Maximum Dose Values Research Articles

From AAA to Acuros XB-clinical implications of selecting either Acuros XB dose-to-water or dose-to-medium

When implementing Acuros XB (AXB) as a substitute for anisotropic analytic algorithm (AAA) in the Eclipse Treatment Planning System, one is faced with a dilemma of reporting either dose to medium, AXB-Dm or dose to water, AXB-Dw. To assist with decision making on selecting either AXB-Dm or AXB-Dw for dose reporting, a retrospective study of treated patients for head & neck (H&N), prostate, breast and lung is presented. Ten patients, previously treated using AAA plans, were selected for each site and re-planned with AXB-Dm and AXB-Dw. Re-planning was done with fixed monitor units (MU) as well as non-fixed MUs. Dose volume histograms (DVH) of targets and organs at risk (OAR), were analyzed in conjunction with ICRU-83 recommended dose reporting metrics. Additionally, comparisons of plan homogeneity indices (HI) and MUs were done to further highlight the differences between the algorithms. Results showed that, on average AAA overestimated dose to the target volume and OARs by less than 2.0%. Comparisons between AXB-Dw and AXB-Dm, for all sites, also showed overall dose differences to be small (<1.5%). However, in non-water biological media, dose differences between AXB-Dw and AXB-Dm, as large as 4.6% were observed. AXB-Dw also tended to have unexpectedly high 3D maximum dose values (>135% of prescription dose) for target volumes with high density materials. Homogeneity indices showed that AAA planning and optimization templates would need to be adjusted only for the H&N and Lung sites. MU comparison showed insignificant differences between AXB-Dw relative to AAA and between AXB-Dw relative to AXB-Dm. However AXB-Dm MUs relative to AAA, showed an average difference of about 1.3% signifying an underdosage by AAA. In conclusion, when dose is reported as AXB-Dw, the effect that high density structures in the PTV has on the dose distribution should be carefully considered. As the results show overall small dose differences between the algorithms, when transitioning from AAA to AXB, no significant change to existing prescription protocols is expected. As most of the clinical experience is dose-to-water based and calibration protocols and clinical trials are also dose-to-water based and there still exists uncertainties in converting CT number to medium, selecting AXB-Dw is strongly recommended.

Lithium fluoride colour centres-based imaging detectors for proton beam characterization at high doses

Proton beams of 7 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films thermally evaporated on glass substrates in the dose range from 103 to 4 × 106 Gy, inducing the formation of stable photoluminescent colour centres (mainly F2 and F3+), emitting in the visible spectral range. Using a conventional fluorescence microscope, the transversal proton beam intensity was mapped by acquiring the photoluminescence image of the irradiated spots. Image analysis allowed measuring the integrated photoluminescence intensity as a function of the irradiation dose: a linear optical response was obtained up to different maximum dose values, after which a quenching was observed, depending on the nature of the samples (crystals or films). The colour centres formation was investigated by optical absorption spectroscopy at room temperature and the Principal Component Analysis was applied to the absorption spectra of irradiated LiF crystals. In samples irradiated at highest doses, it allowed clearly identifying the formation of more complex aggregate defects, which appears strictly related to the observed photoluminescence quenching effect.

The Gradient of γ-Rays Irradiation’s Energy and Its Use in Treatment of Cancer Diseases

The Ionising irradiations used mostly in the treatment of tumoral diseasses are: X, γ, β and e irradiations. The discussion will be about γ irradiations, produced in linear accelerator with photon energy 6 MV and 15 MV. It is important to know the absorption performance before and after the electronic equilibrium. This is a reason that we’ve used the function of dose gradient for irradiations γ. It represents the velocity of dose change as a function of depth in tissue. From skin to maximum dose value, the increase of G-function is more accentuated for γ-rays than for β-particles while after that the G-function decreasing is less sharp for γ-rays. Finally, we’ll discuss about the advantages in terms of radiation protection of γ-rays used in radiotherapy.

