WE-G-218-02: The Measurement of Absorbed Dose to Active Intravenous Cardiac Devices in Patients Undergoing Radiotherapy.

Radiotherapy departments are seeing an increasing number of patients presenting for treatment with active intravenous cardiac devices (AICDs). This is due to an increasing aging population and technology advances in these devices. The AAPM TG-34 addressed the radiation effects in these devices and provided recommendations for patients with these devices undergoing radiotherapy. The current devices utilizing CMOS technology are more sensitive to radiation then the older bipolar devices. Therefore, it is important to estimate the dose to the device to ensure that the radiation does not adversely affect the performance of the device. This is particularly important for patients dependent on their accurate functioning. The dose to the device is generally from secondary radiation and is typically below 0.05 Gy with most of it coming from lower energy scatter radiation. Treatment planning systems can be used to estimate the dose to the device. However, these systems do not accurately calculate doses at distances more than 2-3 cm from the field edge. For these out-of-field dose measurements the dosimeter requires a high sensitivity and a relatively flat energy response. Thermoluminescent detectors and optically stimulated luminescent detectors satisfy these requirements and they are relatively unobtrusive. The newer formulations of radiochromic film with higher sensitivity can also be used. The detector should be place under 0.5 to 1.0 cm of bolus to minimize the contaminating head-scatter electrons provide a more realistic measurement of the dose to the device. LEARNING OBJECTIVES: 1. List the reasons for estimating the dose to the AICDs 2. Provide a discussion on the various methods which include calculations and measurements to estimate the dose to the device as well as the uncertainties in these estimates. 3. Identify detectors that satisfy the requirements for these in-vivo out-of-field dose measurements as well as describe the appropriate correction factors to apply for accurate dose measurements.

SU-E-T-263: Luminescent Dosimetry to Measure the Out-Of-Field Low and High LET Dose Components in High Energy Photon and Proton Therapy Beams.

PURPOSE: Luminescent dosimetry using thermoluminescent detectors (TLDs) and optically stimulated luminescent detectors (OSLDs) were used in mixed radiation fields containing both low LET (photons and protons) and high LET (neutrons)components to obtain their out-of-field absorbed dose, dose equivalent and quality factor. METHOD AND MATERIALS: LiF Thermoluminescent Detectors (TLDs) 600 and 700 chips with dimensions 0.31×0.31×0.038 cm3 were used in a 25.4 cm diameter Bonner sphere centered 42 cm from the isocenter of a 15×x15 cm2 field to measure the secondary doses for 10, 15 and 18 MV photons and a 200 MeV proton therapy beam. From the sensitivity difference to LET radiation between the210 and 280 C peaks in the glow curve, the areas under the peaks were used to obtain the absorbed dose, dose equivalent and QF of the secondary radiation. The OSLD detector measured the low LET dose component to compare with the TLD dose measurement. The neutron calibration of the TLDs was obtained from an Am-Be source at the Argonne National Laboratory. The photon and proton TLD and OSLD calibrations were obtained in 6 MV and 200 MeV beams, respectively. RESULTS: From the two-peak analysis of the TLDs in the Bonner sphere the ratios of the neutron dose to photon dose were 0.001, 0.014 and 0.17 for 10, 15 and 18 MV, respectively. The low LET OSLD measurements agreed within 10% of the TLD results. From the dose equivalent measurements the QFs (+/-14%) obtained were 4.5, 3.9 and 4.0 for these beam energies. For the 200 MeV proton beam the ratio of neutron to proton dose was 0.28 with a measured QF of 13. CONCLUSION: Luminescent detectors in a Bonner Sphere provide measurements of the secondary photon, proton and neutron doses and provide an estimate of the neutron QF.

The dosimetric effects of tissue heterogeneities in intensity-modulated radiation therapy (IMRT) of the head and neck.

