Risk of autoimmune diseases following COVID-19 and the potential protective effect from vaccination: a population-based cohort study

Background

Case reports suggest that SARS-CoV-2 infection could lead to immune dysregulation and trigger autoimmunity while COVID-19 vaccination is effective against severe COVID-19 outcomes. We aim to examine the association between COVID-19 and development of autoimmune diseases (ADs), and the potential protective effect of COVID-19 vaccination on such an association.

Methods

A retrospective cohort study was conducted in Hong Kong between 1 April 2020 and 15 November 2022. COVID-19 was confirmed by positive polymerase chain reaction or rapid antigen test. Cox proportional hazard regression with inverse probability of treatment weighting was applied to estimate the risk of incident ADs following COVID-19. COVID-19 vaccinated population was compared against COVID-19 unvaccinated population to examine the protective effect of COVID-19 vaccination on new ADs.

Findings

The study included 1,028,721 COVID-19 and 3,168,467 non-COVID individuals. Compared with non-COVID controls, patients with COVID-19 presented an increased risk of developing pernicious anaemia [adjusted Hazard Ratio (aHR): 1.72; 95% Confidence Interval (CI): 1.12-2.64]; spondyloarthritis [aHR: 1.32 (95% CI: 1.03-1.69)]; rheumatoid arthritis [aHR: 1.29 (95% CI: 1.09-1.54)]; other autoimmune arthritis [aHR: 1.43 (95% CI: 1.33-1.54)]; psoriasis [aHR: 1.42 (95% CI: 1.13-1.78)]; pemphigoid [aHR: 2.39 (95% CI: 1.83-3.11)]; Graves' disease [aHR: 1.30 (95% CI: 1.10-1.54)]; anti-phospholipid antibody syndrome [aHR: 2.12 (95% CI: 1.47-3.05)]; immune mediated thrombocytopenia [aHR: 2.1 (95% CI: 1.82-2.43)]; multiple sclerosis [aHR: 2.66 (95% CI: 1.17-6.05)]; vasculitis [aHR: 1.46 (95% CI: 1.04-2.04)]. Among COVID-19 patients, completion of two doses of COVID-19 vaccine shows a decreased risk of pemphigoid, Graves' disease, anti-phospholipid antibody syndrome, immune-mediated thrombocytopenia, systemic lupus erythematosus and other autoimmune arthritis.

Interpretation

Our findings suggested that COVID-19 is associated with an increased risk of developing various ADs and the risk could be attenuated by COVID-19 vaccination. Future studies investigating pathology and mechanisms would be valuable to interpreting our findings.

Funding

Supported by RGC Collaborative Research Fund (C7154-20GF).

Safety of BNT162b2 or CoronaVac COVID-19 vaccines in patients with heart failure: A self-controlled case series study

Background

COVID-19 vaccines are important for patients with heart failure (HF) to prevent severe outcomes but the safety concerns could lead to vaccine hesitancy. This study aimed to investigate the safety of two COVID-19 vaccines, BNT162b2 and CoronaVac, in patients with HF.

Methods

We conducted a self-controlled case series analysis using the data from the Hong Kong Hospital Authority and the Department of Health. The primary outcome was hospitalization for HF and the secondary outcomes were major adverse cardiovascular events (MACE) and all hospitalization. We identified patients with a history of HF before February 23, 2021 and developed the outcome event between February 23, 2021 and March 31, 2022 in Hong Kong. Incidence rate ratios (IRR) were estimated using conditional Poisson regression to evaluate the risks following the first three doses of BNT162b2 or CoronaVac.

Findings

We identified 32,490 patients with HF, of which 3035 were vaccinated and had a hospitalization for HF during the observation period (BNT162b2 = 755; CoronaVac = 2280). There were no increased risks during the 0–13 days (IRR 0.64 [95% confidence interval 0.33–1.26]; 0.94 [0.50–1.78]; 0.82 [0.17–3.98]) and 14–27 days (0.73 [0.35–1.52]; 0.95 [0.49–1.84]; 0.60 [0.06–5.76]) after the first, second and third doses of BNT162b2. No increased risks were observed for CoronaVac during the 0–13 days (IRR 0.60 [0.41–0.88]; 0.71 [0.45–1.12]; 1.64 [0.40–6.77]) and 14–27 days (0.91 [0.63–1.32]; 0.79 [0.46–1.35]; 1.71 [0.44–6.62]) after the first, second and third doses. We also found no increased risk of MACE or all hospitalization after vaccination.

Interpretation

Our results showed no increased risk of hospitalization for HF, MACE or all hospitalization after receiving BNT162b2 or CoronaVac vaccines in patients with HF.

Funding

The project was funded by a Research Grant from the 10.13039/501100005407Food and Health Bureau, The Government of the Hong Kong Special Administrative Region (Ref. No. COVID19F01). F.T.T.L. (Francisco T.T. Lai) and I.C.K.W. (Ian C.K. Wong)'s posts were partly funded by the D24H; hence this work was partly supported by AIR@InnoHK administered by Innovation and Technology Commission.

Autoimmune conditions following mRNA (BNT162b2) and inactivated (CoronaVac) COVID-19 vaccination: A descriptive cohort study among 1.1 million vaccinated people in Hong Kong

BACKGROUND

Concerns regarding the autoimmune safety of COVID-19 vaccines may negatively impact vaccine uptake. We aimed to describe the incidence of autoimmune conditions following BNT162b2 and CoronaVac vaccination and compare these with age-standardized incidence rates in non-vaccinated individuals.

METHODS

This is a descriptive cohort study conducted in public healthcare service settings. Territory-wide longitudinal electronic medical records of Hong Kong Hospital Authority users (≥16 years) were linked with COVID-19 vaccination records between February 23, 2021 and June 30, 2021. We classified participants into first/second dose BNT162b2 groups, first/second dose CoronaVac groups and non-vaccinated individuals for incidence comparison. The study outcomes include hospitalized autoimmune diseases (16 types of immune-mediated diseases across six body systems) within 28 days after first and second dose of vaccination. Age-standardized incidence rate ratios (IRRs) with exact 95% confidence intervals (CIs) were estimated using Poisson distribution.

RESULTS

This study included around 3.9 million Hong Kong residents, of which 1,122,793 received at least one dose of vaccine (BNT162b2: 579,998; CoronaVac: 542,795), and 721,588 completed two doses (BNT162b2: 388,881; CoronaVac: 332,707). Within 28 days following vaccination, cumulative incidences for all autoimmune conditions were below 9 per 100,000 persons, for both vaccines and both doses. None of the age-standardized incidence rates were significantly higher than the non-vaccinated individuals, except for an observed increased incidence of hypersomnia following the first dose of BNT162b2 (standardized IRR: 1.47; 95% CI: 1.10-1.94).

CONCLUSIONS

Autoimmune conditions requiring hospital care are rare following mRNA and inactivated COVID-19 vaccination with similar incidence to non-vaccinated individuals. The association between first dose BNT162b2 vaccination and immune-related sleeping disorders requires further research. Population-based robust safety surveillance is essential to detect rare and unexpected vaccine safety events.

