Tissue Heterogeneity Corrections Research Articles

MOSFET Assessment of Radiation Dose Delivered to Mice Using the Small Animal Radiation Research Platform (SARRP)

The Small Animal Radiation Research Platform (SARRP) is a novel isocentric irradiation system that enables state-of-the-art image-guided radiotherapy research to be performed with animal models. This paper reports the results obtained from investigations assessing the radiation dose delivered by the SARRP to different anatomical target volumes in mice. Surgically implanted metal oxide semiconductor field effect transistors (MOSFET) dosimeters were employed for the dose assessment. The results reveal differences between the calculated and measured dose of -3.5 to 0.5%, -5.2 to -0.7%, -3.9 to 0.5%, -5.9 to 2.5%, -5.5 to 0.5%, and -4.3 to 0% for the left kidney, liver, pancreas, prostate, left lung, and brain, respectively. Overall, the findings show less than 6% difference between the delivered and calculated dose, without tissue heterogeneity corrections. These results provide a useful assessment of the need for tissue heterogeneity corrections in SARRP dose calculations for clinically relevant tumor model sites.

SU-E-T-743: The Comparison of Dose Modification Factors for Two Multi-Lumen Brachytherapy Applicators Used in Partial Breast Irradiation

Purpose: When using a multi‐lumen applicator for partial breast irradiation, the desired result may be to produce an asymmetric dose distribution. An asymmetric dose distribution is necessary when the distance between the applicator and/or chest wall is small (<7mm). This small distance causes the air outside the skin and in the lung to act as a low density and poorly scattering medium. This may be problematic since most treatment planning systems do not account for tissue heterogeneity corrections. The purpose of this investigation is to quantify the inaccuracies of the treatment planning system by utilizing a parameter known as the dose modification factor (DMF). Methods: The DMF is defined as the ratio of the dose rate at 1 cm beyond the applicator's surface (PTV) with full scatter to the dose rate with a finite tissue thickness beyond the PTV. The two applicators used were Contura (SensoRx, Inc.) and SAVI (Cianna Medical, Inc.). The treatment planning system used to create symmetric and asymmetric plans was BrachyVision (Varian Medical Systems, Inc.). Results: For both applicators, the DMF increased exponentially as the thickness of tissue beyond the PTV decreased. The maximum measured DMFs for the symmetric plans using the Contura and SAVI applicators are 1.0878 and 1.0962, respectively. The maximum measured DMFs for the asymmetric plans are 1.0858 and 1.1148, respectively. Relative error for all measurements of DMF was less than or equal to ±0.005. Conclusions: With the DMFs mentioned above, it is obvious that there is a difference between the planned and measured dose distributions. The difference is due to the lack of heterogeneity corrections of the treatment planning system. Depending on the amount of tissue beyond the PTV, the planning system may over‐predict the dose by as much as 11%.

SU‐E‐T‐819: Impact of Dental Extractions on Dose Distribution and Planning for Head and Neck Intensity Modulated Radiation Therapy (IMRT)

Purpose: To investigate the equivalence of Head and Neck IMRT plans generated on a 1) pre‐dental extraction CT, with teeth to be removed contoured‐out as compared to 2) an actual post dental procedure CT. Ultimate goal is to improve clinic efficiency and through‐put by planning treatment during the patient recovery time from a dental extraction rather then after. Methods: A jaw phantom (constructed using 28 human teeth embedded in Aqua‐Plast mandible structures, submersed in a H&amp;N water tank) was scanned in a clinical CT and 4 significant teeth (3 molars and a bicuspid; most with metal dental fillings) were contoured‐out; assigned HU=0. Simulating the post‐extraction CT, the 4 teeth were physically removed from the jaw. IMRT plans were then generated on these datasets simulating 5 clinically representative situations; critical structures and artefacts contoured and 3 PTVs of varying sizes representing stage T1, T2 and T3 oral cavity cancers were explored. Planning utilized a Pencil‐Beam Convolution algorithm and the Modified Batho tissue heterogeneity correction. Results: No perceptible differences were noted between IMRT plans optimized on pre‐extraction CTs with extracted teeth contoured‐out and plans optimized on the post‐extraction CTs in DVH and isodose coverage. Furthermore, IMRT plans calculated on post‐extraction datasets using fluences from pre‐extraction plans also showed no significant differences, validating the simulation of physical extraction by the contouring‐out process. Finally, plans calculated with teeth not contoured‐out showed underdosing in nearby regions as well as inferior DVHs. Conclusions: Results indicate that clinically comparable plans can be generated by contouring‐out teeth on a pre‐extraction CT. This suggests that our current practice of scheduling a planning CT at least 2 weeks after dental procedure may be unnecessary; instead this time can be used for treatment planning if swelling/jaw‐changes are deemed insignificant, thus saving significant wait times and increasing patient through‐put and clinic efficiency.