The dosimetric effects of bone and air heterogeneities in head and neck IMRT treatments were quantified. An anthropomorphic RANDO phantom was CT-scanned with 16 thermoluminescent dosimeter (TLD) chips placed in and around the target volume. A standard IMRT plan generated with CORVUS was used to irradiate the phantom five times. On average, measured dose was 5.1% higher than calculated dose. Measurements were higher by 7.1% near the heterogeneities and by 2.6% in tissue. The dose difference between measurement and calculation was outside the 95% measurement confidence interval for six TLDs. Using CORVUS' heterogeneity correction algorithm, the average difference between measured and calculated doses decreased by 1.8% near the heterogeneities and by 0.7% in tissue. Furthermore, dose differences lying outside the 95% confidence interval were eliminated for five of the six TLDs. TLD doses recalculated by Pinnacle3's convolution/superposition algorithm were consistently higher than CORVUS doses, a trend that matched our measured results. These results indicate that the dosimetric effects of air cavities are larger than those of bone heterogeneities, thereby leading to a higher delivered dose compared to CORVUS calculations. More sophisticated algorithms such as convolution/superposition or Monte Carlo should be used for accurate tailoring of IMRT dose in head and neck tumours.

Experimental determination of the overall perturbation factor for the NACP chamber in electron beams for dmax < d < or = d80.

In electron beam dosimetry with an ionization chamber, a factor that corrects for the cavity perturbation of the medium, Prepl, and one to account for the disturbance due to the chamber wall material differing from the medium, Pwall, are required. The overall perturbation correction factor, p(q) = PreplPwall, has been introduced because of the difficulty in separately measuring these two components. An advantage of parallel-plate ionization chambers is that p(q) has been shown to be close to unity at dmax. However, many dosimetry applications require knowledge of the overall perturbation factor at depths greater than dmax. We determined p(q) for the NACP chamber at depths beyond dmax by intercomparing percentage depth dose measurements made with it with those obtained with a PTW/diamond detector for which p(q) was taken as unity at all the measurement depths. Data were obtained at depths corresponding to approximately the 90 and 80 per cent of the dose maxima for 20, 16, 12 and 6 MeV incident electrons. The beam energy at depth, Ed, and the percentage depth-dose gradient varied from 1.4 to 14.3 MeV and 0 to 5.8% mm(-1) respectively. Our results show that within the estimated uncertainty of 1.3%, p(q),NACP is unity over the range of energies and dose gradients studied.

Evaluation of the contribution of capture gamma rays, x-ray leakage, and scatter to the photon dose at the maze door for a high energy medical electron accelerator using a Monte Carlo particle transport code.

A Monte Carlo simulation of the photon dose due to scattered x rays, head leakage photons, and capture gamma rays in the maze of an 18 MeV accelerator facility was carried out. The results of the Monte Carlo simulation were compared with dose measurements made in the maze and also with values calculated using an empirical equation. Agreement within +/-26% was found among the three techniques used to evaluate the capture gamma ray dose. It was found that the empirical equation overestimated the scattered x ray plus head leakage photon dose by a factor as large as 2.9 as compared to the other methods. It was concluded that the photon dose, for mazes greater than 3 m in length, is produced predominately by capture gamma rays.

Measured overall perturbation factors at depths greater than dmax for ionization chambers in electron beams.

In electron beam dosimetry the perturbation effect in the medium by the ionization chamber cavity is accounted for by introducing a replacement correction factor, P(repl). Another perturbation correction factor, denoted as P(wall), is due to the materials of the walls of the parallel-plate chamber differing from the phantom material. Because of the difficulties in separating these two components, we measure the overall perturbation factor, p(q) = P(repl)P(wall). A distinct advantage of parallel-plate ionization chambers over cylindrical chambers is that p(q) has been shown to be close to unity at the standard calibration depth, d(max). However, for many dosimetry applications it is necessary to know the overall perturbation factor at depths greater than d(max). We measured the overall perturbation factor at depths greater than d(max) (approximating the 95%, 90% and 50% depth dose) for a Farmer-type cylindrical ionization chamber and three parallel-plate ionization chambers. We assumed that p(q) for the NACP chamber is unity at these measurement depths. The depth dependence for the other chambers was then measured relative to the NACP chamber. The mean energy at depth, E(d), and percentage depth dose gradient ranges studied were 1.9-18.5 MeV and 0 to 4.5%/mm, respectively. For the other two parallel-plate chambers, we find p(q) to be unity at depths where the percent depth dose is greater than 90%, but it deviates from unity at deeper depths, where the dose gradients exceed about 2.5%/mm. For the cylindrical chamber, p(q) values at depths greater than d(max) were found to be in good agreement with those in TG 21, where the energy at depth, E(d), is used to evaluate p(q).