The effectiveness of exercises on fall and fracture prevention amongst community elderlies: A systematic review and meta-analysis

Objective

To analyze the effectiveness of exercise interventions on falls and fall-related fracture prevention among community-dwelling elderlies.

Methods

Literature search was conducted in Pubmed and Embase. Keywords used for literature search were “fracture” AND “fall” AND “exercise”. Randomized controlled trials involving community-dwelling elderlies older than 60 years old with physical exercises as intervention were included. A systematic review and meta-analysis was performed. The primary outcomes were falls and fractures.

Results

Twelve studies were included and 4784 participants were involved with a mean age of 75.4. The most common exercise interventions were strength and balance exercises. The results of meta-analysis of 11 studies showed that exercise intervention had beneficial effect on fall prevention (RR = 0.71, 95% CI, 0.62–0.82; I² = 24%, p < 0.0001). The effect was better when exercise intervention applied to women participants (RR = 0.64, 95% CI, 0.49–0.83; I² = 28%, p = 0.00009) compared to men and women participants (RR = 0.75, 95% CI, 0.64–0.89; I² = 24%, p = 0.001). The results of meta-analysis of seven studies showed that physical exercise had significant effect on fracture prevention (RR = 0.54, 95% CI, 0.35–0.83; I² = 25%, p = 0.005). However, the effect was significant when exercise intervention applied to women participants only (RR = 0.37, 95% CI, 0.20–0.67; I² = 0%, p = 0.001) but not significant when exercise intervention applied to both genders (RR = 0.80, 95% CI, 0.58–1.09; I² = 0%, p = 0.15).

Conclusion

Exercise interventions, especially the combination of strength and balance training, were effective in preventing falls. Resistance exercises and jumping exercises were effective for fracture prevention among community-dwelling older population. The effectiveness of exercise interventions on fracture prevention have more significant effect on women. Further studies are needed to test the effectiveness of exercise interventions in men.

Translational potential

The use of effective exercises or biophysical interventions including vibration therapy can be incorporated into Fracture Liaison Services to prevent future fall and fracture.

Comparison of BACTEC MGIT 960 system and BACTEC 460 TB system for growth and detection of Mycobacteria from clinical specimens

This study was carried out to compare the performance of BACTEC MGIT 960 with the BACTEC 460 TB for growth and detection of Mycobacteria from human clinical specimens. The BACTEC MGIT 960 instrument is a fully automated system that utilizes MGIT tubes containing an oxygen sensor embedded in silicon at the bottom and filled with 7 mL of modified Middlebrook 7H9 broth. Identical samples were inoculated into the two automated systems and incubated for six weeks. Over a period of three months, 279 specimens were decontaminated and processed according to the standard CDC NALC/NaOH method, using the commercial MycoPrep kit. Forty-two specimens (15%) yielded Mycobacterium tuberculosis; 37 (88%) were detected by the fluorescent BACTEC MGIT 960 and 35 (83%) detected by the radiometric BACTEC 460 TB. Fifteen specimens (5%) yielded Mycobacterium Other Than Tuberculosis (MOTT); 10 (66%) were detected by BACTEC MGIT 960 and 11 (73%) detected by BACTEC 460 TB. The average time to detection and contamination rates and the average time to obtain results of antimicrobial susceptibility tests between the two systems were compared. The performance of the BACTEC MGIT 960 was comparable to the BACTEC 460 TB system which has been the "Gold Standard" for automated detection of TB. The former was more rapid, as sensitive and less labour intensive than the BACTEC 460. Our data demonstrates that the BACTEC MGIT 960 system is an accurate, automated and a non-radioactive alternative to the BACTEC 460 TB for the culture and susceptibility testing of M. tuberculosis.

Inverse planning algorithms for external beam radiation therapy

Intensity-modulated radiation therapy (IMRT) is a new treatment technique that has the potential to produce superior dose distributions to those of conventional techniques. An important step in IMRT is inverse planning, or optimization. This is a process by which the optimum intensity distribution is determined by minimizing (or maximizing) an objective function. For radiation therapy, the objective function is used to describe the clinical goals, which can be expressed in terms of dose and dose/volume requirements, or in terms of biological indices. There are 2 types of search algorithms, stochastic and deterministic. Typical algorithms that are currently in use are presented. For clinical implementations, other issues are also discussed, such as global minimum vs. local minima, dose uniformity in the target and sparing of normal tissues, smoothing of the intensity profile, and skin flash. To illustrate the advantages of IMRT, clinical examples for the treatment of the prostate, nasopharynx, and breast are presented. IMRT is an emerging technique that has shown encouraging results thus far. However, the technique is still in its infancy and more research and improvements are needed. For example, the effects of treatment uncertainties on the planning and delivery of IMRT requires further study. As with any new technology, IMRT should be used with great caution.

The measurement of three dimensional dose distribution of a ruthenium-106 ophthalmological applicator using magnetic resonance imaging of BANG polymer gels

The BANG (MGS Research Inc., Guilford, CT) polymer gel has been used as a dosimeter to determine the three-dimensional (3D) dose distribution of a ruthenium-106 (Ru-106) ophthalmologic applicator. An eye phantom made of the BANG gel was irradiated with the Ru-106 source for up to 1 h. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5 T MR imager using the Hahn spin-echo pulse sequence with a TR of 2000 ms and two TEs of 20 ms and 100 ms. The T(2) values were evaluated on a pixel-by-pixel basis using custom-built software on a DEC alpha workstation and converted to dose using calibration data. Depth doses and isodose lines of the Ru-106 eye-plaque were generated. It is concluded that the BANG gel dosimetry offers the potential for measuring the 3D dose distributions of an ophthalmologic applicator, with high spatial resolution and relatively good accuracy.

Delivery of intensity-modulated radiation therapy with a conventional multileaf collimator: comparison of dynamic and segmental methods

Intensity-modulated radiation therapy (IMRT) can be delivered with a conventional multileaf collimator (MLC), either in dynamic mode (DMLC) or in segmental mode (SMLC, also known as "step-and-shoot"). The advantage of DMLC is its ability to deliver the desired intensity profile produced by inverse planning with a high degree of fidelity. The SMLC method, on the other hand, resembles treatment with multiple static fields, and can be more easily verified. However, the use of SMLC requires that the desired profile be approximated by discrete levels of intensity, which may lead to degradation in the delivered dose distribution. Clearly, the results of SMLC delivery depend on the number of levels and the spatial resolution of the intensity distribution. In this work, we compare the DMLC method and the SMLC method employing different numbers of levels and different spatial resolutions. Three disease sites were studied: prostate, nasopharynx, and breast, with three cases for each. In general, a 5- to 10-level SMLC plan produced results comparable to that from a DMLC plan. The target coverage is improved by increasing the number of levels while critical organs are better protected with finer spatial resolutions. The beam-on-time (MUs) requirement for SMLC was approximately 20% less than DMLC, but the delivery time (in minutes) was about twice as long. Thus, the choice depends on many factors including machine capability, quality assurance, target coverage, critical organ protection, beam-on-time, delivery time, and other clinical considerations.

