Maximum Dose Values Research Articles

SU-E-T-512: Monte Carlo Dose Verification of Pencil Beam Scanning Proton Therapy

Purpose: To verify a clinical pencil (PB) beam dose calculation algorithm for scanned beam intensity modulated proton therapy (IMPT), using TOPAS (TOol for PArticle Simulation), a GEANT4 based Monte Carlo (MC) simulation system. Methods: Seven patients, previously treated with IMPT for various treatment sites and prescriptions, were selected from our patient database. Proton fluence maps of the treated plans were exported for each field from our clinical treatment planning system (ASTROID) and imported to TOPAS along with the patient and beam geometry. The absolute dose distribution of each individual beam was calculated and compared to the PB algorithm‐based calculation from ASTROID. Results: The differences observed in mean and median target doses were less than ±1% for all cases, while D02 and D98 (surrogates for maximum and minimum dose values respectively) differed by less than ±3% for the majority of beams. Differences in the mean dose for the organs at risk (OARs) ranged from −8.9% to 3.7%, with reference to MC calculation, with an average over all the OARs of −0.1%, indicating no systematic over‐or under‐estimation of the dose by the PB algorithm. 3D gamma analysis (2%/2mm) for the PB to MC dose comparison resulted in an average 95.2% (±5.0) of the target volume having an absolute gamma value equal or less than 1 and 99.2% (±1.2%) equal or less than 2. For the healthy tissue receiving at least 5% of the target mean dose, the corresponding percentages were 99.6% (±0.3%) and 99.9% (±0.1%). Conclusion: We have clinically implemented MC for IMPT plan recalculation. Our PB calculation algorithm for IMPT was found to be in overall good agreement with MC calculations. Clinically significant deviations in OAR mean dose can be attributed to lung tissue or bone anatomy in the beam path.

Standardized treatment planning methodology for passively scattered proton craniospinal irradiation

BackgroundAs the number of proton therapy centers increases, so does the need for studies which compare proton treatments between institutions and with photon therapy. However, results of such studies are highly dependent on target volume definition and treatment planning techniques. Thus, standardized methods of treatment planning are needed, particularly for proton treatment planning, in which special consideration is paid to the depth and sharp distal fall-off of the proton distribution. This study presents and evaluates a standardized method of proton treatment planning for craniospinal irradiation (CSI).MethodsWe applied our institution’s planning methodology for proton CSI, at the time of the study, to an anatomically diverse population of 18 pediatric patients. We evaluated our dosimetric results for the population as a whole and for the two subgroups having two different age-specific target volumes using the minimum, maximum, and mean dose values in 10 organs (i.e., the spinal cord, brain, eyes, lenses, esophagus, lungs, kidneys, thyroid, heart, and liver). We also report isodose distributions and dose-volume histograms (DVH) for 2 representative patients. Additionally we report population-averaged DVHs for various organs.ResultsThe planning methodology here describes various techniques used to achieve normal tissue sparing. In particular, we found pronounced dose reductions in three radiosensitive organs (i.e., eyes, esophagus, and thyroid) which were identified for optimization. Mean doses to the thyroid, eyes, and esophagus were 0.2%, 69% and 0.2%, respectively, of the prescribed dose. In four organs not specifically identified for optimization (i.e., lungs, liver, kidneys, and heart) we found that organs lateral to the treatment field (lungs and kidneys) received relatively low mean doses (less than 8% of the prescribed dose), whereas the heart and liver, organs distal to the treatment field, received less than 1% of the prescribed dose.ConclusionsThis study described and evaluated a standardized method for proton treatment planning for CSI. Overall, the standardized planning methodology yielded consistently high quality treatment plans and perhaps most importantly, it did so for an anatomically diverse patient population.