High-energy total body irradiation as preparation for bone marrow transplantation in leukemia patients: treatment technique and related complications.

PURPOSE: Bone marrow transplantation with conditioning regimens that include total-body irradiation (TBI) is widely used in patients with acute lymphoblastic and acute myelocytic leukemias. The major causes of death in this population are relapse of leukemia, infection, and treatment related complications. Our purpose was to achieve a homogenous radiation dose distribution and to minimize the dose to the lungs, liver, and kidneys so that the incidence of organ injury was reduced. METHODS AND MATERIALS: Dose to the bone marrow, midplane, and periphery was quantified by use of thermoluminescent detectors in a bone-equivalent tissue phantom. In an effort to reduce the risk of complications, we treated relapsed or refractory leukemia patients with TBI administered in fractionated, parallel opposed large fields with 24 MV photons, using tissue compensation and partial-transmission lung shielding. Tissue toxicities were then determined. RESULTS: Dose quantitation in bone-equivalent and tissue-equivalent phantoms demonstrated that backscatter and pair production interactions adjacent to bone increased the bone marrow dose by 6 to 11%. At an SSD of 400 cm and at patient diameters of 20 to 40 cm, the percent inhomogeneity across the phantom with 24 MV photons was 0 to 0.3%, compared to 4 to 6% for 6 MV photons. End-organ toxicities consisted of clinical interstitial pneumonitis in six patients, idiopathic interstitial pneumonitis in three patients, renal toxicity in seven patients, and veno-occlusive disease of the liver in one patient. Toxicities did not correlate with fractionation schedule. CONCLUSIONS: Total-body irradiation administered with 24 MV photons increases the dose deposition in bone marrow through pair production and backscatter interactions occurring in bone. Because percent depth dose increases with SSD, the 24 MV beam is more penetrating at a 400 cm distance than at 100 cm and dose homogeneity is improved with higher energies. Thus, the incidence of radiation-mediated injury to lung, liver, and kidney is reduced. This is an effective preparatory regimen for patients with high-risk leukemias requiring bone marrow transplantation.

A new source localization algorithm with no requirement of one-to-one source correspondence between biplane radiographs.

Conventional source localization algorithms require a one-to-one source correspondence between films. This requirement makes source localization cumbersome and error prone because multiple sources must be carefully digitized and some sources can be obscured or missed. A new source localization algorithm is described in this paper. The algorithm fits a ribbon or needle image on film to a linear-quadratic equation, then analytically determines the 3-D ribbon locus by its image on the other projection, and finally localizes the sources in the ribbon by tracing along the ribbon image. Only three points per ribbon per film are required, and corresponding points need not be identified on the other film. Phantom experiments and tests on clinical cases demonstrate that the source localization algorithm can increase the efficiency by a factor of up to 5, improve accuracy to about 1 mm, and reconstruct obscured or shifted sources without decreased accuracy and efficiency. The simplicity and minimal entry of data make this technique desirable for clinical use.

Eye shielding for patients treated with total body irradiation.