Smoothing intensity-modulated beam profiles to improve the efficiency of delivery

Intensity-modulated beam profiles are generated by an inverse planning or optimization algorithm, a process that, being computationally complex and intensive, is inherently susceptible to noise and numerical artifacts. These artifacts make delivery of the beams more difficult, oftentimes for little, if any, observable improvement in the dose distributions. In this work we examine two approaches for smoothing the beam profiles. The first approach is to smooth the beam profiles subsequent to each iteration in the optimization process (method A). The second approach is to include a term within the objective function that specifies the smoothness of the profiles as an optimization criterion (method B). The two methods were applied to a phantom study as well as three clinical sites: paraspinal, nasopharynx, and prostate. For the paraspinal and nasopharynx cases, which have critical organs with low tolerance doses in close proximity, method B produced sharper dose gradients, better target dose homogeneity, and more critical organ sparing. In the less demanding prostate case, the two methods give similar results. In addition, method B is more efficient during optimization, requiring fewer iterations, but less efficient during DMLC delivery, requiring a longer beam-on time.

Monte Carlo evaluation of tissue inhomogeneity effects in the treatment of the head and neck

PURPOSE

To use Monte Carlo dose calculation to assess the degree to which tissue inhomogeneities in the head and neck affect static field conformal, computed tomography (CT)-based 6-MV photon treatment plans.

METHODS AND MATERIALS

We retrospectively studied the three-dimensional treatment plans that had been used for the treatment of 5 patients with tumors in the nasopharyngeal or paranasal sinus regions. Two patients had large surgical cavities. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil-beam dose-calculation algorithm with an equivalent path-length inhomogeneity correction. Each plan employs conformally-shaped 6-MV photon beams. Patient anatomy and electron densities were obtained from the treatment planning CT images. For each plan, the dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and CT images. The Monte Carlo method accurately accounts for the perturbation effects of local tissue heterogeneities. The Monte Carlo calculated dose distributions were compared with those from the clinical treatment planning system.

RESULTS

The degree to which tissue inhomogeneity affects the dose distributions of individual fields varies with the specific anatomic geometry, especially the size and location of air cavities in relation to the beam orientation and field size. Most of the beam apertures completely enclose the air cavities within or adjacent to the gross tumor volume (GTV). Equivalent squares (including blocking) ranged from approximately 5 to 9.5 cm. A common feature observed for individual fields is that the Monte Carlo calculated doses to tissue directly behind and within an air cavity are lower. However, after combining the fields employed in each treatment plan, the overall dose distribution shows only small differences between the two methods. For all 5 patients, the Monte Carlo calculated treatment plans showed a slightly lower dose received by the 95% of target volume (D(95)) than the plans calculated with the pencil-beam algorithm. The average difference in the target volume encompassed by the prescription isodose line was less than 2.2%. The difference between the dose-volume histograms (DVHs) of the GTV was generally small. For the brainstem and chiasm, the DVHs of the two plans were similar. For the spinal cord, differences in the details of the DHV and the dose to 1 cc (D(1cc)) of the structure were observed, with Monte Carlo calculation generally predicting increased dose indices to the spinal cord. However, these changes are not expected to be clinically significant.

CONCLUSION

For 6-MV photons, the effects of both normal tissue inhomogeneities and surgical air cavities on the target coverage were adequately accounted for by conventional pencil beam methods for all of the cases studied. Although differences in details of the DVHs of the normal structures were observed, depending on whether Monte Carlo or pencil-beam algorithm was used for calculation, these differences are not expected to be clinically significant. In general, the pencil-beam calculation corrected for primary attenuation by the equivalent pathlength is a sufficiently accurate method for head-and-neck treatment planning using 6-MV photons.

Treatment planning and delivery of intensity-modulated radiation therapy for primary nasopharynx cancer

PURPOSE

To implement intensity-modulated radiation therapy (IMRT) for primary nasopharynx cancer and to compare this technique with conventional treatment methods.

METHODS AND MATERIALS

Between May 1998 and June 2000, 23 patients with primary nasopharynx cancer were treated with IMRT delivered with dynamic multileaf collimation. Treatments were designed using an inverse planning algorithm, which accepts dose and dose-volume constraints for targets and normal structures. The IMRT plan was compared with a traditional plan consisting of phased lateral fields and a three-dimensional (3D) plan consisting of a combination of lateral fields and a 3D conformal plan.

RESULTS

Mean planning target volume (PTV) dose increased from 67.9 Gy with the traditional plan, to 74.6 Gy and 77.3 Gy with the 3D and IMRT plans, respectively. PTV coverage improved in the parapharyngeal region, the skull base, and the medial aspects of the nodal volumes using IMRT and doses to all normal structures decreased compared to the other treatment approaches. Average maximum cord dose decreased from 49 Gy with the traditional plan, to 44 Gy with the 3D plan and 34.5 Gy with IMRT. With the IMRT plan, the volume of mandible and temporal lobes receiving more than 60 Gy decreased by 10-15% compared to the traditional and 3D plans. The mean parotid gland dose decreased with IMRT, although it was not low enough to preserve salivary function.

CONCLUSION

Lower normal tissue doses and improved target coverage, primarily in the retropharynx, skull base, and nodal regions, were achieved using IMRT. IMRT could potentially improve locoregional control and toxicity at current dose levels or facilitate dose escalation to further enhance locoregional control.

Relative profile and dose verification of intensity-modulated radiation therapy

PURPOSE

To develop a quality assurance (QA) procedure to assess the intensity profile and dosimetry for intensity-modulated (IM) treatment fields using electronic portal imaging devices (EPIDs).

METHODS AND MATERIALS

A series of rapidly acquired (approximately 1/sec) portal images are summed and converted to dose. For relative intensity QA, the intended profile is subtracted point-by-point from the measured profile forming a series of error values. The standard deviation, sigma, of the errors, a measure of the goodness of the match, is minimized by applying a normalization and uniform scatter subtraction from the measured profile. For dose verification (dose to isocenter), an empirically determined phantom-correction factor is added to incorporate the effect of patient presence on EPID readings. Seventy prostate treatment fields were used in a phantom study to verify these approaches. Sensitivity was studied by creating artificial mismatches.

RESULTS

The average sigma for relative profile verification is 3.3% (percentage of average intended intensity) whereas artificial mismatches resulted in sigma values from 5% to 27%. The average isocentric dose calculated from EPID readings is 1.001 relative to the planned dose with a standard deviation of 0.018.

CONCLUSIONS

An EPID can be used for profile verification and absolute isocentric dose measurement for IM fields.