A retrospective analysis of rectal and bladder dose for gynecological brachytherapy treatments with GZP6 HDR afterloading system

AimThe aim of this work is to evaluate rectal and bladder dose for the patients treated for gynecological cancers. BackgroundThe GZP6 high dose rate brachytherapy system has been recently introduced to a number of radiation therapy departments in Iran, for treatment of various tumor sites such as cervix and vagina. Materials and methodsOur analysis was based on dose measurements for 40 insertions in 28 patients, treated by a GZP6 unit between June 2009 and November 2010. Treatments consisted of combined teletherapy and intracavitary brachytherapy. In vivo dosimetry was performed with TLD-400 chips and TLD-100 microcubes in the rectum and bladder. ResultsThe average of maximum rectal and bladder dose values were found to be 7.62Gy (range 1.72–18.55Gy) and 5.17Gy (range 0.72–15.85Gy), respectively. It has been recommended by the ICRU that the maximum dose to the rectum and bladder in intracavitary treatment of vaginal or cervical cancer should be lower than 80% of the prescribed dose to point A in the Manchester system. In this study, of the total number of 40 insertions, maximum rectal dose in 29 insertions (72.5% of treatment sessions) and maximum bladder dose in 18 insertions (45% of treatments sessions) were higher than 80% of the prescribed dose to the point of dose prescription. ConclusionIn vivo dosimetry for patients undergoing treatment by GZP6 brachytherapy system can be used for evaluation of the quality of brachytherapy treatments by this system. This information could be used as a base for developing the strategy for treatment of patients treated with GZP6 system.

SU-E-T-593: VMAT Vs. IMRT for Treatment of Neoplasms of the Brain: Dosimetric Quality and Delivery.

A retrospective study was performed to determine any dosimetric or delivery benefits for treatment planning with intensity-modulated radiotherapy (IMRT) versus volumetric-modulated arc therapy (VMAT) for the treatment of brain neoplasms. Eighteen patients treated with modulated brain radiotherapy treatments were included in this study (primary treatment volumes of 15.3 to 374.9 cc). IMRT and VMAT plans were generated for each patient using the same criteria for prescription coverage and normal tissue sparing. IMRT optimizations ranged from five to seven fields and VMAT from two to four arcs. Plans were generated with Varian Eclipse treatment planning system utilizing AAA-8615 dose calculation algorithm. VMAT optimizations provided limited dosimetric advantages versus IMRT. VMAT provided superior treatment volume coverage (volumes receiving 95%, 100% of prescription dose (V95%, V100%)) over IMRT, but differences were not statistically significant (paired t-test p > 0.05). Relative maximum dose values, conformity and homogeneity indices also exhibited no statistical differences. IMRT plans resulted in similar mean brain minus treatment volume (Brain- TV) dose (mean = 1460.1 vs. 1506.3 cGy; p = 0.056). The volume of Brain- TV receiving 40Gy was lower for VMAT than IMRT (average = 38.96 vs. 44.97 cc; p = 0.050). Maximum skin dose was lower for VMAT (mean = 4568.8 vs. 5063.3cGy; p = 0.006), as well as skin V20Gy (6.56% vs 7.69%; p = 0.027) and V40Gy (0.56% vs. 0.35%; p = 0.017). VMAT plans required fewer fields along with fewer MU than IMRT (mean = 388.2 vs. 721.1 MU, respectively), allowing for approximately 20% faster delivery times. VMAT treatments significantly reduced treatment time due to reduced MU and fewer fields. Certain skin and Brain-TV high dose spread parameters were superior for VMAT as compared to IMRT plans. All other dosimetric parameters tested were statistically equivalent for VMAT and IMRT techniques.

Integral T-Shaped Phantom-Dosimeter System to Measure Transverse and Longitudinal Dose Distributions Simultaneously for Stereotactic Radiosurgery Dosimetry

A T-shaped fiber-optic phantom-dosimeter system was developed using square scintillating optical fibers, a lens system, and a CMOS image camera. Images of scintillating light were used to simultaneously measure the transverse and longitudinal distributions of absorbed dose of a 6 MV photon beam with field sizes of 1 × 1 and 3 × 3 cm2. Each optical fiber has a very small sensitive volume and the sensitive material is water equivalent. This allows the measurements of cross-beam profile as well as the percentage depth dose of small field sizes. In the case of transverse dose distribution, the measured beam profiles were gradually become uneven and the beam edge had a gentle slope with increasing depth of the PMMA phantom. In addition, the maximum dose values of longitudinal dose distribution for 6 MV photon beam with field sizes of 1 × 1 and 3 × 3 cm2 were found to be at a depth of approximately 15 mm and the percentage depth dose of both field sizes were nearly in agreement at the skin dose level. Based on the results of this study, it is anticipated that an all-in-one phantom-dosimeter can be developed to accurately measure beam profiles and dose distribution in a small irradiation fields prior to carrying out stereotactic radiosurgery.