Dosimetric analysis of varying cord planning organ at risk volume in spine stereotactic body radiation therapy

BackgroundSpine stereotactic body radiation therapy (SBRT) is becoming widely accepted as first-line treatment of oligometastatic spine disease as well as in the postoperative setting. The reported incidence of myelopathy is very low and guidelines vary widely on the maximum tolerable dose of the spinal cord and thecal sac. Methods and materialsBetween April 2008 and December 2010, radiation parameters were exported for 46 patients treated with spine SBRT at the Mayo Clinic. Using an in-house data mining program, dose-volume histogram constraints were extracted, including dose prescription, dose fractionation, planning target volume, planning target volume coverage, maximum dose to the cord, D2cc thecal sac, and D2cc spinal cord. Diagnostic magnetic resonance imaging scans and/or computed tomography myelograms were fused with the planning set to delineate the cord and thecal sac for receiver operating characteristic analysis of cord and thecal sac subvolume doses. A high-resolution planning at-risk volume was created in 1-mm increments for cord (1-7 mm) and the thecal sac (1-2 mm) to examine dose gradients that might be correlated with toxicity. ResultsNo patients experienced myelopathy with a median follow-up of 14 months. The most common toxicities were pain and nausea. Median values of D2cc maximum dose (maximum dose received by 2 cc of the organ at risk; biologically equivalent 2-Gy dose maximum [EQD2]) for cord and thecal sac were 38.5 Gy (range, 7.5-67.9 Gy) and 67.7 Gy (range, 15.5-155.8 Gy), respectively. Median values for high-dose subvolumes for cord and thecal sac were 2 times higher than the doses for 5% predicted grade 3 cord toxicity as recommended in the current literature. Cord D0.1cc[EQD2] ≥23.8 Gy was correlated with pain flare (n = 5). Thecal sac D2cc [EQD2] ≥29.3 Gy was a significant indicator of nausea. ConclusionCurrent guidelines may overestimate the risk of myelopathy from spine SBRT. The current study's population included both radiation-naïve and retreatment cases, but no myelopathy was observed despite exceeding recommended spine limits.

Association of VEGFR2 gene polymorphisms with the effect of L-dopa and dyskinesia complications

To explore the association of VEGFR2 gene polymorphisms (rs2305948 and rs1870377) with the effect of levodopa (L-dopa) and dyskinesia in Chinese population and to provide theoretical basis for clinical treatment. By using Taqman MGB analysis and gene sequencing, the rs2305948 and rs1870377 polymorphisms of 69 enrolled Parkinson's disease (PD) patients were detected. Among them, 32 cases developed dyskinesia during 5 years and 37 cases did not develop dyskinesia during 8 years (as the control). There was no significant association between the occurrence of dyskinesia and VEGFR2 polymorphisms at rs2305948 and rs1870377. However, rs1870377 polymorphism of AA showed greater maximum L-dopa dose [(565.00±163.55) mg/d vs (396.88±200.39) mg/d, (300.00±80.18) mg/d, P=0.038] and higher value of Modified Abnormal Involuntary Movement Scale (mAIMS) compared with that with polymorphisms of AT and TT [17.00±5.24 vs 8.94±6.53, 7.86±4.45, P=0.026]. VEGFR2 genes polymorphism (rs1870377) is associated with maximum L-dopa dose and mAIMS value in PD patients.

Are published ESR dose assessments on fossil tooth enamel reliable?