Experimental verification of a CT-based Monte Carlo dose-calculation method in heterogeneous phantoms

To further validate the Monte Carlo dose-calculation method [Med. Phys. 25, 867-878 (1998)] developed at the Memorial Sloan-Kettering Cancer Center, we have performed experimental verification in various inhomogeneous phantoms. The phantom geometries included simple layered slabs, a simulated bone column, a simulated missing-tissue hemisphere, and an anthropomorphic head geometry (Alderson Rando Phantom). The densities of the inhomogeneity range from 0.14 to 1.86 g/cm3, simulating both clinically relevant lunglike and bonelike materials. The data are reported as central axis depth doses, dose profiles, dose values at points of interest, such as points at the interface of two different media and in the "nasopharynx" region of the Rando head. The dosimeters used in the measurement included dosimetry film, TLD chips, and rods. The measured data were compared to that of Monte Carlo calculations for the same geometrical configurations. In the case of the Rando head phantom, a CT scan of the phantom was used to define the calculation geometry and to locate the points of interest. The agreement between the calculation and measurement is generally within 2.5%. This work validates the accuracy of the Monte Carlo method. While Monte Carlo, at present, is still too slow for routine treatment planning, it can be used as a benchmark against which other dose calculation methods can be compared.

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.

A patient-specific Monte Carlo dose-calculation method for photon beams

A patient-specific, CT-based, Monte Carlo dose-calculation method for photon beams has been developed to correctly account for inhomogeneity in the patient. The method employs the EGS4 system to sample the interaction of radiation in the medium. CT images are used to describe the patient geometry and to determine the density and atomic number in each voxel. The user code (MCPAT) provides the data describing the incident beams, and performs geometry checking and energy scoring in patient CT images. Several variance reduction techniques have been implemented to improve the computation efficiency. The method was verified with measured data and other calculations, both in homogeneous and inhomogeneous media. The method was also applied to a lung treatment, where significant differences in dose distributions, especially in the low-density region, were observed when compared with the results using an equivalent pathlength method. Comparison of the DVHs showed that the Monte Carlo calculated plan predicted an underdose of nearly 20% to the target, while the maximum doses to the cord and the heart were increased by 25% and 33%, respectively. These results suggested that the Monte Carlo method may have an impact on treatment designs, and also that it can be used as a benchmark to assess the accuracy of other dose calculation algorithms. The computation time for the lung case employing five 15-MV wedged beams, with an approximate field size of 13 X 13 cm and the dose grid size of 0.375 cm, was less than 14 h on a 175-MHz computer with a standard deviation of 1.5% in the high-dose region.

A gradient inverse planning algorithm with dose-volume constraints

An inverse planning algorithm for determining the intensity-modulated beams that will most closely generate a desired dose distribution is presented. The algorithm is three-dimensional and does not explicitly depend on beam energies and modalities. It allows a single prescription dose or a window of acceptable doses to be specified for the target, with additional constraints to account for under- or over-dosing. For the protection of organs at risk, it provides maximum-dose and dose-volume constraints. The latter apply to the entire volume of the organ exposed to the corresponding dose levels. Several levels of each type of constraint, with varying penalty weights, may be specified for each organ. The objective function that serves as the measure of the goodness of the solution is of the least-squares type and is minimized using conjugate gradient methods. Typical clinical cases involving 40,000 points and 4000 rays to be determined require about 10 min of CPU time on a DEC AlphaStation. Results are presented for two clinical sites, prostate and lung. The optimization algorithm yielded plans that featured higher target dose homogeneity, compared with the human planner's plan, while selectively sparing more of the normal organs at the desired dose regions.

Total body irradiation with an arc and a gravity-oriented compensator

PURPOSE

To deliver uniform dose distributions for total-body irradiation (TBI) with an arc field and a gravity-oriented compensator. This technique allows the patient to be treated lying on the floor in a small treatment room.

METHODS AND MATERIALS

Through the sweeping motion of the gantry, a continuous arc field can deliver a large field to a patient lying on the floor. The dose profile, however, would not be uniform if no compensator were used, due to the effects of inverse square variation of beam intensity with distance as well as the slanted depth in patient. To solve this problem, a gravity-oriented compensator made of cerrobend alloy was designed. This compensator has a cross-section of an inverted isosceles triangle, with the apex always pointing downward, due to gravity. By properly selecting the thickness of the compensator, the width of the base, and the distance between the pivots to the base, the difference in the path length through the compensator can be made just right to compensate the effects of inverse-square and slanted depth, thus producing a uniform dose profile.

RESULTS

Arc fields with a gravity-oriented compensator were used for 6, 10, 15, and 18 MV photon beams. The arc field can cover a patient with a height up to 180 cm. The field width was chosen from 32 to 40 cm at the machine isocenter. The optimal thickness of the compensator was found to be 2.5 cm, and its base was 25 cm wide. The distance from the pivot points to the flat surface of the compensator proximal to the beam ranges from 13 to 14 cm for different beam energies. The dose uniformity at a depth of 10 cm is within +/-5% for all beam energies used in this study.

CONCLUSIONS

Highly uniform dose profiles for TBI treatments can be delivered with an arc and a gravity-oriented compensator. The proposed technique is simple and versatile. A single compensator can be used for all energies, because the amount of compensation can be adjusted by changing the distance to the pivot and/or the field size.

Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate

PURPOSE

To improve the local control of patients with adenocarcinoma of the prostate we have implemented intensity modulated radiation therapy (IMRT) to deliver a prescribed dose of 81 Gy. This method is based on inverse planning and the use of dynamic multileaf collimators (DMLC). Because IMRT is a new modality, a major emphasis was on the quality assurance of each component of the process and on patient safety. In this article we describe in detail our procedures and quality assurance program.

METHODS AND MATERIALS

Using an inverse algorithm, we have developed a treatment plan consisting five intensity-modulated (IM) photon fields that are delivered with DMLC. In the planning stage, the planner specifies the number of beams and their directions, and the desired doses for the target, the normal organs and the "overlap" regions. Then, the inverse algorithm designs intensity profiles that best meet the specified criteria. A second algorithm determines the leaf motion that would produce the designed intensity pattern and produces a DMLC file as input to the MLC control computer. Our quality assurance program for the planning and treatment delivery process includes the following components: 1) verification of the DMLC field boundary on localization port film, 2) verification that the leaf motion of the DMLC file produces the planned dose distribution (with an independent calculation), 3) comparison of dose distribution produced by DMLC in a flat phantom with that calculated by the treatment planning computer for the same experimental condition, 4) comparison of the planned leaf motions with that implemented for the treatment (as recorded on the MLC log files), 5) confirmation of the initial and final positions of the MLC for each field by a record-and-verify system, and 6) in vivo dose measurements.

RESULTS

Using a five-field IMRT plan we have customized dose distribution to conform to and deliver 81 Gy to the PTV. In addition, in the overlap regions between the PTV and the rectum, and between the PTV and the bladder, the dose is kept within the tolerance of the respective organs. Our QA checks show acceptable agreement between the planned and the implemented leaf motions. Correspondingly, film and TLD dosimetry indicates that doses delivered agrees with the planned dose to within 2%. As of September 15, 1996, we have treated eight patients to 81 Gy with IMRT.