The incidence of cataracts in patients who have received total body irradiation (TBI) is about 20% and increases to 40% if the patient is treated for graft-versus-host disease. At our institution, all TBI patients are treated with two lateral opposed 24 MV photon fields. No attempt is usually made to shield the eyes during the TBI treatment because of the amount of lead required to adequately attenuate megavoltage photon beams, the difficulty in properly positioning an eye shield and the possibility of compromising the effectiveness of the treatment. However, we were asked to treat a TBI patient who is a professional pilot, and thus his livelihood is contingent upon maintaining perfect vision. A custom eye shield was constructed out of lead and ionization chamber and film measurements were performed under TBI conditions to determine the thickness and location of the eye block to optimize the competing effects of increased scatter and attenuation from the lead. Phantom data were also obtained for 6 MV irradiation for comparison with 24 MV. In-vivo patient and phantom measurements with thermoluminescent dosimeters showed that with visual positioning of the eye block the dose was reduced from 16 to 20% across the orbits of both eyes.

Patterns of failure following total body irradiation and bone marrow transplantation with or without a radiotherapy boost for advanced neuroblastoma.

PURPOSE: To evaluate the patterns of failure and outcome of patients undergoing high-dose chemotherapy, total body irradiation (TBI), and bone marrow transplantation (BMT) for advanced/relapsed pediatric neuroblastoma, with emphasis on the impact of a radiotherapy boost to primary and metastatic sites. METHODS AND MATERIALS: Between May 1986 and June 1993, 26 patients with advanced neuroblastoma underwent high-dose chemotherapy and TBI followed by BMT at our institution. The majority of patients were over the age of 2 years (73%) and were Stage IV at diagnosis (81%). Multiple metastatic sites were involved including bone (17), bone marrow (15), distant nodes (11), liver (5), lung (4) and brain (1). Twenty patients (77%) received cyclophosphamide (50 mg/kg x 4 days) and TBI as consolidation therapy. TBI was delivered to a total dose of 12 Gy given in 2 Gy twice daily (b.i.d.) fractions over the 3 days preceding bone marrow infusion. A local radiotherapy boost of 8-24 Gy was given to 13 out of 26 patients (50%) to the primary and/or metastatic sites immediately prior to or following induction chemotherapy according to physician judgement. Sites not amenable to a radiotherapy boost included the bone marrow, diffuse/bilateral lung involvement, and multiple bone metastases (> four sites). RESULTS: The actuarial overall survival of the 26 patients was 40.4% at 3 and 5 years, with a progression-free survival at 5 years of 38.5%. Six patients died of transplant-related toxicity (23%). The use of cyclophosphamide as high-dose consolidation chemotherapy was significantly better than other multidrug regimens used in terms of overall survival (p < 0.0001) and progression-free survival (p = 0.0004). The presence of liver involvement prior to BMT was a significant adverse prognostic factor by multivariate analysis. Of the 20 patients surviving the transplant, 10 (50%) underwent a local radiotherapy boost. The patterns of failure were as follows: 3 out of 10 "boost" patients failed overall, none in previous (old) sites of disease only, 1 in new sites only, and 2 in old and new sites; 6 out of 10 "no boost" patients failed overall, 4 in old sites only, none in new sites only, and 2 in old and new sites. There was a trend toward improved 5-year progression-free survival in patients surviving the transplant that received a boost (68% vs. 33%, p = 0.24). A failure analysis was also performed for each of the 59 initially involved sites, of which the majority (64%) were amenable to a radiotherapy boost. Overall, there is a trend toward less failure in sites amenable to a radiotherapy boost that were irradiated (1 out of 10) vs. those not irradiated (6 out of 28). Failure in the liver occurred in three out of four of the patients with liver involvement that did not receive boost radiotherapy, whereas all seven patients with distant nodal involvement were controlled without a boost. Long-term sequelae include learning difficulties (2), cataract formation (1), and hearing loss (2). Sequelae attributable to a radiotherapy boost occurred in only one patient who received whole brain radiotherapy and developed a cataract and learning difficulties. CONCLUSION: We have found an actuarial 5-year survival rate of 40.4% for patients with advanced neuroblastoma treated with BMT, which compares favorably with results of other published series. Disease recurrence following BMT was most common in previous sites of disease. The majority (64%) of these sites were amenable to a radiotherapy boost. An analysis of failure suggests that a low-dose radiotherapy boost improves control of these sites.

