Michael D.C. Evans

Accurate skin dose measurements using radiochromic film in clinical applications

Megavoltage x-ray beams exhibit the well-known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. In addition to films, we also used an Attix parallel-plate chamber and a home-built extrapolation chamber to cover tissue-equivalent depths in the range from 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR-T model GAFCHROMIC films. The correction factors for the exit skin dose due to the build-down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.

Spectroscopic characterization of a novel electronic brachytherapy system

The Axxent developed by Xoft Inc. is a novel electronic brachytherapy system capable of generating x-rays up to 50 keV. These low energy photon-emitting sources merit attention not only because of their ability to vary the dosimetric properties of the radiation, but also because of the radiobiological effects of low energy x-rays. The objective of this study is to characterize the x-ray source and to model it using the Geant4 Monte Carlo code. Spectral and attenuation curve measurements are performed at various peak voltages and angles and the source is characterized in terms of spectrum and half-value layers (HVLs). Also, the effects of source variation and source aging are quantified. Bremsstrahlung splitting, phase-space scoring and particle-tagging features are implemented in the Geant4 code, which is bench-marked against BEAMnrc simulations. HVLs from spectral measurements, attenuation curve measurements and Geant4 simulations mostly agree within uncertainty. However, there are discrepancies between measurements and simulations for photons emitted on the source transverse plane (90 degrees).

The prediction of late rectal complications in patients treated with high dose-rate brachytherapy for carcinoma of the cervix

PURPOSE The aim of this work is to investigate an unusually high rate of late rectal complications in a group of 43 patients treated with concomitant irradiation and chemotherapy for carcinoma of the cervix between December 1988 and April 1991, with a view to identifying predictive factors. METHODS AND MATERIALS The biologically effective dose received by each patient to the rectal reference point defined by the International Commission of Radiation Units and Measurements, Report 38, were calculated. Radiotherapy consisted of 46 Gy external beam irradiation plus three high dose-rate intracavitary treatments of 10 Gy each prescribed to point A. Cisplatin 30 mg/m2 was given weekly throughout the duration of the irradiation. The results have been compared to data from 119 patients treated with irradiation alone to assess the confounding effect of the cisplatin. RESULTS The relationship between the biologically effective dose delivered to the rectal reference point and the development of late complications shows a strong dose-response with a threshold for complications occurring at approximately 125 Gy3 corresponding to a brachytherapy dose of approximately 8 Gy per fraction. This value is approximately the same biologically effective dose threshold as that found for external beam irradiation in the head and neck region. The data from the group of patients treated without cisplatin is comparable to the data from the first group of patients in the lower dose ranges; the higher doses were not used and thus are not available for comparison. CONCLUSION Using the linear quadratic model applied to our clinical results, we have established a threshold for late rectal complications for patients treated with external beam irradiation and high dose-rate brachytherapy for carcinoma of the cervix. This threshold is consistent with similar data for external beam irradiation in the head and neck region.

Rectal complications in patients with carcinoma of the cervix treated with concomitant cisplatin and external beam irradiation with high dose rate brachytherapy: A dosimetric analysis