CONCLUSION

For complex planning problems where the surrounding normal tissues place severe constraints on the prescription dose, IMRT provides a powerful and efficient solution. Given a comprehensive and rigorous quality-assurance program, the intensity-modulated fields can be efficaciously and accurately delivered using DMLC. IMRT treatment is now ready for routine implementation on a large scale in our clinic.

Lying-on position of total skin electron therapy

PURPOSE

A technique for whole-body electron therapy with the patient in a lying position has been developed. This technique allows Total Skin Electron Therapy (TSET) for those patients who were previously unable to be treated in a conventional standing position.

METHODS AND MATERIALS

This study was carried out on a Varian 2100C linear accelerator with a 6 MeV high dose rate electron beam. The collimator was open to a width of 36 x 36 cm. There were two main procedures, with six dual-field techniques: 1) two static AP/PA vertical dual fields (VDF): the patient laid on the floor transversely under the collimator when the gantry was in a vertical position. A 0.6 cm acrylic board was placed 15 cm away from the patient, then the gantry was rotated 25 degrees clockwise and counterclockwise to treat the patient in the supine and prone positions, respectively. 2) Four oblique junction fields (OJF): the patient laid on the floor in a prone and supine position parallel to the wave guide at (227 - body thickness x tan 60 degrees) cm away from the vertical axis of the gantry, then the gantry was rotated 60 degrees toward the patient. A 0.6 cm acrylic board was placed 15 cm away from the patient perpendicular to the beam. The patient was move along the field central axis. It allowed the patient's body to be within the 160 cm effective treatment profile. When the patient's body axis move 5 degrees toward the lateral side of the field central axis, we could obtain a better dose distribution in the vertex of the scalp and the soles of the feet. The angle of the VDF was measured by chamber detectors to obtain the effective treatment profile. Likewise, the optimal profile for the OJF was determined by the same procedures. The Rando phantom was used to measure the superficial dose of the body.

RESULTS

The dimension of effective treatment profile for the VDF was 188 x 72 cm at 87% dose level For the OJF, we had to move the patient along the field central axis to obtain the effective treatment profile in a 180 x 85 cm dimension at a 87% dose level. The vertex and sole dose measured in this setup was in the range of 80-88%.

CONCLUSIONS

The empirical data showed that the lying-on position for TSET was technically feasible. The dose distribution in the body surface was also compatible with the Stanford standing technique. The nonambulatory skin malignancy patient can be treated in a comfortable and reproducible position.

Implementation of a Monte Carlo dosimetry method for patient-specific internal emitter therapy

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose-response relationship, as well as to avoid critical organ toxicity. This work describes the implementation of a dosimetry method that accounts for the radionuclide decay spectrum, and patient-specific activity and density distributions. The dosimetry algorithm is based on a Monte Carlo procedure that simulates photon and electron transport and scores energy depositions within the patient. The necessary input information may be obtained from a registered set of CT and SPECT or PET images. The algorithm provides the absorbed dose rate for the radioactivity distribution provided by the SPECT or PET image. The algorithm was benchmarked by reproducing dosimetric quantities using the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom and was used to calculate absorbed dose distributions for representative case studies.

The use of diode dosimetry in quality improvement of patient care in radiation therapy

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST). Correction factors of clinical relevance were quantified to convert the diode readings into patient dose. The results of dose measurements on 6 patients undergoing external beam radiation therapy for carcinoma of the prostate on three different therapy units are presented. Field shaping during treatments was accomplished either by multileaf collimation or by cerrobend blocking. A deviation of less than +/-4% between the measured and prescribed patient doses was observed. The results indicate that the diodes exhibit excellent linearity, dose reproducibility, minimal anisotropy, and can be used with confidence for patient dose verification. Furthermore, diodes render real time verification of dose delivered to patients.

A Monte Carlo approach to patient-specific dosimetry

In internal emitter therapy, an accurate description of the absorbed dose distribution is necessary to establish an administered dose-response relationship, as well as to avoid critical organ toxicity. Given a spatial distribution of cumulated activity, an absorbed dose distribution that accounts for the effects of attenuation and scatter can be obtained using a Monte Carlo method that simulates particle transport across the various densities and atomic numbers encountered in the human body. Patient-specific information can be obtained from CT and SPECT or PET imaging. Since the data from these imaging modalities is discrete, it is necessary to develop a technique to efficiently transport particles across discrete media. The Monte Carlo-based algorithm presented in this article produces accurate absorbed dose distributions due to patient-specific density and radionuclide activity distributions. The method was verified by creating CT and SPECT arrays for the Medical Internal Radionuclide Dose (MIRD) Committee's Standard Man phantom, and reproducing the spatially averaged specific absorbed fractions reported in MIRD Pamphlet 5. The algorithm was used to investigate the implications of replacing a mean absorbed dose with a distribution, and of neglecting atomic number and density variations for various patient geometries and energies. For example, the I-131 specific absorbed fraction for spleen to liver is the same as for liver to spleen, yet the distributions were different. Furthermore, neglecting atomic number variations across the vertebral bone led to an overestimation of I-125 absorbed dose by an order of magnitude, while no error was observed for I-131.

Conformal radiation treatment of prostate cancer using inversely-planned intensity-modulated photon beams produced with dynamic multileaf collimation

PURPOSE

To implement radiotherapy with intensity-modulated beams, based on the inverse method of treatment design and using a multileaf collimation system operating in the dynamic mode.

METHODS AND MATERIALS

An algorithm, based on the inverse technique, has been integrated into the radiotherapy treatment-planning computer system in our Center. This method of computer-assisted treatment design was used to derive intensity-modulated beams to optimize the boost portion of the treatment plan for a patient with a T1c cancer of the prostate. A dose of 72 Gy (in 40 fractions) was given with a six-field plan, and an additional 9 Gy (in five fractions) with six intensity-modulated beams. The intensity-modulated fields were delivered using dynamic multileaf collimation, that is, individual leaves were in motion during radiation delivery, with the treatment machine operating in the clinical mode. Exhaustive quality assurance measurement and monitoring were carried out to ensure safe and accurate implementation.

RESULTS

Dose distribution and dose-volume histogram of the "inverse method" boost plan and of the composite (72 Gy primary + 9 Gy boost) plan were judged clinically acceptable. Compared to a manually designed boost plan, the inverse treatment design gave improved conformality and increased dose homogeneity in the planning target volume. Film and ion chamber dosimetry, performed prior to the first treatment, indicated that each of the six intensity-modulated fields was accurately produced. Thermoluminescent dosimeter (TLD) measurements performed on the patient confirmed that the intended dose was delivered in the treatment. In addition, computer-aided treatment-monitoring programs assured that the multileaf collimator (MLC) position file was executed to the specified precision. In terms of the overall radiation treatment process, there will likely be labor savings in the planning and the treatment phases.