SU‐E‐T‐317: MOSFET Assessment of Radiation Dose Delivered to Mice Using the Small Animal Radiation Research Platform (SARRP)

Purpose: MOSFETS were employed in this study for in situ assessment of radiation dose delivered by the Small Animal Radiation Research Platform (SARRP) to mouse anatomical target volumes: brain, left kidney, left lung, liver, pancreas and prostate. Method and Materials:MOSFET detectors were calibrated with conversion factors (CF) determined for each MOSFET as a function of depth in tissue equivalent material. Calibrated MOSFET was surgically implanted post‐euthanasia into the mouse target volume, with minimal organ disruption. Using CBCT imaging and robotic motion, the mouse was then positioned with implanted MOSFET at the isocenter of the SARRP. The depth of the MOSFET from the CBCT image and corresponding CF were used in the SARRP dose calculation program to calculate the target dose for 220 kVp and 13 mA, without heterogeneity correction. Irradiations were conducted using a 0.15 cm copper filter. Three readings per MOSFET were obtained for each organ of three euthanized male mice. The measured dose was determined by averaging the MOSFET readings for that organ. The percentage difference between the measured and calculated dose was determined. Results: The results showed differences between the calculated and measured dose of between: +1.2 to 4.6 %, −3.3 to +4.8 %, +0.9 to +5.1 %, −5.9 to +2.5 %, ‐2.3 to +1.0 %, and −4.3 to +1.4 %, for the left kidney, liver, pancreas, prostate, left lung, and brain, respectively. Conclusions: The study indicates that, for 220 kVp x‐rays, the radiation dose delivered by the SARRP to characteristic target volumes in mice is within 6% of the calculated dose. The results provide a useful assessment of the need for tissue heterogeneity corrections in SARRP dose calculations for clinically relevant tumor model sites. The work also demonstrates the feasibility of using MOSFET dosimeters for in situ assessment of SARRP radiation dose delivery.

Multicenter phase II study of cetuximab (C) with concomitant radiotherapy (RT) followed by consolidation chemotherapy (CT) in locally advanced non-small cell lung cancer (NSCLC).

7019 Background: C, an anti-EGFR monoclonal antibody, enhances the efficacy of both RT and CT. We evaluated a novel strategy of RT plus C followed by consolidation CT plus C for the treatment of stage III NSCLC. Methods: Eligible were pts with unresectable stage IIIA or IIIB NSCLC, ECOG performance status (PS) 0-1, no history of pulmonary fibrosis and adequate end organ function. Pts were treated with chest RT (73.5 Gy in 35 fractions/7 weeks, once daily, with lung and tissue heterogeneity corrections (63 Gy without corrections) and C 250 mg/m2 weekly during RT (after a loading dose of 400 mg/m2 the week prior to starting RT). C was continued after RT completion and during consolidation therapy for 26 weeks. Paclitaxel 200 mg/m2 and carboplatin AUC of 6 were given for 3 cycles (every 3 weeks) after 4-6 weeks from RT. Primary endpoint was the overall survival. Results: Thirty-eight eligible pts were analyzed (2 ineligible pts were excluded from analysis). 25 males; median age 67 yrs; PS 0: 19, PS 1: 19; 14 adenocarcinomas, 10 squamous, 14 non-otherwise specified; Stage: 13 IIIA, 25 IIIB. Median corrected RT dose: 73.5 Gy (56-73.5); median C doses: 18. 33 pts received at least 1 cycle of consolidation. Grade 3/4 toxicities during RT/C were (in > 1 pt): pulmonary embolism (2/2), infection (2/1), fatigue (2/0) and rash (3/0). Grade 3/4 toxicities during consolidation: neutropenia (7/9), neutropenic fever (0/2), lymphopenia (4/0), infection (0/1), rash (2/0). One pt died of pneumonitis, possibly related to C. Best response using RECIST in 31 evaluable pts: 4 CR, 17 PR, 4 SD, 6 PD. With a median follow up of 17.75 months, the median OS and PFS were 17.1 and 9.3 months, respectively. Pts with EGFR positive tumors by FISH (17 of 28 tested) had a trend towards better PFS (p=0.07). Conclusions: C with RT followed by consolidation therapy was associated with acceptable toxicities and efficacy comparable to RT/CT. There was no associated grade 3-4 esophagitis. Supported in part by P50 CA090440.

