradiation cases were: (1) posterior fields of single lung with field sizes of 4×4 and 10×10cm 2 (2) lateral fields of thorax with 4×4 and 10×10cm 2 field sizes. Results TPS calculations involving ETAR and MB methods were in close agreement with Monte Carlo results and measurements for a 10×10 cm 2 field size at both energies. For a field size of 4×4cm 2 the maximum differences in local dose between TPS calculations and measurement were +33% (MB) and +28% (ETAR). Also, they ignored lung dose reduction due to lateral electronic equilibrium for small field size. Similar results would be expected for other TPSs implementing these algorithms. MC calculations were in excellent agreement with measurement, showing local differences of no more than 2% for all measured points. Conclusions Our study findings showed great differences between both analytical methods and measurements for 4×4cm 2 field sizes for points in the lung. Our study recommends using the MC method for small-field lung dose calculations.

SU‐FF‐T‐143: Evaluation of the Lung Dose Calculation Accuracy for An IMRT Planning System

Purpose: Image guided treatment of lung provides the ability to deliver dose precisely to the target, however, lung dose calculation is often difficult and might be the weakest link now in the chain of lung IGRT. The purpose in this work is to find out the accuracy of a common TPS system in lung dose calculation. Method and Materials: So far lung dose measurements have been done only in slab geometry and for a single beam. In this work, we used a realistic lung phantom and delivered all fields of realistic IMRT plans. The phantom is supplied with cylindrical inserts, made of lung, bone, and tissue, which were used to load the dosimetry equipments. The dose to the phantom was calculated, for 10 IMRT treatment plans, with the Corvus system. Validation of the calculated dose was performed with LiF thermoluminescent dosimeters (TLDs) measurement, Ionization chamber measurement, and Monte Carlo simulation. For each treatment plan, the dose was verified at points located in lung, bone, and tissue. Results: The comparison of the collected data shows that the dose to the lung calculated with Corvus was overestimated by 10% relative to the Monte Carlo results, and by 7% relative to the chamber measurements. The TLDs dose results show better agreement to the Monte Carlo results than to the Corvus results. In bone the Monte Carlo agreed well with both TLDs and chamber measurements (within 3.5%) while Corvus dose was 6.5% different. The dose to the tissue shows good agreement (within 2%) between all the dosimetry tools. Conclusion: The dose calculation accuracy in lung has been estimated for IMRT planning system. It indicates some better dose algorithms might be needed in order to have an accuracy of a few percent.

The effect of finite patient dimensions and tissue inhomogeneities on dosimetry planning of 192Ir HDR breast brachytherapy: A Monte Carlo dose verification study

To evaluate the accuracy of clinical dosimetry planning using commercially available treatment planning systems in (192)Ir high-dose-rate (HDR) breast brachytherapy, with emphasis on skin dose, in view of potential uncertainties owing to the patient finite dimensions and the presence of the lung. A patient-equivalent mathematical phantom was constructed on the basis of the patient computed tomography scan used in the clinical treatment planning procedure. The actual treatment plan delivered to the patient, involving an implant of six plastic catheters and 26 programmed source dwell positions, was simulated by means of the Monte Carlo method. Results are compared with corresponding dose calculations of a commercially available treatment planning system in the form of prescribed dose percentage isodose contours and cumulative dose-volume histograms. The comparison of Monte Carlo results and treatment planning system calculations revealed that all percentage isodose contours greater than 60% of the prescribed dose are not affected by the finite breast dimensions or the presence of the lung. Treatment planning system calculations overestimate dose in the lung as well as lower isodose contours at points lying both close to the breast or lung surface and relatively away from the implant. In particular, skin dose is overestimated by 5% in the central breast region and within 10% at all other points. Dose-volume histogram and all other relevant planning quality indices for the planning target volume calculated by the treatment planning system are credible. Skin and lung dose calculations by the treatment planning system can be thought of as a conservative approach in view of the reported dose overestimation.

Theoretical foundation for a simple method for simultaneous measurements of the unattached fraction and activity median diameter of attached radon progeny

Calculation of lung dose from established lung dosimetry models requires use of the unattached fraction of potential alpha energy concentration (PAEC) of radon progeny and the activity median diameter (AMD) of attached radon progeny, in addition to the total PAEC. In the present work, for indoor environments without the nucleation mode of aerosols, a method based on the wire screen penetration theory using two wire screens and a filter is proposed for simultaneous measurements of these two parameters. It is shown that the traditional wire screen method can overestimate or sometimes underestimate the unattached fraction, depending on the properties of the wire screen and the true unattached fraction. The present method eliminates such uncertainties.