Dosimetric analysis of trigeminal nerve, brain stem doses in CyberKnife radiosurgery of trigeminal neuralgia

CyberKnife radiosurgery treatment of Trigeminal neuralgia (TN) is performed as a non-invasive image guided procedure. The prescription dose for TN is very high. The brainstem is the adjacent critical organ at risk (OAR) which is prone to receive the very high target dose of TN. The present study is to analyze the dose distribution inside the tiny trigeminal nerve target and also to analyze the dose fall off in the brain stem. Seven TN cases treated between November 2010 and January 2012 were taken for this study retrospectively. The treatment plans were analyzed for target dose conformity, homogeneity and dose coverage. In the brainstem the volume doses D1%, D2% were taken for analyzing the higher doses in the brain stem. The dose fall off was analyzed in terms of D5% and D10%. The mean value of maximum dose within the trigeminal nerve target was 73.5±2.1Gy (P=0.0007) and the minimum dose was 50.0±4.1Gy (P=0.1315). The mean conformity index was 2.19 and the probable reason could be the smallest CyberKnife collimator of 5mm used in the treatment plan. The mean D1%, of the brainstem was 10.5± 2.1Gy (P=0.5316) and the mean value of the maximum point dose within the brainstem was 35.6±3.8Gy. This shows the degree of dose fall off within the brainstem. Though the results of the present study are showing superior sparing of brain stem and reasonable of target coverage, it is necessary to execute the treatment plan with greater accuracy in CyberKnife as the immobilization is noninvasive and frameless.

SU‐E‐T‐159: Average and Maximum Doses in Thin Fricke Layers Irradiated with Soft X‐Rays

Purpose: To determine the relationship between maximum and average doses in thin Fricke layers irradiated with soft x‐rays. Methods: The MC code PENELOPE was used to modeling a Fricke solution pillbox‐like volume 0.30 cm thick and 1.5 cm diameter embedded in the center and flushing the surface of a 0.50‐cm thick, 1125 cm3 solid water thank filled with water. The pillbox was covered with a 0.0030 cm thick polystyrene film and the reference point for the absorbed dose calculations was its geometric center for all the simulations performed at a source‐detector distance of 30.0 cm using the PTB (Physikalisch‐Technische Bundesanstalt‐Germany courtesy: R. Kramer) calibration therapy TW30 kV, TW50 kV TW 70kV and TW100 kV spectra. The same procedure was followed with pillbox thickness varying from 0.0010 to 0.40 cm. For each x‐ray quality, the average energy per unit mass (average dose) in the pillboxes were normalized for the 0.30 cm thick Fricke solution and their maximum average dose values were assigned as being the CPE region. The effect of the beam divergence on the average dose and the water/Fricke dose relationship are being investigated. Results: As the thickness of the pillbox increases, the average dose increases, reaching a maximum after descending due to attenuation in the solution. With an uncertainty of ±0.3%, a deviation between the maximum and average doses of 17.0% (0.010‐cm depth), 5.2% (0.025‐cm depth), 0.23% (0.15‐cm depth) and 0.00% (0.30‐cm depth) were observed for 30, 50, 70 and 100 kV, respectively. Conclusions: As expected, different thickness of Fricke solution are required to determine the absorbed dose under CPE condition. Nevertheless, the data provided in this work may serve for absolute calibration of the Fricke solution using a similar setup, radiation qualities and the calculated relationship between maximum and average dose presented herein.