We performed dose recovery tests on proto-historic enamel samples that were dosed to approximately 200 Gy to present equivalents of Middle to Late Pleistocene fossil teeth. The experimental results confirmed that the ESR dose response of fossil tooth enamel can be best described by a double saturating exponential (DSE) function. When using a DSE for dose evaluation, it is necessary to irradiate samples to very high doses (at least 20 kGy, ideally up to 50 kGy) and use at least 15 additive dose steps.In past studies, dose estimations were mostly carried out using a single saturating exponential (SSE) function and maximum doses (Dmax) in the range of around five to ten times the equivalent dose, usually less than 5 kGy. To address the question whether older dose estimations have to be disregarded as unreliable, we carried out computer simulations using DSE dose response curves which were then fitted with SSEs. Artificial data sets were generated for a wide range of preset dose values between 10 and 5000 Gy. The results indicate a strong correlation between the magnitude of the DE value with systematic and random errors. Reliable doses can be obtained with SSEs for preset dose values of <1000 Gy using 10 exponentially distributed dose points with maximum dose values in the range of up to ten times the DE value. Higher preset doses (>1 kGy) require modified Dmax values. Although here the application of DSEs is preferable, the use of SSEs can nevertheless provide reliable results with reasonably small systematic errors for doses of up to 2000 Gy, provided Dmax is adjusted to about 1*Dset and measurement precision is better than 0.3%. The overall uncertainty remains acceptable (∼10%) using DSEs for DE values of up to 5000 Gy, provided the measurement precision is better than 0.5% and up to 20 dose steps are used. 5000 Gy is apparently the maximum dose than can be reliably assessed with ESR.Finally, we provide some detailed recommendations to ensure reliable dose evaluations with either the SSE or DSE functions. These recommendations can also be used to evaluate the reliability of previously published ESR dose assessments.

Assessment of Radioactive Gaseous Effluent Released From Nuclear Power Plant Ninh Thuan 1 under Scenario of Ines-Level 5 Nuclear Accident

Based on guides RG 1.109, RG 1.111 published by United States Nuclear Regulatory Commission (USNRC)~our~research~concentrates on assessing radiation doses caused by radioactive substances released from the nuclear power plant (NPP) Ninh Thuan 1 to the environment under scenario of an INES-level 5 nuclear accident caused by two incidents of the station black out (SBO) and loss of coolant accident (LOCA) using software~RASCAL4.3~provided by the Emergency Operations Center of USNRC. The plant Ninh Thuan 1 is assumed to use the VVER-1200 technology with a total power of 2400 MW\(_{e}\). The input data for the model calculations is based on building the accident scenario, the technical parameters of VVER-1200 technology and the meteorology. The meteorological data on dry and rainy seasons, which are typical for the Ninh Thuan region was also considered. The \(X/Q\) (s/m\(^{3}\)) quality and the maximum dose values were calculated within an area of 40 km radius from the NPP site, where \(X/Q\) (s/m\(^{3}\)) is the ratio of activity concentration to release rate. Based on the obtained results on dose distribution the necessary measures for nuclear emergency preparedness have been proposed according to the IAEA recommendations.

SU-E-T-272: Direct Verification of a Treatment Planning System Megavoltage Linac Beam Photon Spectra Models, and Analysis of the Effects On Patient Plans

Purpose: For the photon dose calculation Philips Pinnacle Treatment Planning System (TPS) uses collapsed cone convolution algorithm, which relies on energy spectrum of the beam in computing the scatter component. The spectrum is modeled based on Linac’s standard commissioning data and typically is not independently verified. We explored a methodology of using transmission measurements in combination with regularization data processing to unfold Linac spectra. The measured spectra were compared to those modeled by the TPS, and the effect on patient plans was evaluated. Methods: Transmission measurements were conducted in narrow-beam geometry using a standard Farmer ionization chamber. Two attenuating materials and two build -up caps, having different atomic numbers, served to enhance discrimination between absorption of low and high-energy portions of the spectra, thus improving the accuracy of the results. The data was analyzed using a regularization technique implemented through spreadsheet-based calculations. Results: The unfolded spectra were found to deviate from the TPS beam models. The effect of such deviations on treatment planning was evaluated for patient plans through dose distribution calculations with either TPS modeled or measured energy spectra. The differences were reviewed through comparison of isodose distributions, and quantified based on maximum dose values for critical structures. While in most cases no drastic differences in the calculated doses were observed, plans with deviations of 4 to 8% in the maximum dose values for critical structures were discovered. The anatomical sites with large scatter contributions are the most vulnerable to inaccuracies in the modeled spectrum. Conclusion: An independent check of the TPS model spectrum is highly desirable and should be included as part of commissioning of a new Linac. The effect is particularly important for dose calculations in high heterogeneity regions. The developed approach makes acquisition of megavoltage Linac beam spectra achievable in a typical radiation oncology clinic.