PURPOSE This paper reports a dosimetric study of 43 patients treated with a combination of concomitant radiotherapy and chemotherapy (cisplatin) for locally advanced carcinoma of the cervix with the aim of investigating the correlation between the radiation dose to the rectum and the incidence of late rectal complications. METHODS AND MATERIALS Radiotherapy consisted of 46 Gy external beam irradiation plus three high dose rate intracavitary treatments given weekly, concurrent with the last 3 weeks of external beam therapy, to a total dose of 30 Gy to point A. Cisplatin 30 mg/m2 was given weekly throughout the duration of the external beam irradiation. The brachytherapy irradiated volume was reconstructed from the orthogonal treatment radiographs to accurately locate the reference points defined by the International Commission of Radiation Units and Measurements (ICRU), report 38. The doses calculated at these points were compared to in vivo dose measurements performed immediately prior to treatment. RESULTS The group of patients who were calculated to have received a dose to the rectal reference point greater than the prescribed point A dose (9/13) had a significantly greater probability of development of late rectal complications compared to the group of patients who were calculated to have received less than the prescribed point A dose at this rectal point (7/30), p = 0.003. There was no correlation of rate of complication with the in vivo measured dose in the rectum, stage of disease, or age. At 40 months post treatment, the group of patients receiving the higher dose to the rectal reference point had an actuarial rate of serious (Grade 3 and 4) rectal complications of 46% compared to a rate of 14% in the remainder. In terms of survival, the group of patients receiving the higher dose to the rectal reference point have all survived, whereas the group of patients receiving the lower dose to the rectal reference point have a significantly different rate of survival of 72%, p = 0.046. CONCLUSION This investigation has revealed a significant correlation between the dose calculated at the rectal point defined by the ICRU and the incidence of late rectal complications in patients with carcinoma of the cervix undergoing concomitant radiotherapy and chemotherapy. Thus, this rectal reference point appears to be a useful prognostic indicator of late rectal complications in these patients and we recommend that the brachytherapy dose delivered to this rectal point be limited to the dose prescribed to point A for treatment regimens using three fractions of 8-10 Gy each, limiting the total dose to this point, including the external beam component, to 76 Gy. Further study will be required to determine whether this rule should be applied to patients receiving irradiation alone.

High dose rate afterloading intracavitary therapy in carcinoma of the cervix

From January 1984 through December 1986, 87 patients with previously untreated carcinoma of the cervix received external beam pelvic irradiation and high dose rate intracavitary therapy (HDRT). There were 18 Stage IIA patients, 39 Stage IIB, and 30 Stage IIIB. The median age was 60 years and the median follow-up time was 42 months for patients at risk. Radiotherapy consisted of external megavoltage irradiation to the whole pelvis (median dose 4600 cGy) combined with one (6 patients), two (51 patients), or three (30 patients) HDRT insertions. A high dose rate remote afterloading unit with 60Co sources was used to deliver the HDRT. The prescribed dose to point A was between 800 and 1000 cGy per treatment. The dose rate at point A initially was approximately 150 cGy/min and dropped to approximately 100 cGy/min during the duration of the study. Treatments with multiple fractions were given at weekly intervals. The overall actuarial survival at 5 years was 88% for Stage IIA, 64% for Stage IIB and 32% for Stage IIIB patients. Pelvic recurrence remained the major cause of failure. Grade III and IV late complications included proctitis and bowel obstruction in six patients each. We conclude that HDRT results are similar to those obtained with conventional low dose rate intracavitary systems. HDRT is cost effective and minimizes exposure to personnel. Several questions, such as the total number of insertions required, dose per HDRT insertion, and optimal HDRT insertion schedule remain unanswered and further experience is needed to better clarify these issues.

Surface applicators for high dose rate brachytherapy in aids-related Kaposi's sarcoma

PURPOSE The development of commercially available surface applicators using high dose rate remote afterloading devices has enabled radiotherapy centers to treat selected superficial lesions using a remote afterloading brachytherapy unit. The dosimetric parameters of these applicators, the clinical implementation of this technique, and a review of the initial patient treatment regimes are presented. METHODS AND MATERIALS A set of six fixed-diameter (1, 2, and 3 cm), tungsten/steel surface applicators is available for use with a single stepping-source (Ir-192, 370 GBq) high dose rate afterloader. The source can be positioned either in a parallel or perpendicular orientation to the treatment plane at the center of a conical aperture that sits at an SSD of approximately 15 mm and is used with a 1-mm thick removable plastic cap. The surface dose rates, percent depth dose, and off-axis ratios were measured. A custom-built, ceiling-mounted immobilization device secures the applicator on the surface of the patients lesion during treatment. RESULTS Between November 1994, and September 1996, 16 AIDS-related Kaposis sarcoma patients having a total of 120 lesions have been treated with palliative intent. Treatment sites were distributed between the head and neck, extremity, and torso. Doses ranged from 8 to 20 Gy, with a median dose of 10 Gy delivered in a single fraction. Treatments were well tolerated with minimal skin reaction, except for patients with lesions treated to 20 Gy who developed moderate/severe desquamation. CONCLUSION Radiotherapy centers equipped with a high dose rate remote afterloading unit may treat small selected surface lesions with commercially available surface applicators. These surface applicators must be used with a protective cap to eliminate electron contamination. The optimal surface dose appears to be either 10 or 15 Gy depending upon the height of the lesion.