A comparison of methods for calibrating parallel-plate chambers/.

All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel-plate ionization chambers, but the method of determining their value of Ngas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at dmax with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60Co for megavoltage chamber calibrations, two alternate procedures based on exposures in-air, or in-phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured Ngas using each of these techniques for six of the most commonly used and commercially-available parallel-plate ionization chambers. The paired cylindrical and parallel-plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for Ngas that are within 1% of each other.

Computer-aided geometric reconstruction of Fletcher-Suit source positions.

A computer-aided method for reconstruction of the source positions of a Fletcher-Suit applicator has been developed. The tandem source positions are determined by digitizing of the tip and two arbitrary points on the shaft from each of two orthogonal simulator films. A colpostat source position is reconstructed by digitizing of a single point on the endcap and three arbitrary points on the cylindrical sidewalls of the colpostat on the films. This computer-aided method considers the true projection geometry and applicator shape and permits localization of the source positions to within a mean error of less than 1 mm. Compared with the conventional method, the new approach (1) is more time efficient because only a few easily identified points are digitized, (2) allows localization when the tandem sources are shifted by a spacer or when colpostat sources are difficult to visualize on the lateral film, and (3) is more accurate than the conventional technique because no manual drawing of source positions on films is involved.

Experimental determination of fluence perturbation factors for five parallel-plate ionization chambers.

The calibration of parallel-plate chambers for absolute dosimetry is an unsettled matter. The medical physics community has not yet agreed on a practical method of obtaining Ngas, although several researchers are working on this problem. If the photon and electron fluence perturbation factors, KwallKcomp, were known for chambers of standard construction with full buildup provision, then an in-air Co-60 calibration could be applied to these, as is done with cylindrical chambers. We have obtained such correction factors for five commercially available chambers based on measurements in air and in homogeneous phantoms relative to matched cylindrical chambers of known dosimetric parameters. For three of the chambers (Markus, Holt and Exradin) we find that KwallKcomp = 1.000 +/- 0.008, in excellent agreement with available results from Monte Carlo calculations. The values for the other two chambers (NACP and Capintec) are different than 1. Our results are compared to recently published values, both calculated and measured.

How does rHuEPO effect D/P creatinine ratios?

Initially starting CAPD patients on EPO was concerning after hearing reports of hemodialysis patients stating that they "may need more dialysis". The rationale given was that with a higher hematocrit the percentage of plasma in whole blood would decrease, leading to an increase of red cell mass. This decreased plasma volume and increased viscosity would lead to a slower blood flow ultimately resulting in less efficient dialysis. Assessing CAPD patients' peritoneal efficiency was the next step. We obtained pre and post-EPO PETs and evaluated. The initial results showed that D/P creatinine ratios were dropping as our Hcts increased, and ultrafiltration results projected an improvement. What remained unanswered was what took place over extended periods of time on EPO therapy. We examined twelve patients over a period of 27 months. Each patient received 4 exchanges per day using 1500 to 2500 volume. PET tests were performed on each patient prestudy, and at months three, six, and 25-27. Initially each patient received EPO 4000 units, 3/week, SQ. EPO easily increased and maintained our patients' hematocrits within 12 weeks after starting the study. D/P creatinine ratios initially dropped but as our study continued there was a return of D/P creatinine ratios to 6% greater than baseline. One report suggests that EPO may have a direct vasoconstricting effects on blood vessels caused by the stimulation of calcium toward the cell. Vasoconstriction of the vessels would lead to a decrease in exchangeable surface area resulting in a decreased D/P creatinine ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

3-D treatment planning and dose delivery verification integrating a variety of state-of-the-art techniques: a case report.