Assessment of radioactive gaseous effluent released from Ninh Thuan 1 nuclear power plant under scenario of INES-level 6 nuclear accident

Based on guides RG 1.109, RG 1.111 published by United States Nuclear Regulatory Commission (USNRC) our research concentrates on assessing radiation doses caused by radioactive substances released from the Ninh Thuan 1 nuclear power plant (NPP) to the environment under scenario of an INES-level 6 nuclear accident caused by the Station Black Out (SBO) incident and the loss of coolant accident (LOCA) phenomenon using software RASCAL4.3 provided by the Emergency Operations Center of USNRC. The NinhThuan 1 plant is assumed to use the VVER-1200 technology with a total power of 2400 MWe. The input data for the model calculations is based on building the accident scenario, the technical parameters of VVER-1200 technology and the meteorology. In this work the meteorological data on dry and rainy seasons which are typical for the NinhThuan region was considered. The maximum dose values were calculated within an area of 40 km radius from the NPP site. The obtained calculation results showed that the dose distributions affected by meteorological conditions. In the rainy season the dose values near the plant are higher than those in the dry season, especially at the distances less than 10 kilometers from the plant the total effective dose equivalence (TEDE) values can be from about 3.5 to 15 times higher than the dose limit for publics. As a result, it is requested to follow the evolution of the accident in order to issue timely the appropriate response measures based on the guidance specified by the Circular 25/2014/TT-BKHCN.

Toxicité aiguë cutanée de l’irradiation mammaire avec modulation d’intensité avec technique de champ dans le champ (optimisation avec pré-segmentation)

PurposeThe optimization with presegmentation irradiation technique (Dosisoft™), used in treatment of breast carcinomas, facilitates the treatment delivery and improves radioprotection. The objective of our study was to evaluate the potential impact of this innovative technique on the acute skin toxicity especially on the rate of moist desquamation during irradiation of the whole breast after conservative surgery. Patients and methodsThe scores of acute skin toxicity observed at 50Gy in 103 patients treated with the presegmentation technique were compared to those of 101 patients with similar breast size treated with a classic 3D technique. All patients received 50Gy/2Gy per fraction, 5days a week using 4MV photon beam. The boost on the tumoural bed was realized according to conventional technique. Using the NCI-CTCAE V4, the skin toxicity was recorded every week during the medical visit. Moreover, the following factors that could induce skin toxicity have been studied: breast size, body mass index, age, prior chemotherapy, concurrent administration of trastuzumab, hypertension, diabetes, smoking habits and statin uptake. ResultsThe incidence of moist desquamation observed in all sites, has been decreased to only 9.8% in the presegmentation group versus 16.8% in the test group, the difference being not statistically significant OR=0.53 [0.23; 1.22] (P=0.13). In univariate analysis the presegmentation technique enabled a significant decrease of 4.4Gy in mean, of the value of maximum dose (P=0.001). The other risk factors of skin toxicity are the increase of breast size (P<0.001), a high body mass index (P<0.001), hypertension (P=0.03) and concurrent administration of trastuzumab (P=0.07). In multivariate analysis, the two remaining significant factors are breast size (OR=1.004 [1.002; 1.006]) and trastuzumab administration (OR=4.95 [1.17; 20.79]). ConclusionThe comparison of the skin toxicity induced by the presegmentation or the reference technique shows a trend regarding the improvement of the skin tolerance when using the presegmentation technique, thus pleading in favour of its use considering its dosimetric interest and the improvement of radioprotection of the patient. The next step will consist to experiment the simultaneous boost on tumoural bed using this technique.

Dosimetric and radiobiologic comparison of 3D conformal, IMRT, VMAT and proton therapy for the treatment of early-stage glottic cancer.