CONCLUSIONS

We have placed into clinical use an integrated system of conformal radiation treatment that incorporated the inverse method of treatment design and the use of dynamic multileaf collimation to deliver intensity-modulated beams. The system can provide better treatment design, which can be implemented reliably and safely. We are hopeful that improved treatment efficacy will result.

Radionuclide photon dose kernels for internal emitter dosimetry

Photon point dose kernels and absorbed fractions were generated in water for the full photon emission spectrum of each radionuclide of interest in nuclear medicine, by simulating the transport of particles using Monte Carlo. The kernels were then fitted to a mathematical expression. Absorbed fractions for point sources were obtained by integrating the kernels over spheres. Photon dose kernels and absorbed fractions were generated for the following radionuclides: I-123, I-124, I-125, I-131, In-111, Cu-64, Cu-67, Ga-67, Ga-68, Re-186, Re-188, Sm-153, Sn-117m, Tc-99m. The Monte Carlo simulation was verified by comparing the dose kernels to published monoenergetic photon kernels. Further validation was obtained by generating an I-125 brachytherapy seed kernel and comparing it with published data. Since Monte Carlo simulation was initialized by sampling from the complete photon spectra of these radionuclides, interpolation between monoenergetic kernels and absorbed fractions was not required. The absorbed-fraction due to uniform spherical distributions can be directly applied for use in internal dosimetry. In addition, the kernels can be used as input for three-dimensional internal dosimetry calculations.

Testing of dynamic multileaf collimation

It has been shown that intensity-modulated fields have the potential to deliver optimum dose distributions, i.e., high dose uniformity in the target and lower doses in the surrounding critical organs. One way to deliver such fields is by using dynamic multileaf collimation (DMLC). This capability is already available in research mode on some treatment machines. While much effort has been devoted to developing algorithms for DMLC, the mechanical reliability of this new treatment delivery mode has not been fully studied. In this work, we report a series of tests designed to investigate the mechanical aspects of DMLC and their implications on dosimetry. Specifically, these tests were designed to examine (1) the stability of leaf speed, (2) the effect of lateral disequilibrium on dose profiles between adjacent leaves, (3) the significance of acceleration and deceleration of leaf motion, (4) the effect of positional accuracy and rounded-end of the leaves, and (5) create a simple test pattern that may serve as a basis for routine quality assurance checks. Results of these tests are presented. The implications on dosimetry and consideration for the design of leaf motion are discussed.

Dosimetric verification of intensity-modulated fields

The optimization of intensity distributions and the delivery of intensity-modulated treatments with dynamic multi-leaf collimators (MLC) offer important improvements to three-dimensional conformal radiotherapy. In this study, a nine-beam intensity-modulated prostate plan was generated using the inverse radiotherapy technique. The resulting fluence profiles were converted into dynamic MLC leaf motions as functions of monitor units. The leaf motion pattern data were then transferred to the MLC control computer and were used to guide the motions of the leaves during irradiation. To verify that the dose distribution predicted by the optimization and planning systems was actually delivered, a homogeneous polystyrene phantom was irradiated with each of the nine intensity-modulated beams incident normally on the phantom. For each exposure, a radiographic film was placed normal to the beam in the phantom to record the deposited dose. The films were calibrated and scanned to generate 2-D isodose distributions. The dose was also calculated by convolving the incident fluence pattern with pencil beams. The measured and calculated dose distributions were compared and found to have discrepancies in excess of 5% of the central axis dose. The source of discrepancies was suspected to be the rounded edges of the leaves and the scattered radiation from the various components of the collimation system. After approximate corrections were made for these effects, the agreement between the two dose distributions was within 2%. We also studied the impact of the "tongue-and-groove" effect on dynamic MLC treatments and showed that it is possible to render this effect inconsequential by appropriately synchronizing leaf motions. This study also demonstrated that accurate and rapid delivery of realistic intensity-modulated plans is feasible using a dynamic multi-leaf collimator.

Generation of arbitrary intensity profiles by combining the scanning beam with dynamic multileaf collimation

An algorithm, which combines the scanning beam with dynamic collimation to generate any arbitrary intensity profile, is presented. The desired intensity profile is assumed to be piecewise linear. The dynamic collimation method used is the "sliding window." The algorithm can be used either for a given scanning beam profile or to simultaneously determine the scanning beam profile and the leaf motions required to generate the desired intensity profile, which minimize the total treatment time. The limitations imposed by the physics of an elementary beam are taken into account. The algorithm is an iterative one, with typical calculation times being of the order of a few milliseconds.

A Monte Carlo model of photon beams used in radiation therapy

A generic Monte Carlo model of a photon therapy machine is described. The model, known as McRad, is based on EGS4 and has been in use since 1991. Its primary function has been the characterization of the incident photon fluence for use by dose calculation algorithms. The accuracy of McRad is examined by comparing the dose distributions in a water phantom generated using only the Monte Carlo data with measured dose distributions for two machines in our clinic; a 6 MV Varian Clinac 600C and the 15 MV beam from a Clinac 2100C. The Monte Carlo generated dose distributions are computed using a dose calculation algorithm based on the use of differential pencil beam kernels. It was found that the match to measured data could be improved if the model is tuned by adjusting the energy of the electron beam incident on the target. The beam profiles were found to be more sensitive indicators of the electron beam energy than the depth dose curves. Beyond the depths reached by contaminant electrons, the computed and measured depth dose curves agree to better than 1%. The comparison of beam profiles indicate that in regions up to within 1 cm of the field edge, the measured and computed doses generally agree to within 2%-3%.

Effect of ocular implants of different materials on the dosimetry of external beam radiation therapy

PURPOSE

To study the attenuation and scattering effects of ocular implants, made from different materials, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams used in orbital radiotherapy.

METHODS AND MATERIALS

Central axis depth-dose measurements were performed in a polystyrene phantom with embedded spherical ocular implants using film dosimetry of a 6 MV photon beam and electron beams of 6, 9, and 12 MeV energy. The isodose distributions were also calculated by a computerized treatment planning system using computerized tomography (CT) scans of a polystyrene phantom that had silicone, acrylic, and hydroxyapatite ocular implants placed into it.

RESULTS

Electron beam dose distributions display distortions both on the measured and calculated data. This effect is most accentuated for the hydroxyapatite implants, for which the transmissions through ocular implants are on the order of 93% for the 6 MV photon beam, and range from 60% for 6 MeV electrons to 90% for 12 MeV electrons.

CONCLUSION

We studied the effect of ocular implants of various materials, embedded in a polystyrene phantom, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams. Our investigations show that while 6 MV photons experience only a few percent attenuation, lower energy electron beam with 60% transmission is not a suitable choice of treating tumors behind the ocular implants.