Evaluation of pencil beam convolution and anisotropic analytical algorithms in stereotactic lung irradiation

The aim of this study was to evaluate differences in dose distributions in stereotactic body radiation therapy treatment plans for lung tumors calculated with pencil beam convolution (PBC) algorithm with modified Batho power law (MBPL) versus heterogeneity corrected anisotropic analytical algorithm (AAA) of the Varian Eclipse treatment planning system. The four-dimensional computed tomography images from 20 patients with lung cancer were used to create treatment plans. Plans used five to seven nonopposing coplanar 6 MV beams. Plans generated with the PBC algorithm and MBPL for tissue heterogeneity corrections were optimized to deliver 60 Gy in three fractions to at least 95% of the planned target volume, and the normal tissue doses for spinal cord, esophagus, heart, and ipsilateral bronchus were restricted to less than 18, 27, 30, and 30 Gy, respectively. Plans were recalculated with AAA, retaining identical beam arrangements, photon beam fluences, and monitor units. The pencil beam plans, designed to deliver 60 Gy, delivered on average 51.6 Gy when re-calculated with the AAA, suggesting a reduction of at least 10% to prescription dose is appropriate when calculating with the AAA.

Early Pulmonary Toxicity of Lung Stereotactic Body Radiation Therapy (SBRT) Delivered as Consecutive Daily Fractions

Early Pulmonary Toxicity of Lung Stereotactic Body Radiation Therapy (SBRT) Delivered as Consecutive Daily Fractions

Analysis of Radiation Pneumonitis (RP) Incidence in a Phase I Stereotactic Body Radiotherapy (SBRT) Dose Escalation Study for Multiple Metastases

Analysis of Radiation Pneumonitis (RP) Incidence in a Phase I Stereotactic Body Radiotherapy (SBRT) Dose Escalation Study for Multiple Metastases

Applications of tissue heterogeneity corrections and biologically effective dose volume histograms in assessing the doses for accelerated partial breast irradiation using an electronic brachytherapy source

A low-energy electronic brachytherapy source (EBS), the model S700 Axxent™ x-ray device developed by Xoft Inc., has been used in high dose rate (HDR) intracavitary accelerated partial breast irradiation (APBI) as an alternative to an Ir-192 source. The prescription dose and delivery schema of the electronic brachytherapy APBI plan are the same as the Ir-192 plan. However, due to its lower mean energy than the Ir-192 source, an EBS plan has dosimetric and biological features different from an Ir-192 source plan. Current brachytherapy treatment planning methods may have large errors in treatment outcome prediction for an EBS plan. Two main factors contribute to the errors: the dosimetric influence of tissue heterogeneities and the enhancement of relative biological effectiveness (RBE) of electronic brachytherapy. This study quantified the effects of these two factors and revisited the plan quality of electronic brachytherapy APBI. The influence of tissue heterogeneities is studied by a Monte Carlo method and heterogeneous ‘virtual patient’ phantoms created from CT images and structure contours; the effect of RBE enhancement in the treatment outcome was estimated by biologically effective dose (BED) distribution. Ten electronic brachytherapy APBI cases were studied. The results showed that, for electronic brachytherapy cases, tissue heterogeneities and patient boundary effect decreased dose to the target and skin but increased dose to the bones. On average, the target dose coverage PTV V100 reduced from 95.0% in water phantoms (planned) to only 66.7% in virtual patient phantoms (actual). The actual maximum dose to the ribs is 3.3 times higher than the planned dose; the actual mean dose to the ipsilateral breast and maximum dose to the skin were reduced by 22% and 17%, respectively. Combining the effect of tissue heterogeneities and RBE enhancement, BED coverage of the target was 89.9% in virtual patient phantoms with RBE enhancement (actual BED) as compared to 95.2% in water phantoms without RBE enhancement (planned BED). About 10% increase in the source output is required to raise BED PTV V100 to 95%. As a conclusion, the composite effect of dose reduction in the target due to heterogeneities and RBE enhancement results in a net effect of 5.3% target BED coverage loss for electronic brachytherapy. Therefore, it is suggested that about 10% increase in the source output may be necessary to achieve sufficient target coverage higher than 95%.