This study aims to compare dosimetrically and radiobiologically 3D conformal, intensity modulated radiation therapy (IMRT), RapidArc (RA) volumetric modulated arc therapy and proton therapy techniques for early-stage glottic cancer. Ten patients were retrospectively selected. Photon treatment planning was performed using Eclipse External Beam Planning, and proton planning was performed using CMS Xio. The minimum, mean and maximum dose values for planning target volume (PTV), mean and maximum dose values for organ at risk, % of volume of PTV receiving at least 95% of the prescription dose, and D20, D50 and D90 of carotid arteries were compared. Biological response models of tumour control probabilities and normal tissue complication probabilities were calculated. IMRT, RA and proton plans versus three-dimensional conformal radiotherapy (3D-CRT) plans consistently provided superior PTV coverage and decreased mean dose to the thyroid and carotid arteries. All these three modalities showed superiority with less variation among themselves compared with 3D-CRT plans. Clinical investigation is warranted to determine if these treatment approaches will translate into a reduction in radiation therapy-induced toxicities.

Acute and late side effects to salivary glands and oral mucosa after head and neck radiotherapy in children and adolescents. Results of the "Registry for the evaluation of side effects after radiotherapy in childhood and adolescence".

The registry for the evaluation of side effects after radiotherapy in childhood and adolescence (RiSK) was established to prospectively characterize radiation-associated side effects. The purpose of this analysis was to characterize side effects after radiotherapy to the head and neck in children and adolescents. Radiation doses have been collected across Germany since 2001. Acute and late side effects were characterized. Until January 2010, 133 patients (median age, 12.7 years) were recruited who had received radiotherapy to the salivary glands. Toxicity evaluation was available for 114 patients (median follow-up, 2.9 years). Acute and late toxicity significantly depended on the maximum radiation dose to the salivary glands. An increase of the mean value of maximum dose of 1 Gray (Gy) to the submandibular glands resulted in an odds ratio of 1.04 (range, 1.00-1.08; p = .039) for acute toxicities of the salivary glands and 1.08 (range, 1.03-1.13; p = .001) for acute mucosal toxicities. These data can be used for an individual risk assessment in pediatric head and neck radiotherapy.

SU-E-T-104: Development of 3 Dimensional Dosimetry System for Gamma Knife

Purpose: The aim of this study was to develop a new 3 dimensional dosimetry system to verify the dosimetric accuracy of Leksell Gamma Knife-Perfexion™ (LGK) (Elekta, Norcross, GA). Methods: We designed and manufactured a lightweight dosimetry instrument to be equipped with the head frame to LGK. It consists of a head phantom, a scintillator, a CCD camera and a step motor. The 10×10 cm2 sheet of Gd2O3;Tb phosphor or Gafchromic EBT3 film was located at the center of the 16 cm diameter hemispherical PMMA, the head phantom. The additional backscatter compensating material of 1 cm thick PMMA plate was placed downstream of the phosphor sheet. The backscatter plate was transparent for scintillation lights to reach the CCD camera with 1200×1200 pixels by 5.2 um pitch. With This equipment, 300 images with 0.2 mm of slice gap were acquired under three collimator setups (4mm, 8mm and 16mm), respectively. The 2D projected doses from 3D distributions were compared with the exposured film dose. Results: As all doses normalized by the maximum dose value in 16 mm setup, the relative differences between the equipment dose and film dose were 0.2% for 4mm collimator and 0.5% for 8mm. The acquisition of 300 images bymore » the equipment took less than 3 minutes. Conclusion: The new equipment was verified to be a good substitute to radiochromic film, with which required more time and resources. Especially, the new methods was considered to provide much convenient and faster solution in the 3D dose acquisition for LGK.« less

SU-E-T-228: Liquid Ionisation Chamber Array and MicroDiamond Measurements with the CyberKnife System