A patient previously treated with radiation for base-of-tongue cancer presented with recurrent disease seven years later. The spinal cord had received tolerance dose. Using state-of-the-art treatment planning techniques, including beam's-eye-view and volumetrics, dose-volume histograms, split field technique, mixed energies, and beam intensity modulation (with a compensator), we achieved uniform dose coverage of the target in 3-D. This was verified in vivo with thermoluminescence dosimeters positioned in the esophagus by means of a nasogastric tube that ran centrally through the target volume. The various techniques applied will be presented with a discussion of the rationale used in each step of plan optimization and verification.

Measurement of the replacement correction factor for parallel-plate chambers in electron fields.

When parallel-plate chambers are used for dosimetry in electron fields, the AAPM dosimetry protocol recommends a value of 1.0 for the replacement correction factor, P(repl),pp,E, until further data become available. Here, P(repl),pp,E for five commercially available parallel-plate chambers was measured as a function of electron energy from a nominal value of 5.5 to 22 MeV by comparison with a cylindrical chamber whose P(repl),cyl,E was obtained from data in the protocol. Since this method is based on the concept of a constant value for Ngas,pp, the energy and modality dependence of Ngap,pp is also investigated for these chambers for Co-60, 4-, 6-, 24-MV photons and for 22-MeV electrons. It is found that for three of the chambers P(repl),pp,E is independent of energy, consistent with unity within one or two standard deviations (s.d.). For the fourth chamber P(repl),pp,E is similarly consistent with one above 10 MeV, but decreases at lower energies, while for the fifth one it shows a systematic drop with decreasing energy.

Dosimetry of Sr-90 ophthalmic applicators.

Sr-90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLD's), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported. Here we describe a technique to obtain the peak dose rate at the applicator surface using LiF TLD's. The technique can be used for the calibration of flat as well as curved surface applicators. Results for two flat and three concave applicators are presented. Our measurement of the surface dose rate for one of the flat applicators is compared with those obtained by four other institutions, each using different dosimetric devices.

Follow-up study to assess line changes with peritonitis.

Most centers change the transfer set each time a patient develops peritonitis. The underlying rationale is that development of intraluminal biofilm may favor the adherence of microorganisms to the transfer set and reinfect the patient. The purpose of this study was to assess the necessity of a line change with each episode of peritonitis. We examined 63 consecutive episodes of peritonitis which occurred over a period of 58 months in 23 of our CAPD patients. Each patient performed 3 to 4 exchanges per day. Two patients used the Abbott sterile connection device, the remainder used a Baxter system. In 33 of these episodes a line change was performed (group A) and in 30 it was not (group B). The follow-up period after each episode of peritonitis was 4 weeks. There were 3 episodes of relapsing peritonitis (same organism within 4 weeks of termination of antibiotic treatment) at 3 weeks after finishing intraperitoneal antibiotic therapy in group A and no episodes of relapsing peritonitis in group B. In conclusion, according to our data it appears that a line change may not always be necessary for the adequate management of peritonitis in CAPD patients.

Output calibration in solid water for high energy photon beams.

The AAPM Protocol recommends the use of water, polystyrene or acrylic media for measuring the output of high energy photon beams. It provides the appropriate restricted mass stopping powers and mass energy absorption coefficients for converting the dose to these media to dose to water. A water-equivalent solid has been developed for dosimetric applications [C. Constantinou, F. Attix, and B. Paliwal, Med. Phys. 9, 436 (1982)]. Calculated values for the restricted mass stopping powers and mass energy absorption coefficients have been published for this material [A. Ho and B. Paliwal, Med. Phys. 13, 403 (1986)]. The accuracy of these calculations was investigated by making output measurements, following the Protocol, with a Farmer type chamber in four materials for Co-60, 4, 6, 10, 18, and 24 MV photon beams. The results show that the scaled dose to water for the different media agree to better than 1%, and the analysis supports the methodology of the Protocol for obtaining the dose to water from the different media.