SU-GG-T-442: Dose Verification of SRS Monte Carlo Plan with a Moving Anthropomorphic Phantom

Purpose: Dose verification of Stereotactic Radiosurgery plan using Monte Carlo for tissue heterogeneity correction with a moving anthropomorphic phantom Method and Materials: An anthropomorphic lung phantom has gold fiducials, TLD capsules and GAFChromic films imbedded in a target in the left lung, TLD capsules are also inserted in heart and cord structures. The phantom was scanned, CT images were sent to Cyberknife Multiplan for planning. Target and critical structures were contoured. A SRS treatment plan with no tissue heterogeneity correction was generated to give 6Gy to 95% of the target, 5.4Gy to 99% of the target, no point > 2cm from target > 3.5Gy, conformai index < 1.2, cord < 1.8Gy, heart < 3Gy, 10% of whole lung < 2Gy. The plan was recalculated with Monte Carlo with 2% uncertainty for tissue heterogeneity correction for dose delivery. During delivery the phantom moved in superior/inferior and anterior/posterior direction, Cyberknife Synchrony tracking system took orthogonal x-ray images of the phantom while infra-red camera tracked LED diodes placed on the phantom to track phantom motion in real time. After the fiducials were identified by the tracking program and correlated with the motion of LED diodes, a motion model of the target was built; radiation started with the Cyberknife robot following the predicted target motion and adjusted its position during irradiation. Throughout delivery, the motion model was updated and adjusted with new x-ray images of the fiducials. Results: Target TLD was 98.3% of Monte Carlo dose; heart and cord TLD were within 7% acceptability. Profiles from 3 orthogonal films were displaced L/R 0mm/3mm, P/A 1mm/5mm and I/S 1mm/2mm from treatment plan profiles, within 5mm acceptability. Conclusion: Cyberknife Multiplan with Monte Carlo accurately predicts dose in heterogeneous tissue; Cyberknife Synchrony tracking system delivers dose accurately to a target in a moving phantom.

SU‐GG‐T‐399: Dosimetric Comparison of Monte Carlo and Ray Tracing of Cyberknife T Spine Treatment Plans

Purpose: Cyberknife treatment involves many beams passing through lung before reaching the T spine target therefore tissue heterogeneity correction will have major effect on dose distribution. This study compares dose to the target and to critical structure for T Spine treatment plans generated from Multiplan with Monte Carlo and ray tracing for tissue heterogeneity correction. Method and Materials: Clinical T spine treatment plans based on ray tracing algorithm for tissue density correction were recalculated with Monte Carlo at high resolution with 2% uncertainty. Results including target dose, Conformity Index, target coverage and cord dose based on the two calculation methods are compared. Results: Based on clinical cases, the ratio of Monte Carlo results to ray tracing results are: target minimum dose 94% +/− 6%, target mean dose 97% +/− 2%, target maximum dose 101% +/− 3%, Conformity Index 93% +/− 3%, coverage 95% +/− 2% and cord maximum dose 96% +/− 3%. Monte Carlo plans tend to have more dose heterogeneity with lower target minimum dose, lower target mean dose and similar target maximum dose. Coverage and Conformity Index are lower for Monte Carlo by 5% to 7% because the optimized ray tracing plans are no longer optimal when recalculated with Monte Carlo. Conclusion: Ray tracing treatment plan tends to under dose the T spine target and cord by 3% to 4%. At the lung/bone interface, the isodose line tends to pull toward the vertebral body and the target voxels closest to the lung will have the biggest dose drop therefore the lower target minimum dose. It is recommended to start Multiplan optimization with Monte Carlo for T spine treatment plan in order to get realistic dose to target and cord.

Monte Carlo calculation of dose distribution in early stage NSCLC patients planned for accelerated hypofractionated radiation therapy in the NCIC-BR25 protocol

The dosimetric consequences of plans optimized using a commercial treatment planning system (TPS) for hypofractionated radiation therapy are evaluated by re-calculating with Monte Carlo (MC). Planning guidelines were in strict accordance with the Canadian BR25 protocol which is similar to the RTOG 0236 and 0618 protocols in patient eligibility and total dose, but has a different hypofractionation schedule (60 Gy in 15 fractions versus 60 Gy in 3 fractions). A common requirement of the BR25 and RTOG protocols is that the dose must be calculated by the TPS without tissue heterogeneity (TH) corrections. Our results show that optimizing plans using the pencil beam algorithm with no TH corrections does not ensure that the BR25 planning constraint of 99% of the PTV receiving at least 95% of the prescription dose would be achieved as revealed by MC simulations. This is due to poor modelling of backscatter and lateral electronic equilibrium by the TPS. MC simulations showed that as little as 75% of the PTV was actually covered by the 95% isodose line. The under-dosage of the PTV was even more pronounced if plans were optimized with the TH correction applied. In the most extreme case, only 23% of the PTV was covered by the 95% isodose.