Sensitivity Study to Evaluate the Dosimetric Impact of Off-Axis Ratio Profiles Misalignment on TomoTherapy Second Dose Validation

Accurate dose planning and delivery are very important in the intensity modulated radiation therapy. For helical TomoTherapy dose validation, a TomoTherapy second check software, called MU-Tomo, has been developed using archived patient documents, initial coordinates and planned dose of the point of calculation, and common dosimetric functions. Based on this software, sensitivity studies on 50 patient cases have been evaluated to show the impact of off-axis ratio profile misalignment on point dose calculation. Off-axis ratio is defined as the dose profile normalized to its maximum dose value. Sensitivity studies were done for three scenarios: oscillating the fluctuation regions of two off-axis profiles, shifting the profiles, and rotating the profiles. The result of the oscillation trial is linear along the change of longitudinal off-axis ratio (OARy), while oscillating the lateral off-axis ratio (OARx) has little influence on the dose calculation. For shifting, the variation in the percentage difference from the non-shifting value is about 15 times larger in OARy modification than in OARx modification. Rotating OARx by +/- 6' gave less than 1.5% +/- 0.20% difference compared to the non-rotating value. Rotating OARy by +/- 1' changes the result more than 5% +/- 2.69%. Therefore, for helical TomoTherapy dose validation, commissioned OARy profiles are more sensitive than OARx to oscillation, shifting and rotating. As a result, different tolerances for OARx and OARy may be required for annual quality assurance.

Preliminary analysis of intensity modulated radiation therapy for 20 patients with nasopharyngeal carcinoma

Objective:To report the preliminary results of intensity modulated radiation therapy (IMRT) for 20 nasopharyngeal carcinoma patients.Methods:A total of 20 patients with nasopharyngeal carcinoma received IMRT in our hospital between January 2007 and April 2008.Five patients were of stage Ⅱ,13 patients were of stage Ⅲ,and 2 patients were of stage Ⅳ.The prescribed dose 69.96 Gy was delivered to the gross tumor volume (PTV) and positive neck nodes (PTVnd); 59.36 Gy to the clinical target volume (PTV1),covering the upper neck and area around the nasopharynx; and 50.96 Gy to the low neck and supraclavicular area (PTV2).The dose to 50% of the parotid was ≤35 Gy.The maximum dose to the lens,pituitary gland,temporo -mandibular joint,mandible,and temporal lobe was 9,54,60,70,and 60 Gy.The maximum dose to the brainstem,spinal cord,optic nerve and optic chiasma (PRV) was 54,40,54,and 54 Gy,respectively.All of the patients received 1 or 2 circles of chemothrapy before IMRT.Results:The median follow-up time was 14 months.The one-year overall survival was 94.1%.One patient died of osseous metastasis and respiratory failure and 3 patients developed distant metastasis.Acute toxicity was mostly Grade Ⅰ to Grade Ⅱ.Seventeen patients had grade Ⅰ xerostomia.Three patients suffered from grade Ⅰ acute oral mucosa reaction and 12 patients had grade Ⅱ acute oral mucosa reaction.Analysis of the dose-volume histograms (DVHs) showed that the mean dose delivered to the PTV,PTVnd,PTV1 and PTV2 was 73.4,74.1,67.8,and 54.1 Gy,respectively. The median dose to 50% of the right and left parotid glands was 43.9 Gy and 41.9 Gy,respcetively.The average value of maximum dose to the left and right lens was 8.06 and 8.12 Gy,respectively.The average value of maximum dose to the brainstem,spinal cord,left and right optic nerve and optic chiasma PRV was 60.6, 46.6,50.0,55.0,and 56.0 Gy,respectively.Conclusion:IMRT can achieve satisfactory dose distribution to nasopharyngeal carcinoma and surrounding tissues in NPC patients,protect normal tissues during the treatment and improve local control rate.

Lung Tumor Treatment with Very High Energy Electron Beams of 150-250 Mev as Compared to Conventional Megavoltage Photon Beams

Lung Tumor Treatment with Very High Energy Electron Beams of 150-250 Mev as Compared to Conventional Megavoltage Photon Beams

Patient skin dose assessment during CT-guided interventional procedures

The purpose of this study was to investigate patient skin doses in the most frequent computed tomography-guided interventional radiology procedures. The study includes 14 abdominal biopsies and 15 radiofrequency (RF) ablations. Patient skin dose was assessed by slow verification films. The technical factors recorded were: tube kilovoltage (kVp), tube load (mAs), slice thickness and the total number of slices (S). The median value of maximum entrance skin dose (ESD(M)) in biopsy was 108 mGy (9.5-282 mGy). S had a range of 4-152 with a median of 44 slices. Significant correlation was found between S and dose (r = 0.80). Median ESD(M) in RF ablation was 238 mGy (104-500 mGy). S had a range of 108-175 with a median of 155 slices. No significant correlation was found between S and dose. The large range of results show that a more extensive investigation should be performed. In summary, diagnostic procedures such as biopsy present patient skin doses that are relatively low. On the other hand, patient skin dose in RF ablation can be high and can reach values higher than the value that a slow verification film is capable of measuring (1.2 Gy). For these procedures, an alternative method should be investigated.