Purpose: The aim of this study was to measure the dose profile and output factors with a CyberKnife accelerator using a TM60019 microDiamond detector and a 1000SRS liquid chamber array (both PTW Freiburg, Germany). Methods: An MP3 water phantom (PTW, Freiburg) was positioned along the robotic world coordinate system. The TM60019 detector was adjusted to the center of the according fields and the semiconductor axis was aligned with the beam direction. Profiles at 5cm water depth and SSD = 80 cm were measured along the robotic x axis and y axis for the cylindrical collimators of the CyberKnife (diameter 60, 50, 40, 30, 20, 15, 12.5, 10, 7.5 and 5mm). To determine the output factors the dose profile was measured at 0.1 mm steps around the field center to find the maximum dose value. The liquid chamber array (1000SRS) measurement was performed with the same setup, but with RW3 buildup. Results: The 1000SRS measurements closely conform with the TM60019 profile measurement in all profile regions and for all collimator sizes. The profile measurement is influenced by the almost equal spatial resolution of the TM60019 detector (radius of the sensitive area 1.1mm) and of the 1000SRS liquid chamber array (single chamber width 2.3mm). The measured dose profiles have not been corrected for this limited spatial resolution. Rather we purpose to consider that spatial dose averaging over 2 mm wide regions might be justified in view of patient positioning inaccuracies and of the spaces in tissue participating in the biological radiation responses. Conclusion: The 1000SRS data points conform with the TM60019 profile measurements at all profile regions showing the applicability of liquid ion chamber arrays with the CyberKnife system.

External evaluation of the Radiation Therapy Oncology Group brachial plexus contouring protocol: Several issues identified

The aims of the study were to evaluate interobserver variability in contouring the brachial plexus (BP) using the Radiation Therapy Oncology Group (RTOG)-approved protocol and to analyse BP dosimetries. Seven outliners independently contoured the BPs of 15 consecutive patients. Interobserver variability was reviewed qualitatively (visually by using planning axial computed-tomography images and anteroposterior digitally reconstructed radiographs) and quantitatively (by volumetric and statistical analyses). Dose-volume histograms of BPs were calculated and compared. We found significant interobserver variability among outliners in both qualitative and quantitative analyses. These were most pronounced for the T1 nerve roots on visual inspection and for the BP volume on statistical analysis. The BP volumes were smaller than those described in the RTOG atlas paper, with a mean volume of 20.8 cc (range 11-40.7 cc) compared with 33 ± 4 cc (25.1-39.4 cc). The average values of mean dose, maximum dose, V60Gy, V66Gy and V70Gy for patients treated with conventional radiotherapy and IMRT were 42.2 Gy versus 44.8 Gy, 64.5 Gy versus 68.5 Gy, 6.1% versus 7.6%, 2.9% versus 2.4% and 0.6% versus 0.3%, respectively. This is the first independent external evaluation of the published protocol. We have identified several issues, including significant interobserver variation. Although radiation oncologists should contour BPs to avoid dose dumping, especially when using IMRT, the RTOG atlas should be used with caution. Because BPs are largely radiologically occult on CT, we propose the term brachial-plexus regions (BPRs) to represent regions where BPs are likely to be present. Consequently, BPRs should in principle be contoured generously.

Hypofractionated helical tomotherapy (75 Gy at 2.5 Gy per fraction) for localized prostate cancer: long-term analysis of gastrointestinal and genitourinary toxicity.

BackgroundThis study is a report on the long-term analysis of acute and late toxicities for patients with localized prostate cancer treated with hypofractionated helical tomotherapy.MethodsFrom January 2008 through August 2013, 70 patients with localized prostate cancer were treated definitively with hypofractionated helical tomotherapy. The helical tomotherapy was designed to deliver 75 Gy in 2.5 Gy per fraction to the prostate gland, 63 Gy in 2.1 Gy per fraction to the seminal vesicles, and 54 Gy in 1.8 Gy per fraction to the pelvic lymph nodes. Incidence rates and predictive factors for radiation toxicities were analyzed retrospectively.ResultsThe incidences of grades 0, 1, and 2 acute gastrointestinal (GI) toxicity were 51.4%, 42.9%, and 5.7%, and those of acute genitourinary (GU) toxicity were 7.1%, 64.3%, and 28.6%, respectively. The maximum dose of rectum and bladder V40 and V50 were significant predictive factors for acute GI and GU toxicity. The cutoff value of rectum maximum dose and bladder V40 and V50 by receiver-operating characteristic curves analysis were 76.5 Gy, 17.3%, and 10.2%, respectively. The incidences of grades 0, 1, and 2 late GI toxicity were 82.0%, 14.0%, and 4.0%, and those of late GU toxicity were 18.0%, 56.0%, and 26.0%, respectively. Rectum V70 and bladder V70 and V75 were significant predictive factors for late GI and GU toxicity. The cutoff value of rectum V70 and bladder V70 and V75 by receiver-operating characteristic curves analysis was 2.8%, 2.8%, and 1.0%, respectively.ConclusionHypofractionated helical tomotherapy using a schedule of 75 Gy at 2.5 Gy per fraction had favorable acute and late toxicity rates and no serious complication, such as grade 3 or worse toxicity. To minimize radiation toxicities, constraining the rectum maximum dose to less than 76.5 Gy, rectum V70 to less than 2.8%, bladder V40 to less than 17.3%, bladder V50 to less than 10.2%, bladder V70 to less than 2.8%, and bladder V75 to less than 1.0% would be necessary.