Regional Human Lung Dose Following Inhalation of Radioactive Particles at Ages One Month to Adulthood

Abstract A model was developed to relate respiratory tract deposition of inhaled aerosol particles to age at exposure, beginning at 1 month old. The model was expanded to include calculation of radiation dose to the lung when the exposure was continuous through 70 years of life. An aerosol simulating 239PuO2 particles of 0.3μm aerodynamic diameter and in an air concentration of 1 Bq m–3 was used to establish the cumulative radiation doses over the lifetime exposure, using retention half-lives of 1, 10, 100, and 1000 days in the pulmonary region. A computer program was written to use the model in deposition calculations. The program is sufficiently flexible to accept all variables pertinent to lung deposition and dose calculations as a function of age at exposure.

Lung dose calculations for inhaled alpha emitters using personal computers

Lung dosimetry of inhaled alpha-emitters is performed with complex models in scientific research work, while calculations for practical applications in radiation protection are use equations with simple multiplicative factors. Because of the many parameters involved in lung dose calculations and the variation of exposure conditions the use of such factors is not an appropriate way to get reliable information about radiation risks. The wide spread of computer systems and the fact that computers are very powerful nowadays makes it possible to do more sophisticated calculations with a computersystem. Basic requirements for practical applications of dosimetric models is the simplicity in handling and performing the calculations. The use of a computer system allows to do calculations with a preprogrammed model by defining and adapting only the relevant parameters. The structure of the model and the implementation on small computers as well as the parameters involved in lung dosimetry are discussed in this paper.

Accuracy of lung dose calculations for large-field irradiation with 6-MV x rays.

In large-field irradiations of the upper half-body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half-body) fields of 6-MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bjärngard), the simple scaled tissue-maximum ratio and the detailed equivalent tissue-air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.

Improved lung dose calculation using tissue-maximum ratios in the Batho correction.

We have reexamined the Batho power law for computing the dose within and beyond lung irradiated with small and large fields of cobalt-60 and 6-MV x rays. Using slab phantoms consisting of two materials, agreement between calculated and measured doses was within 2% inside lung for 6-MV x irradiation, but much poorer (9%) for cobalt-60 irradiation. For cobalt-60 irradiation, tissue-air ratios (TARs) were used initially in the Batho equation, while for 6-MV x rays, tissue-maximum ratios (TMRs) were used. When we substituted TMR values instead of TAR values for cobalt-60, we found marked improvement by nearly 5% in the accuracy of dose calculated within lung. This was confirmed by numerical comparison of the Batho expression with an analytic solution of the primary and first-scattered radiation. We therefore encourage the use of TMRs for cobalt-60 radiation, especially for larger radiation fields, and provide measured data tables for field sizes up to 50 X 50 cm2, and depths up to 30 cm. In addition to unifying the dosimetry for all megavoltage irradiation, this approach improves the accuracy of doses calculated within lung.

Lung dose calculations using computerized tomography: Is there a need for pixel based procedures?

Radiotherapy of tumors within the thorax often requires radiation beams that traverse lung tissues. Not only do lungs consist of large volumes of low density tissues but, in addition, lung tissues are more radiosensitive than most other tissues. Computed tomography (CT) provides the detailed anatomic and geometric data ideally suited for accurate lung dose calculations. However, most radiotherapy centers do not have direct access to a CT scanner for planning purposes. In addition, it is argued by some that it is not necessary to use the CT pixel data directly for lung dose calculations but that it is sufficient to have the lung geometry outlined and to assume some average bulk density (not related to that specific patient) for the total lung volume. This study analyzes the potential errors in lung dose calculations when different lung density assumptions are made. The conclusions show that different levels of accuracy can be achieved with different levels of sophistication in lung density assumptions. However, the delivery of radiation absorbed dose to lung to an accuracy of 5% for all patients will require the detailed anatomical density data that are provided by CT.

Cellular lung dosimetry for inhaled radon decay products as a base for radiation-induced lung cancer risk assessment. II. Microdosimetric calculations.

Lung dose calculations for inhaled radon decay products presented in part I have revealed that mean basal cell doses are significantly dependent on various personal and environmental factors. Whereas these macroscopic dosimetric methods have been applied with great success to radiation protection problems, the interpretation of radiobiological effects, such as lung cancer incidence, needs some refinement of these methods. Energy deposition at the microscopic level as the physical input quantity and radiation carcinogenesis as the biological endpoint are by nature stochastic processes. Therefore, a microdosimetric model was developed taking into consideration the randomness of physical and biological parameters involved, Part II of the paper presents results on specific energy distributions in lung cells, demonstrating that single event density distributions together with the number of cells receiving single hits represent more appropriate parameters than mean radiation doses.