Quality assurance for radiotherapy in prostate cancer: Point dose measurements in intensity modulated fields with large dose gradients

Purpose: We aimed to evaluate an optimization algorithm designed to find the most favorable points to position an ionization chamber (IC) for quality assurance dose measurements of patients treated for prostate cancer with intensity-modulated radiotherapy (IMRT) and fields up to 10 cm × 10 cm. Methods and Materials: Three cylindrical ICs (PTW, Freiburg, Germany) were used with volumes of 0.6 cc, 0.125 cc, and 0.015 cc. Dose measurements were made in a plastic phantom (PMMA) at 287 optimized points. An algorithm was designed to search for points with the lowest dose gradient. Measurements were made also at 39 nonoptimized points. Results were normalized to a reference homogeneous field introducing a dose ratio factor, which allowed us to compare measured vs. calculated values as percentile dose ratio factor deviations ΔF (%). A tolerance range of ΔF (%) of ±3% was considered. Results: Half of the ΔF (%) values obtained at nonoptimized points were outside the acceptable range. Values at optimized points were widely spread for the largest IC (i.e., 60% of the results outside the tolerance range), whereas for the two small-volume ICs, only 14.6% of the results were outside the tolerance interval. No differences were observed when comparing the two small ICs. Conclusions: The presented optimization algorithm is a useful tool to determine the best IC in-field position for optimal dose measurement conditions. A good agreement between calculated and measured doses can be obtained by positioning small volume chambers at carefully selected points in the field. Large chambers may be unreliable even in optimized points for IMRT fields ≤10 cm × 10 cm.

Mechanical and dose delivery accuracy evaluation in radiosurgery using polymer gels

The polymer gel–magnetic resonance imaging (MRI) dosimetry technique was used to evaluate the mechanical and dose delivery accuracy in Leksell gamma‐knife stereotactic radiosurgery for the treatment of multiple targets. Two different polymer gel dosimeter formulations reported in the literature were prepared in‐house. A plan for the treatment of four brain metastases (targets) was generated. It involved the delivery of four 8‐mm collimator shots using different prescription isodose lines and different prescription doses for each target, keeping the maximum dose constant for all targets. A sample of each gel formulation was irradiated using a custom‐made phantom with an experimental procedure capable of testing the increased nominal mechanical accuracy of stereotactic radiosurgery. The irradiated dosimeters were evaluated using a clinical 1.5 T MR imager. Result manipulation in 3D allowed for the determination of the mechanical accuracy in the delivery of each shot through the comparison of measured versus planned shot center coordinates. Dose delivery accuracy was also evaluated by comparison of maximum dose values measured at the center of each shot as well as dose distribution measurements, with corresponding treatment‐planning calculations. Polymer gel dosimetry was found capable of verifying the complete chain of radiosurgery treatment in gamma‐knife applications involving the irradiation of multiple targets.PACS numbers: 87.53.Dq, 87.53.Ly, 87.53.Xd

Natural radiation exposure in the Campo Arañuelo region in the surroundings of Almaraz nuclear power station (Spain)

During 1998 regional surveys were conducted to evaluate natural radiation exposure of people in the vicinity of the six Spanish nuclear power stations. Indoor radon, external gamma dose rates outdoors and indoors and radioactivity in soil were measured in these surveys. The highest mean annual effective dose to the population was found in the surroundings of the Almaraz nuclear power plant in the province of Cáceres and was mainly due to the presence of high radon concentrations in homes. In order to make a more accurate assessment of the dose coming from radon in this area, a new and more extensive survey on indoor radon with a total of 380 measurements was carried out in 2000 in the Campo Arañuelo region around the Almaraz nuclear power station. From the results obtained in this survey, a population-weighted mean annual effective dose from radon exposure of 1.7 mSv y −1, 44% lower than that previously reported in 1998, was estimated for the whole Campo Arañuelo region. The maximum dose value due to radon exposure, about 4 mSv y −1, was reached in the so-called La Vera area located in the north of this region. The relationship between the indoor radon concentrations experimentally measured, the geological characteristics of this geographic area and the information provided by the radiation map of the Spanish MARNA Project is also analyzed, confirming that La Vera area should be considered as a high radon level area.