Beam characteristics of a new generation 50 MeV racetrack microtron

The first of a new generation of microtron accelerators has been installed and tested. It is currently in use for multisegment conformal radiotherapy at our institution. The unit produces x rays and electrons from 10 to 50 MeV in 5 MeV increments. It incorporates a 64 leaf, doubly focused multileaf collimator (MLC), which can be used to shape x-ray and electron beams. Both x-ray and electron beams are produced by magnetically scanning the electron beams from the accelerator. The new generation unit incorporates a purging magnet to sweep away any primary or secondary electrons that pass through the target(s). In this paper, the beam characteristics of the accelerator that were studied during acceptance testing are described. Representative examples of depth doses, beam profiles, output factors, and elementary beam distributions are presented and discussed, in comparison with the earlier generation of microtron accelerators and with other radiotherapy machines.

Mean mass energy absorption coefficient ratios for megavoltage x-ray beams

Mean mass energy absorption coefficient ratios of acrylic, polystyrene, and water to air, were calculated using Monte Carlo generated energy spectra. The energy spectra were calculated for 4- to 50-MV x-ray beams, from machines using flattening filters and scanning beams. The validity of these spectra was verified by comparing the measured ionization ratios with the calculated values. The agreement was found to be within 1.9%. For beams of energy below 6 MV, our estimates of the mean mass energy absorption coefficient ratios agree well with those recommended by the TG-21 protocol. For higher energy beams, the discrepancy increases to about 3%. It was found that the discrepancy is attributable to the different spectra used in these calculations.

Analysis of the photon beam treatment planning data for a scanning beam machine

The characteristics of photon beams from the Scanditronix MM50 radiation therapy machine that are necessary for treatment planning are described. The MM50 uses a scanning beam instead of a conventional flattening filter to achieve flat dose distributions. At each beam energy, a scan pattern is chosen, depending on the field size; the small scan pattern (S) is used for field sizes up to 10 x 10 cm, the medium scan pattern (M) is used for field sizes up to 20 x 20 cm, and the large scan pattern (L) is used for the larger field sizes. The dose distributions of the beams associated with the 10 MV S, M, and L scan patterns, the 25 MV S, M, and L patterns, and the 50 MV S and M patterns are described. The data reported includes central axis data, beam profiles, and output factors. In addition to the measured data, our dose calculation model requires a pencil beam kernel for each beam. The kernel is constructed using the average photon energy spectrum, which is generated using a Monte Carlo simulation of the MM50. The simulation, based on EGS4, is also used to generate the radial variation of fluence and energy fluence, which is required by a new dose calculation model that does not require the measurement of beam profiles. The Monte Carlo generated data; the photon energy spectrum, the fluence, and the energy fluence are presented.

Initial clinical experience with computer-controlled conformal radiotherapy of the prostate using a 50-MeV medical microtron

PURPOSE

We have described previously a model for delivering computer-controlled radiation treatments. We report here on the implementation and first year's clinical experience with such treatments using a 50 MeV medical microtron.

METHODS AND MATERIALS

The microtron is equipped with a multileaf collimator and is capable of setting up and treating a sequence of fixed fields called segments, under computer control. An external computer derives machine parameters for the segments from a three-dimensional treatment planning system, transfers them to the microtron control computer, checks the machine settings before allowing dose delivery to begin, and records the treatment. We describe the patient treatment methodology, portal film acquisition, electronic portal imaging, and quality assurance.

RESULTS

Patient treatments began in July 1992, comprising six-segment conformal treatments of the prostate. Using the recorded treatment data, the system performance has been examined and compared to other treatment machines. The average treatment time is 10 min, of which 4 min is for computer-controlled setup and irradiation; the remaining time is for patient positioning and checking of clearances. Long-term reproducibility of computer-controlled setup of the gantry and multileaf position is better than 0.5 degrees and 1 mm, respectively. Termination due to a machine fault has occurred in 5.5% of treatments, improving to 2.5% in recent months.

CONCLUSION

Our initial experience indicates that computer-controlled segmental therapy can be performed reliably on a routine basis. Treatment times with the microtron are significantly shorter than with conventional linacs, and setup accuracy is consistent with that needed for conformal therapy. We believe that treatment times can be further improved through software upgrades and integration of electronic portal imaging.

Beam profiles along the nonwedged direction for large wedged fields

Beam profiles along the nonwedged direction of a wedged field produced by a linear accelerator exhibit more "sagging" than that of an open field at the same depth. For large fields, the profiles of open and wedged fields can differ by as much as 7%. The extra "sagging" of wedged profiles is mainly due to the difference in penetration between on- and off-axis rays caused by the variation of beam quality across the field. An algorithm was developed to estimate an "effective" depth such that the profile of a wedged field can be approximated by the open-field profile at the effective depth. The algorithm was verified by measured beam profiles for 6- and 15-MV x-ray beams for 15 degree, 30 degree, 45 degree, and 60 degree wedges.

Dose calculation for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations

A dose calculation algorithm has been developed for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations. First, an in-air fluence distribution is constructed based on the dynamic motion of the jaws or leaves, taking into account the variation of output with field size defined by the jaws. The fluence distribution is then convolved with the appropriate pencil beam kernel to give correction factors which are used to calculate the dose distribution for an intensity-modulated photon field. The proposed algorithm is strictly valid in homogeneous media only, patient heterogeneity correction is accounted for in an approximate manner. Dose distributions at several depths and for several field sizes were calculated for 6- and 15-MV x-ray beams for a set of standard wedges produced by dynamic jaws. Measurements were made with film and an ion chamber. Comparisons between calculated and measured data show good agreement (within 2%) for both dose profiles and wedge factors. Similar calculations and measurements were also made for a 25-MV intensity-modulated photon field produced by dynamic motion of a multileaf collimator. Agreement between calculations and measurements is also good (within 3%). The "tongue-and-groove" effect associated with a multileaf collimator design is also examined using a ring-shaped field produced by matching two component fields. The computation time for a dynamic-collimated field is the same as that for an irregular field shaped by conventional blocks. The algorithm is applicable to any pattern of jaw or multileaf motions. The strengths and remaining problems of the algorithm are discussed.

Generation of arbitrary intensity profiles by dynamic jaws or multileaf collimators

An algorithm, which calculates the motions of the collimator jaws required to generate a given arbitrary intensity profile, is presented. The intensity profile is assumed to be piecewise linear, i.e., to consist of segments of straight lines. The jaws move unidirectionally and continuously with variable speed during radiation delivery. During each segment, at least one of the jaws is set to move at the maximum permissible speed. The algorithm is equally applicable for multileaf collimators (MLC), where the transmission through the collimator leaves is taken into account. Examples are presented for different intensity profiles with varying degrees of complexity. Typically, the calculation takes less than 10 ms on a VAX 8550 computer.

Perspectives of multidimensional conformal radiation treatment

We consider the present technological advancement that underlies the implementation of computer-controlled conformal radiotherapy. We also consider the developments in modern biology that may provide input to therapy planning. The concept of multidimensional conformal radiotherapy is advanced, which integrates geometrical precision and biological conformality, to optimize the treatment planning for individual patients, with a view to improve the overall success of radiotherapy.