Impact of tissue heterogeneity corrections in stereotactic body radiation therapy treatment plans for lung cancer

This study aims at evaluating the impact of tissue heterogeneity corrections on dosimetry of stereotactic body radiation therapy treatment plans. Four-dimensional computed tomography data from 15 low stage non-small cell lung cancer patients was used. Treatment planning and dose calculations were done using pencil beam convolution algorithm of Varian Eclipse system with Modified Batho Power Law for tissue heterogeneity. Patient plans were generated with 6 MV co-planar non-opposing four to six field beams optimized with tissue heterogeneity corrections to deliver a prescribed dose of 60 Gy in three fractions to at least 95% of the planning target volume, keeping spinal cord dose <10 Gy. The same plans were then regenerated without heterogeneity correction by recalculating previously optimized treatment plans keeping identical beam arrangements, field fluences and monitor units. Compared with heterogeneity corrected plans, the non-corrected plans had lower average minimum, mean, and maximum tumor doses by 13%, 8%, and 6% respectively. The results indicate that tissue heterogeneity is an important determinant of dosimetric optimization of SBRT plans.

Monte Carlo vs. Pencil Beam Algorithm with and without Tissue Heterogeneity Correction in Stereotactic Body Radiation Therapy for Lung Tumors

Monte Carlo vs. Pencil Beam Algorithm with and without Tissue Heterogeneity Correction in Stereotactic Body Radiation Therapy for Lung Tumors

Stereotactic Body Radiation Therapy for Medically Inoperable Early-stage Lung Cancer Patients: Analysis of RTOG 0236

R. D. Timmerman, R. Paulus, J. Galvin, J. Michalski, W. Straube, J. Bradley, A. Fakiris, A. Bezjak, G. Videtic, H. Choy University of Texas Southwestern Medical Center, Dallas, TX, Radiation Therapy Oncology Group (RTOG) Headquarters, Philadelphia, TN, Thomas Jefferson University, Philadelphia, PA, Washington University, St. Louis, MO, Indiana University, Indianapolis, IN, Princess Margaret Hospital, Toronto, ON, Canada, Cleveland Clinic Foundation, Cleveland, OH

SU-FF-T-633: Impact of Tissue Heterogeneity Corrections in Stereotactic Body Radiation Therapy Treatment Plans for Non Small Cell Lung Cancer Patients

Purpose: To study the effect of tissue heterogeneity corrections on the dosimetry of stereotactic body radiation therapy treatment plans for patients with non-small cell lung cancer. Materials and Methods: Data from 15 stage I non-small cell lung patients was used. Treatment planning was done with 6 MV non-opposing coplanar beams using Varian Eclipse TPS (PBC Algorithm with Modified Batho Power Law). Prescription dose was 60 Gy in 3 fractions with tissue heterogeneity corrections. Plans were normalized to deliver prescription dose to 95% of the planning target volume (PTV) maintaining the spinal cord dose under 10 Gy. Separate plans were generated by recalculating the optimized treatment plans without heterogeneity correction while keeping identical beam arrangements, monitor units, and fluences. Results: Compared with the heterogeneity corrected plans, the uncorrected plans had lower minimum, mean, and maximum tumor doses by at least 12.5%, 7.7%, and 6.0%. The PTV receiving less than prescription dose was significantly higher (>25%) in the uncorrected plans. The percentage of uninvolved lung volume receiving minimum doses of 10 Gy, 15 Gy, 20 Gy, and 30 Gy was ⩽ 10% for both treatment plans. However, the uncorrected plans calculated slightly less dose to normal lung. The average maximum spinal cord dose was 8.56 Gy and 9.73 Gy for corrected and uncorrected plans respectively. Conclusions: Treatment plans optimized with tissue heterogeneity corrections were generated to satisfy planning goals and constraints. Re-calculating such plans without tissue heterogeneity corrections and keeping identical conditions of beam and monitor units resulted in target underdosing because the planning system assumed that more tissue attenuation was present than is actually the case. By extension, failure to use tissue heterogeneity correction to plan complex beam arrangements to treat tumors within the lung will lead to delivery of higher doses than are prescribed to less than optimum dose distributions.