Optimal source position for irradiation of coronary bifurcations in endovascular brachytherapy with catheter based beta or iridium-192 sources.

Background and purpose Intracoronary brachytherapy after percutaneous transluminal coronary angioplasty (PTCA) is usually performed with catheter-based treatment techniques in a straight vessel segment. There is a growing interest for treatment of bifurcations, which requires consecutive positioning of the source in main vessel and side branch. Materials and methods In-house developed software (IC-BT doseplan) is used to explore the optimal positioning of the source in modelled bifurcations with different shape for the source types available in our hospital, i.e. 90Sr/ 90Y, 32P and 192Ir. The results were summarised in look-up tables. The usefulness of these look-up tables was tested on various clinical examples. Results Tabulated results for the modelled bifurcations yield an estimation of the distance between the sources (gap width) in relation to the geometry and source type: 90Sr/ 90Y gap range 3–8.5 mm, 32P gap range 2–7 mm and 192Ir gap range 3.5–8 mm. The average dose relative to 2 mm from the source axis is: 90Sr/ 90Y, (mean±SD) 120±40%; 32P, 125±50% and 192Ir, 120±22%. The look-up tables also provide the coarse location and value of maximum and minimum dose: 90Sr/ 90Y, 220–60%, 32P, 230–55% and 192Ir, 170–85%. It appeared that the look-up tables provide a good approximation of the optimal gap width in the clinical examples. Conclusions Tabulated optimal gap widths are very useful for quick estimation of the required gap width for a given bifurcation and source type, in case the prescribed dose in both vessels is the same. In unfavourable geometries there is a risk of local underdosage. Individual treatment planning using a program such as IC-BT doseplan is then recommended.

Skin radiation injuries in patients following repeated coronary angioplasty procedures.

This study investigates the incidence of skin injuries and retrospectively estimates skin doses in a sample of patients who had multiple coronary angiographies and who underwent more than four percutaneous transluminal coronary angioplasties (PTCAs), performed primarily by the same team of cardiologists in a university hospital. A database of 7824 PTCAs performed during the last 14 years was analysed. Patients were selected and reviewed by a cardiologist and two radiotherapists with experience in radiation-induced skin injuries. A retrospective analysis of skin doses was performed using data from the patients' files and from the quality assurance (QA) programme of the hospital, which includes periodic patient dose measurements. 14 patients were included in the study. Each patient had undergone between 4 and 14 coronary angiographies and between 5 and 10 PTCAs, performed over a period of 2-10 years. The estimated mean dose-area product per procedure was 46 Gy cm(2) for coronary angiography and 82 Gy cm(2) for PTCA. Mean values of maximum skin dose per procedure were 217 mGy for the diagnostic studies and 391 mGy for the PTCAs. Only a slight radiation skin injury was clinically demonstrated in one patient with a history of 10 coronary angiographies and 10 PTCAs (estimated maximum skin dose 9.5 Gy). Another patient who underwent 14 coronary angiographies and 10 PTCAs (estimated maximum skin dose 7.3 Gy) showed a slight telangiectasia and discrete pigmentation. Another patient with a cutaneous lupus erythematosus showed pigmentation in the area of the radiation field following seven coronary angiographies and six PTCAs (estimated maximum skin dose 5.6 Gy), as expected bearing in mind that skin tolerance to high doses may be altered for patients with this pathology. Each of the remaining 11 patients with no skin injuries had undergone between 5 and 7 PTCAs and between 5 and 14 additional angiographies. None of the 14 patients reported acute skin injuries and no necrosis or radiodermatitis was observed.