The use of a multi-leaf collimator for conformal radiotherapy of carcinomas of the prostate and nasopharynx

We investigate the use of a multi-leaf collimator for conformal radiation therapy of carcinomas of the prostate and of the nasopharynx. Following verification of dose calculation algorithms for multi-leaf collimated fields using film dosimetry, we compute dose distributions for multi-field conformal treatment using fields shaped with either the multi-leaf collimator or conventional cerrobend blocks. We compare the two sets of treatment plans using graphical isodose displays, tissue specific dose volume histograms, tumor control probabilities, and normal tissue complication probabilities. We also incorporate setup errors into the calculated dose distributions to assess the effect of treatment uncertainties on the various criteria. Based on these comparisons, we conclude that for multi-field conformal radiotherapy for these two disease sites, the use of multi-leaf collimation is equivalent to that of conventional cerrobend blocks.

Stereotactic treatment of brain tumors with radioactive implants or external photon beams: radiobiophysical aspects

We perform calculations, based on the linear-quadratic model, to assess the biologically effective doses (BED) of tumor and normal tissue in the stereotactic irradiation of brain tumors with either radioactive implants or radiosurgery techniques. Treatment protocols for radiosurgery and radioactive implants, as obtained from the literature, are reviewed and compared. A figure of merit is defined to be the ratio of tumor to normal tissue BED, expressed in units of Gy10/Gy3. These comparisons indicate a clear radiobiological advantage for brachytherapy, unless the radiosurgery is to be delivered in a large number of fractions. The differences in dose uniformity, and in the volume of normal tissue encompassed by the high dose regions, are factors that may also influence clinical results.

Beam characteristics of a new model of 6-MV linear accelerator

This paper describes the beam characteristics and dosimetry measurements performed on the 6-MV photon beam of a new model of linear accelerator, three of which were recently introduced and installed in our institution. Percent depth dose and tissue maximum ratio tables for a variety of field sizes and depths, as well as other parameters used for treatment planning are presented. These accelerators are the first of their kind using both hardware and software tools to control interlocks. Checking procedures for these interlocks are available from the authors upon request. Comparison of characteristic parameters between these three new 6-MV linear accelerators and with the 6-MV beams of two other accelerators is also made.

Diode dosimetry of models 6711 and 6712 125I seeds in a water phantom

Two-dimensional relative dose distributions have been measured around 125I brachytherapy seeds. The two seed models studied, models 6711 and 6712, were manufactured by the 3M Company. Silicon detectors immersed in water phantoms were used to measure the dose. A computerized data acquisition system that controlled the radial position of the diode and the angular rotation of the seed, as well as a manually controlled system were used to collect and store the data. Our results show that the two seed models have relative dose distributions which are quite similar; however, the absolute dose distributions are sufficiently different to warrant separate look-up tables for the two seed models. Additionally, our results are compared with dose distribution data previously obtained for the model 6711 seed.

Interinstitutional experience in verification of external photon dose calculations

Under the auspices of NCI contracts, four institutions have collaborated to assess the accuracy of the pixel-based dose calculation methods they employ for external photon treatment planning. The approach relied on comparing calculations using each group's algorithm with measurements in phantoms of increasing complexity. The first set of measurements consisted of ionization chamber measurements in water phantoms in normally incident square fields, an elongated field, a wedged field, a blocked field, and an obliquely incident beam. The second group of measurements was carried out using thermoluminescent dosimeters in phantoms designed to investigate the effects of surface curvature, high density heterogeneities, and low density heterogeneities. The final study tested the entire treatment planning system, including CT data conversion, in an anthropomorphic phantom. Overall, good agreement between calculation and measurements was found for all algorithms. Regions in which discrepancies were observed are pointed out, areas for algorithm improvement are identified and the clinical import of algorithm accuracy is discussed.

A comprehensive three-dimensional radiation treatment planning system

A comprehensive software system has been developed to allow 3-dimensional planning of radiation therapy treatments using the extensive anatomical information made available by imaging modalities such as CT and MR. Biological structures of interest and tumor volumes are defined by outlines drawn on a sequence of CT slices. Beam set-ups may then be determined in three dimensions by displaying the structure contours in a beam's eye view, or in two dimensions using a single CT cut. Each beam defined may be shaped by the specification of block aperture contours, and its intensity may be modified with the use of planar compensators. 3D dose calculation algorithms are discussed. To evaluate the calculation results, dose volume histograms are provided, as well as various types of displays in two and three dimensions, including dose on arbitrarily oriented planes, dose on the surface of anatomical objects, and isodose surfaces. Computer generated beam films are also available as an aid in patient set-up verification. These tools, and others, provide the basis for a comprehensive 3D system that can be used throughout the treatment planning process.

Extraction of pencil beam kernels by the deconvolution method

A method has been developed to extract pencil beam kernels from measured broad beam profiles. In theory, the convolution of a symmetric kernel with a step function will yield a function that is symmetric about the inflection point. Conversely, by deconvolution, the kernel may be extracted from a measured distribution. In practice, however, due to the uncertainties and errors associated with the measurements and due to the singularities produced in the fast Fourier transforms employed in the deconvolution process, the kernels thus obtained and the dose distributions calculated therefrom, often exhibit erratic fluctuations. We propose a method that transforms measured profiles to new, modified distributions so that they satisfy the theoretical symmetry condition. The resultant kernel from the deconvolution is then free of fluctuations. We applied this method to compute photon and electron dose distributions at various depths in water and electron fluence distributions in air. The agreement between measured and computed profiles is within 1% in dose or 1 mm in distance in high dose gradient regions.

The effect of angular spread on the intensity distribution of arbitrarily shaped electron beams

Knowledge of the relative intensity distribution at the patient's surface is essential for pencil beam calculations of three-dimensional dose distributions for arbitrarily shaped electron beams. To calculate the relative intensity distribution, the spatial spread resulting from angular spread is convolved with a two-dimensional step function whose shape corresponds to the applicator aperture. Two different approaches to obtain angular spread or the equivalent spatial spread are investigated. In the first method, the pencil beam angular spread is assumed to be Gaussian in shape. The angular spread constants (sigma theta) are then obtained from the slopes of measured intensity profiles. In the second method, the angular spread, in the form of an array of numerical values, is obtained by the deconvolution of measured intensity profiles. After obtaining the angular spread, the calculation for convolution is done in a number of parallel planes normal to the central axis at various distances from the electron collimator. Intensity at any arbitrary point in space is computed by interpolating between intensity distributions in adjacent planes on either side of the point. The effects of variations in angular spread as a function of field size for two treatment machines, one with a scanned electron beam and the other with a scattering foil, have been studied. The consequences of assuming angular spread to be of Gaussian shape are also examined. The electron intensity calculation techniques described in this paper apply primarily to methods of dose calculations that employ pencil beams generated using Monte Carlo simulations.

Dose computations for asymmetric fields defined by independent jaws

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.