SU-FF-T-570: A Dosimetric Comparison as a Function of Respiration and Tumor Location in Stereotactic Body Radiation Therapy Treatment Plans For Patients with Non Small Cell Lung Cancer

Purpose: To calculate and compare doses to the tumor and normal tissues at full respiratory cycle, inhalation, and exhalation as a function of the tumor site and location within the thorax. Materials and Methods: 20 patients were scanned using 4D CT techniques. Treatment plans were created with five to seven 6 MV non-opposing coplanar beams using Varian Eclipse Treatment Planning System with tissue heterogeneity corrections for three respiratory phase combinations. A 7 mm margin was given to the internal target volume to create the planning target volume. The prescribed dose was 60 Gy in 3 fractions. Optimization goals and normal tissue constraints were the same as in the RTOG 0236 criteria. Statistical analysis used a mixed effect ANOVA model, and a 2-sided 0.05 alpha level was used for hypothesis testing. Results: Lower peripheral tumors had three times the displacement of the upper peripheral tumors. Central tumors received lower minimum doses than the peripheral tumors. When tumors were in the peripheral region, the uninvolved lung mean dose was significantly different for all phases of respiration (p=0.008), specially, when the tumors were in the upper peripheral region (p=0.04), and the doses were higher (>26%) in the lower peripheral region compared to the upper one for all phases of respiration. The V15, V20, and V30 for uninvolved lung were below 10%. Patients with central tumors received higher maximum doses in the esophagus than patients with peripheral tumors. For the ipsilateral bronchus, patients with central tumors received higher mean and maximum doses than patients with peripheral tumors. There were no statistical differences in the mean and maximum doses to the heart or to the spinal cord. Conclusions: Lesions in the lower part of the lung are more impacted by respiratory motion. Central lesions receive less tumor dose to preserve normal tissue dose limits.

SU‐FF‐T‐422: Dependency of Proton Dose On Tissue Composition Investigated Using Monte Carlo Models of Hounsfield Number Conversion and Cadaver‐Based Anatomical Data

Purpose: In this study, we performed dependency study of proton dose on tissue composition using Monte Carlo models of Hounsfield number conversion and cadaver‐based anatomical data Method and Materials: Monte Carlo methods provide the most accurate radiation dose calculation technology as it take into account detailed materials properties, such as materials composition, mass density and interaction cross section. Monte Carlo simulation calculates the energy deposit per mass of each small volume (voxel) after a patient is presented by a large number of voxels. Two methods of building patient‐specified Monte Carlo models have been used in this study: one is to convert patient's CT Hounsfield numbers to materials; the other way is assign anatomical detailed materials using cadavers' segments. Dose distribution and dose volume histogram were compared based on the Monte Carlo models. Results: The dose distribution at the iso‐center slice, the 95% did not cover conformally to the ROI for Hounsfield MC model with shifting 2∼3 mm superior to the ROI. Dose volume history for planning tumor volume (PTV), Brain, Pituitary and Chiasm were used for evaluating the effect of tissue composition. The mean doses difference for PTV was 2.1% for the cadaver‐based MC and Hounsfield conversion MC model. The mean dose difference for Brain, Pituitary and Chiasm was less than 1.0%. Conclusion: Proton radiation dose was calculated and closely compared using two Monte Carlo models: one from CT Hounsfield number conversion and the other from human anatomically detailed Cadaver segments. It is found that the effect of different tissue composition on proton radiation dose calculation is complex involving organs at risk. Our method using cadaver‐based Monte Carlo model for proton dose calculation was shown to be suitable for benchmarking other Monte Carlo dose calculation methods and for providing tissue heterogeneity correction due to the effect of tissue composition.

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source

Tissue heterogeneity is commonly observed in clinical brachytherapy source implants. In some cases, an obvious variance in dose distribution will be caused by application of the AAPM recommended formulas, which are only valid for a homogeneous medium. By taking soft tissue as the base and considering adipose, air and bone as the separate interlayers, an evaluation model is established and dose correction formulas are derived for a single 125I source in this study. The applicability of correction formulas is validated at different interlayer thicknesses and angles by Monte Carlo (MCNP 4C) simulations. The results show fairly good agreement between correction formulas and MC simulations with overall maximum variances of 1.62 ± 0.06, 5.93 ± 0.23, and 8.41 ± 0.33% for adipose, air, and bone, respectively.

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source