Chest wall irradiation with MLC-shaped photon and electron fields

Purpose: To improve the treatment technique for chest wall irradiation, using the multileaf collimator (MLC) of the MM50 Racetrack Microtron to shape both photon and electron beams, and to check the dose delivery in the match-line region of these fields for the routine and improved technique. Methods and Materials: Using diode and film phantom measurements, the optimal number of photon beam segments and their positions relative to the electron beam were determined. On phantoms, and during actual patient treatment using in vivo dosimetry, the dose homogeneity in the match-line region was determined for both the routine and improved techniques. Results: Three photon beam segments (9-mm gap, perfect match, and 9-mm overlap) were used to match the electron beam, resulting in minimum-maximum dose values in the match-line region of 88–109%, compared to 80–115% for the routine technique (2 photon beam segments). During patient treatment, the average minimum and maximum dose values were 95% and 115%, respectively, compared to 78% and 127%, respectively, for the routine technique. The interfraction variation in dose delivery was reduced from 11.0% (1 SD) to 4.6% (1 SD). The actual treatment time was reduced from 10 to 4.5 min. Conclusion: Using the MLC of the MM50 to shape both photon and electron beams, an improved treatment technique for chest wall irradiation was developed, which is less labor intensive, faster, and yields a more homogeneous, and better reproducible dose delivery.

Reference dose levels for patients undergoing common diagnostic X-ray examinations in Irish hospitals.

Wide variations in patient dose for the same type of X-ray examination have been evident from various international dose surveys. Reference dose levels provide a framework to reduce this variability and aid in the optimization of radiation protection. The aim of this study was to establish, for the first time, a baseline for national reference dose levels in Ireland for four of the most common X-ray examinations: chest, abdomen, pelvis and lumbar spine. Measurements of entrance surface dose using thermoluminescent dosemeters (TLDs) for these four X-ray examinations were performed on 10 patients in each of 16 randomly selected hospitals. This represented 42% of Irish hospitals applicable to this study. Results have shown wide variation of mean hospital doses, from a factor of 3 for an anteroposterior lumbar spine to a factor of 23 for the chest X-ray. The difference between maximum and minimum individual patient dose values varied up to a factor of 75. Reasons for these dose variations were complex but, in general, low tube potential, high mAs and low filtration were associated with high-dose hospitals. This study also demonstrated lower reference dose levels of up to 40% when compared with those established by the UK and the Commission of the European Communities for four out of six projections. Only the chest X-ray exhibited a similar reference level to those established elsewhere. This emphasizes the importance of each country establishing its own reference dose levels that are appropriate to their own radiographic techniques and practices in order to optimize patient protection.

The use of a transverse CT image for the estimation of the dose given to the rectum in intracavitary brachytherapy for carcinoma of the cervix

Background and purpose: The three-dimensional (3D) dose distribution in combination with 3D anatomy of 13 patients treated for cervical carcinoma with intracavitary brachytherapy was analyzed. The aim of this study was to determine the correlation between a dose value obtained from the integral dose volume histogram (DVH) of the rectum and (a) the Nederlands Kanker Instituut (NKI) point of reference for the rectum dose ( R) and (b) the highest dose to the frontal rectum wall in the transverse CT slice near the top of the vagina through point R. Results: The correlation between the DVH rectum dose value for 2 cm 3 in the highest dose region and the rectum dose at point R was poor (regression coefficient 0.50). On the contrary, however, the correlation between the DVH rectum dose value for 2 cm 3 in the highest dose region and the maximum rectum dose value in a transverse CT slice through point R was good (regression coefficient 0.90). Conclusions: The maximal rectum dose value obtained from a transverse CT slice near the top of the vagina through point R was found to be a more representative point for the rectal dose burden and might therefore show a good correlation with complications. The point of reference for the rectal dose ( R) was found not to be a reliable estimation of the maximal dose in the rectum.

Radiation-induced attenuation in detectors using Cerenkov radiator or plastic scintillator

Some radiation detectors, using Cerenkov radiator or plastic scintillator, operate at very high dose rates. The accumulated dose greatly affects their performance. In fast and high gamma pulse environments, detector linearity is of great concern. In this paper we give some results that show the discrepancy which occurs when the transient radiation-induced attenuation is too high. We also give a model that can be used to determine maximum dose values for gamma pulse measurements and to correct spoiled data. >