Bolus-dependent dosimetric effect of positioning errors for tangential scalp radiotherapy with helical tomotherapy

The dosimetric effect of errors in patient position is studied on-phantom as a function of simulated bolus thickness to assess the need for bolus utilization in scalp radiotherapy with tomotherapy. A treatment plan is generated on a cylindrical phantom, mimicking a radiotherapy technique for the scalp utilizing primarily tangential beamlets. A planning target volume with embedded scalplike clinical target volumes (CTVs) is planned to a uniform dose of 200cGy. Translational errors in phantom position are introduced in 1-mm increments and dose is recomputed from the original sinogram. For each error the maximum dose, minimum dose, clinical target dose homogeneity index (HI), and dose-volume histogram (DVH) are presented for simulated bolus thicknesses from 0 to 10mm. Baseline HI values for all bolus thicknesses were in the 5.5 to 7.0 range, increasing to a maximum of 18.0 to 30.5 for the largest positioning errors when 0 to 2mm of bolus is used. Utilizing 5mm of bolus resulted in a maximum HI value of 9.5 for the largest positioning errors. Using 0 to 2mm of bolus resulted in minimum and maximum dose values of 85% to 94% and 118% to 125% of the prescription dose, respectively. When using 5mm of bolus these values were 98.5% and 109.5%. DVHs showed minimal changes in CTV dose coverage when using 5mm of bolus, even for the largest positioning errors. CTV dose homogeneity becomes increasingly sensitive to errors in patient position as bolus thickness decreases when treating the scalp with primarily tangential beamlets. Performing a radial expansion of the scalp CTV into 5mm of bolus material minimizes dosimetric sensitivity to errors in patient position as large as 5mm and is therefore recommended.

Dosimetric impact of reduced nozzle-to-isocenter distance in intensity-modulated proton therapy of intracranial tumors in combined proton-carbon fixed-nozzle treatment facilities

BackgroundIn combined proton-carbon fixed-nozzle treatment facilities with raster scanning delivery, the scattering of proton pencil beams caused by nozzle elements and the relatively large nozzle-to-isocenter distance cause a beam broadening. This may pose limitations to the achievable dose conformity. One way to counteract this effect is by delivering the treatment in a position closer to the nozzle than the room isocenter. Purpose of this study was to assess the potential dosimetric benefit of such solution, in terms of dose conformity and normal tissue sparing, in intensity-modulated proton therapy (IMPT) of intracranial tumors.Material and methodsFor 12 patients with intracranial lesions, IMPT-plans were created at two treatment positions: nozzle-to-treatment-isocenter distance: 100 cm (room isocenter) and nozzle-to-treatment-isocenter distance: 60 cm. The resulting plans were compared in terms of dose distributions, dose-volume histograms and selected dosimetric indexes.ResultsWith comparable target coverage, statistically significant normal tissue sparing was achieved through the reduction of the distance between nozzle and treatment isocenter. The decrease in mean dose (Dmean) was 12.5% to the whole brain, 16.2% to the brainstem, 9.7% and 15.4% to the temporal lobes, 10.0% and 12.9% to the hippocampi, 11.8% and 12.5% to the optic nerves and 0.2% to the chiasm. The volume receiving at least 10% of the prescribed dose (V10%) was reduced by more than 10% for most organs at risk (OARs). The maximum dose (Dnear-max) values to most OARs remained without significant difference.ConclusionA reduced distance between nozzle and treatment isocenter leads to steeper lateral dose gradients and significantly reduces the volume of OARs adjacent to the target, which receives low to intermediate doses. Technical solutions shifting the treatment isocenter closer to the nozzle should be considered in clinical situations, where critical OARs are adjacent to the beam channel and where the integral dose should be minimized.