Absorbed dose to water reference dosimetry using solid phantoms in the context of absorbed-dose protocols.

For reasons of phantom material reproducibility, the absorbed dose protocols of the American Association of Physicists in Medicine (AAPM) (TG-51) and the International Atomic Energy Agency (IAEA) (TRS-398) have made the use of liquid water as a phantom material for reference dosimetry mandatory. In this work we provide a formal framework for the measurement of absorbed dose to water using ionization chambers calibrated in terms of absorbed dose to water but irradiated in solid phantoms. Such a framework is useful when there is a desire to put dose measurements using solid phantoms on an absolute basis. Putting solid phantom measurements on an absolute basis has distinct advantages in verification measurements and quality assurance. We introduce a phantom dose conversion factor that converts a measurement made in a solid phantom and analyzed using an absorbed dose calibration protocol into absorbed dose to water under reference conditions. We provide techniques to measure and calculate the dose transfer from solid phantom to water. For an Exradin A12 ionization chamber, we measured and calculated the phantom dose conversion factor for six Solid Water phantoms and for a single Lucite phantom for photon energies between 60Co and 18 MV photons. For Solid Water of certified grade, the difference between measured and calculated factors varied between 0.0% and 0.7% with the average dose conversion factor being low by 0.4% compared with the calculation whereas for Lucite, the agreement was within 0.2% for the one phantom examined. The composition of commercial plastic phantoms and their homogeneity may not always be reproducible and consistent with assumed composition. By comparing measured and calculated phantom conversion factors, our work provides methods to verify the consistency of a given plastic for the purpose of clinical reference dosimetry.

Treatment results of high dose rate brachytherapy in patients with carcinoma of the cervix.

PURPOSE The combination of external beam irradiation and low-dose-rate brachytherapy is known to be an effective form of treatment in carcinoma of the cervix and any change from this well-established therapeutic combination must be able to equal or improve the treatment results. Since 1984 we have been using high dose rate brachytherapy in conjunction with external beam irradiation for patients with carcinoma of the cervix. This paper reports our long term treatment results in terms of local disease control, survival, and complications. METHODS AND MATERIALS Between January 1984 and December 1989, 187 previously untreated patients with carcinoma of the cervix underwent combined external beam irradiation and high dose rate brachytherapy. The International Federation of Gynecology and Obstetrics stage distribution of patients was as follows: I B = 15, II A = 35, II B = 68, III A = 9, III B = 54, IV A = 6. External beam irradiation to the whole pelvis was delivered by megavoltage irradiation with once-a-day fractionation, to a median dose of 4600 cGy. High dose rate brachytherapy was delivered by a high-dose-rate remote controlled afterloading unit, containing 20 spherical Cobalt 60 sources with a nominal activity of 19 GBq (0.5 Ci) at the time of installation, giving a typical dose rate to point A of 160 cGy/min, decreasing to about 80 cGy/min at the end of the 5-year study. One to 3 high dose rate brachytherapy treatments delivering 800 to 1000 cGy to point A were given weekly concurrently with the last 2 to 3 weeks of radiation therapy, or following its completion. Maximum rectal and bladder doses were routinely measured for each treatment. RESULTS Overall 5-year actuarial survivals were as follows: I B = 72%, II A = 65%, II B = 66%, III A = 66%, III B = 45%. Five-year actuarial pelvic control rates were as follows: I B = 66%, II A = 83%, II B = 78%, III A = 88%, III B = 40%. At a median follow-up time of 54 months for patients at risk, 23 patients developed 25 complications attributable to radiotherapy (13 rectal, 3 bladder, 8 small bowel, 1 fistula) at a median time of 18 months following completion of treatment. Thirteen complications (7.6%) were grades 3 or 4. Patients with Stage II disease had a higher incidence of complications than patients with Stages I and III disease (p < 0.05). Rectal complications were significantly higher in patients who received a total rectal dose > 5400 cGy (p = 0.045). CONCLUSION High-dose-rate brachytherapy treatment results are comparable to those obtained with low dose rate brachytherapy techniques. The use of three high dose rate brachytherapy insertions is a practical, economical, and safe treatment for patients with carcinoma of the cervix.

A Protocol of Dual Prophylaxis for Venous Thromboembolism Prevention in Gynecologic Cancer Patients

OBJECTIVE: To evaluate a quality improvement protocol for venous thromboembolism prevention in postoperative gynecologic cancer patients. METHODS: On January 1, 2006, we initiated a universal protocol of dual prophylaxis with sequential compression devices and three times daily heparin (or daily low molecular weight heparin) until discharge in gynecologic cancer patients having major surgery. Patients with both malignancy and age over 60 years (or history of prior clot) were discharged on 2 weeks of anticoagulant. Before January 2006, all patients were given sequential compression devices starting before the induction of anesthesia, continuing until discharge from the hospital. Records of gynecologic cancer service patients admitted in 2005 and 2006 were reviewed, excluding patients with a history of heparin-induced thrombocytopenia or those admitted on an anticoagulant. Any pulmonary embolism or deep vein thrombosis diagnosed within 6 weeks of surgery was identified. We performed &khgr;2 and Wilcoxon rank sum tests as well as multivariable regression analysis for confounders. RESULTS: Six of the 311 women meeting inclusion criteria in 2006 (1.9%) and 19 of 294 (6.5%) in 2005 had venous thromboembolism (odds ratio 0.33, 95% confidence interval 0.12–0.88, multivariable analysis adjusting for baseline differences between the groups). Heparin was given to 98.1% of patients in the hospital in 2006, and 91.1% of those meeting high-risk criteria were discharged on an anticoagulant. No differences in major bleeding complications were seen between years. CONCLUSION: A protocol of dual prophylaxis with prolonged prophylaxis in high-risk patients was successfully implemented and was associated with a significant reduction in the rate of venous thromboembolism without increasing bleeding complications. LEVEL OF EVIDENCE: II

The influence of phantom size on output, peak scatter factor, and percentage depth dose in large‐field photon irradiation

Machine outputs, peak scatter factors, and central axis percentage depth dose distributions were measured for various phantom sizes in large radiation fields produced at extended distances by cobalt, 6-MV, and 10-MV photon beams. The results can be applied to practical total body irradiation procedures which usually involve treatment volumes smaller than the actual field sizes in order to provide a uniform total body exposure to radiation. Our study addresses the question of the appropriate phantom dimension to be used in the calibration of photon beams employed in total body irradiations. The measurements show that the machine outputs are only slightly dependent on phantom size; the percentage depth dose distributions, however, are strongly dependent on the phantom size, suggesting that machine data for total body irradiations should be measured in phantoms whose dimensions approximate the patient during the total body irradiation. Peak scatter factors measured in large-field/small-phantom configurations link up well with the published small-field/large-phantom data. The finite patient thickness lowers the dose to points close to the beam exit surface by a few percent, when compared to dose measured at the same depths in infinitely thick phantoms. The surface doses in large radiation fields are essentially independent of phantom cross sections and range from 40% for the 10-MV beam, to 65% for the 6-MV beam and 80% for the cobalt beam.