Evaluation of Maximum Patient Skin Dose Arising from Interventional Cardiology Using Thermoluminescence Dosimeter in Mashhad, Iran

Introduction The increasing practice of interventional fluoroscopy in diagnosis and treatment of cardiovascular disease has risen attention to improve radiation protection of patients and cardiologists in these relatively high dose techniques. Therefore, nowadays there is an emphasis on the measurement of radiation dose received by patients and cardiologists arising from the relevant procedures. Materials and Methods Maximum skin dose of 90 patients in two hospitals in Mashhad have been measured by a grid of 30 thermoluminescent dosimeters (TLDs). The X-ray units were Axiom Artis Siemens in both hospitals which were equipped with integrated dose area product (DAP) meters. The procedures were divided into two groups: diagnostic procedures (angiography and angiography with measurement of left or right ventricle and pulmonary artery) and therapeutic procedures (angioplasty with or without dilatation or stent and angiography with angioplasty). DAP value, fluoro time, and cumulative dose at Interventional Reference Point (CDIRP) were also registered for each procedure. Results The mean values of maximum skin dose (MSD) and DAP for diagnostic procedures were 68.51 mGy and 20.96 Gy.cm2, respectively and for therapeutic procedures 344.18 mGy and 70.94 Gy.cm2, respectively. A good correlation was found between MSD and DAP (R=0.88) but correlation between MSD and CDIRP was stronger (R=0.90). Conclusion MSD values did not exceed the 2000 mGy dose threshold for deterministic effects. The highest MSD obtained for diagnostic procedures was 229.40 mGy and for therapeutic procedures it was 820.50 mGy. The results show that CDIRP can be a fairly good estimate of MSD.

External validation of the lumbosacral plexus‐contouring protocol developed by Yi et al. (IJROBP 2012; 84: 376–82) for pelvic malignancies

To evaluate interobserver variability in contouring lumbosacral plexuses (LSP) using the protocol developed by Yi et al. (IJROBP 2012; 84: 376-82) and to review LSP dosimetries for conventional radiotherapy and intensity-modulated radiotherapy (IMRT) for pelvic cancers. Using the above-mentioned protocol, seven outliners independently contoured LSPs of 10 consecutive patients (five patients treated with conventional radiotherapy and five with IMRT). Interobserver variability was reviewed visually by using planning axial CT images and anteroposterior digitally reconstructed radiographs. Dosimetries of LSPs were also calculated and compared. There was a notable learning curve for each outliner; duration to outline the first patient was 45-185 minutes, versus 15-50 minutes after six patients. We found significant interobserver variability among outliners below the level of the S2 nerve roots. The LSP volumes (mean volume range of 40.9-58.4 cc) were smaller than those described in the atlas paper (71-138 cc). The mean values of mean dose, maximum dose, V40 Gy, V50 Gy and V55 Gy, respectively, for patients treated with conventional radiotherapy versus those treated with IMRT were 35.5 Gy versus 33.6 Gy, 52.2 Gy versus 52.2 Gy, 61.3% versus 54.4%, 14.9% versus 18.8% and 0% versus 2.5%. We conclude that the protocol developed by Yi et al. is a useful set of guidelines but suggest that additional at-risk components of the LSP also be contoured. We recommend that radiation oncologists practise 'nerve-sparing' radiotherapy by contouring LSPs, especially when using IMRT. We propose the term 'lumbosacral plexus regions' (LSPRs) to highlight the fact that LSPs are not always radiologically visible, only the regions where they are likely to be present.