Dose levels outside radiotherapy beams

Monte Carlo evaluation of out-of-field dose in 18 MV pelvic radiotherapy using a simplified female MIRD phantom

This study was devoted to determining the unwanted dose due to scattered photons to the out-of-field organs and subsequently estimate the risk of secondary cancers in the patients undergoing pelvic radiotherapy. A typical 18 MV Medical Linear Accelerator (Varian Clinac 2100 C/D) was modeled using MCNPX^®code to simulate pelvic radiotherapy with four treatment fields: anterior-posterior, posterior-anterior, right lateral, left lateral. Dose evaluation was performed inside Medical Internal Radiation Dose (MIRD) revised female phantom. The average photon equivalent dose in out-of-field organs is 8.53 mSv Gy^-1, ranging from 0.17 to 72.11 mSv Gy^-1, respectively, for the organs far from the Planning Treatment Volume (Brain) and those close to the treatment field (Colon). Evidence showed that colon with 4.3049% and thyroid with 0.0020% have the highest and lowest risk of secondary cancer, respectively. Accordingly, this study introduced the colon as an organ with a high risk of secondary cancer which should be paid more attention in the follow-up of patients undergoing pelvic radiotherapy. The authors believe that this simple Monte Carlo (MC) model can be also used in other radiotherapy plans and mathematical phantoms with different ages (from childhood to adults) to estimate the out-of-field dose. The extractable information by this simple MC model can be also employed for providing libraries for user-friendly applications (e.g. '.apk') which in turn increase the public knowledge about fatal cancer risk after radiotherapy and subsequently decrease the concerns in this regard among the public.

A review of uncertainties in radiotherapy dose reconstruction and their impacts on dose-response relationships

Proper understanding of the risk of radiation-induced late effects for patients receiving external photon beam radiotherapy requires the determination of reliable dose-response relationships. Although significant efforts have been devoted to improving dose estimates for the study of late effects, the most often questioned explanatory variable is still the dose. In this work, based on a literature review, we provide an in-depth description of the radiotherapy dose reconstruction process for the study of late effects. In particular, we focus on the identification of the main sources of dose uncertainty involved in this process and summarise their impacts on the dose-response relationship for radiotherapy late effects. We provide a number of recommendations for making progress in estimating the uncertainties in current studies of radiotherapy late effects and reducing these uncertainties in future studies.

Fetus absorbed dose evaluation in head and neck radiotherapy procedures of pregnant patients

In this work the head and neck cancer treatment of a pregnant patient was experimentally simulated. A female anthropomorphic Alderson phantom was used and the absorbed dose to the fetus was evaluated protecting the patient's abdomen with a 7cm lead layer and using no abdomen shielding. The target volume dose was 50Gy. The fetus doses evaluated with and without the lead shielding were, respectively, 0.52±0.039 and 0.88±0.052cGy.

Peripheral dose measurements for 6 and 18 MV photon beams on a linear accelerator with multileaf collimator

Peripheral dose (PD) to critical structures outside treatment volume is of clinical importance. The aim of the current study was to estimate PD on a linear accelerator equipped with multileaf collimator (MLC). Dose measurements were carried out using an ionization chamber embedded in a water phantom for 6 and 18 MV photon beams. PD values were acquired for field sizes from 5 x 5 to 20 x 20 cm2 in increments of 5 cm at distances up to 24 cm from the field edge. Dose data were obtained at two collimator orientations where the measurement points are shielded by MLC and jaws. The variation of PD with the source to skin distance (SSD), depth, and lateral displacement of the measurement point was evaluated. To examine the dependence of PD upon the tissue thickness at the entrance point of the beam, scattered dose was measured using thermoluminescent dosemeters placed on three anthropomorphic phantoms simulating 5- and 10-year-old children and an average adult patient. PD from 6 MV photons varied from 0.13% to 6.75% of the central-axis maximum dose depending upon the collimator orientation, extent of irradiated area, and distance from the treatment field. The corresponding dose range from 18 MV x rays was 0.09% to 5.61%. The variation of PD with depth and with lateral displacements up to 80% of the field dimension was very small. The scattered dose from both photon beams increased with the increase of SSD or tissue thickness along beam axis. The presented dosimetric data set allows the estimation of scattered dose outside the primary beam.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

Dose reconstruction for therapeutic and diagnostic radiation exposures: use in epidemiological studies

This paper describes methods developed specifically for reconstructing individual organ- and tissue-absorbed dose of radiation from past exposures from medical treatments and procedures for use in epidemiological studies. These methods have evolved over the past three decades and have been applied to a variety of medical exposures including external-beam radiation therapy and brachytherapy for malignant and benign diseases as well as diagnostic examinations. The methods used for estimating absorbed dose to organs in and outside the defined treatment volume generally require archival data collection, abstraction and review, and phantom measurements to simulate past exposure conditions. Three techniques are used to estimate doses from radiation therapy: (1) calculation in three-dimensional mathematical computer models using an extensive database of out-of-beam doses measured in tissue-equivalent materials, (2) measurement in anthropomorphic phantoms constructed of tissue-equivalent material, and (3) calculation using a three-dimensional treatment-planning computer. For diagnostic exposures, doses are estimated from published data and software based on Monte Carlo techniques. We describe and compare these methods of dose estimation and discuss uncertainties in estimated organ doses and potential for future improvement. Seven epidemiological studies are discussed to illustrate the methods.

Peripheral dose from uniform dynamic multileaf collimation fields: implications for sliding window intensity-modulated radiotherapy

The increase in the number of monitor units in sliding window intensity-modulated radiotherapy, compared with conventional techniques for the same target dose, may lead to an increase in peripheral dose (PD). PD from a linear accelerator was measured for 6 MV X-ray using 0.6 cm3 ionization chamber inserted at 5 cm depth into a 35 cm x 35 cm x 105 cm plastic water phantom. Measurements were made for field sizes of 6 cm x 6 cm, 10 cm x 10 cm and 14 cm x 14 cm, shaped in both static and dynamic multileaf collimation (DMLC) mode, employing strip fields of fixed width 0.5 cm, 1.0 cm, 1.5 cm, and 2.0 cm, respectively. The effect of collimator rotation and depth of measurement on peripheral dose was investigated for 10 cm x 10 cm field. Dynamic fields require 2 to 14 times the number of monitor units than does a static open field for the same dose at the isocentre, depending on strip field width and field size. Peripheral dose resulting from dynamic fields manifests two distinct regions showing a crest and trough within 30 cm from the field edge and a steady exponential fall beyond 30 cm. All dynamic fields were found to deliver a higher PD compared with the corresponding static open fields, being highest for smallest strip field width and largest field size; also, the percentage increase observed was highest at the largest out-of-field distance. For 6 cm x 6 cm field, dynamic fields with 0.5 cm and 2 cm strip field width deliver PDs 8 and 2 times higher than that of the static open field. The corresponding factors for 14 cm x 14 cm field were 15 and 6, respectively. The factors by which PD for DMLC fields increase, relative to jaws-shaped static fields for out-of-field distance beyond 30 cm, are almost the same as the corresponding increases in the number of monitor units. Reductions of 20% and 40% in PD were observed when the measurements were done at a depth of 10 cm and 15 cm, respectively. When the multileaf collimator executes in-plane (collimator 90 degrees) motion, peripheral dose decreases by as much as a factor of 3 compared with cross-plane data. The knowledge of PD from DMLC field is necessary to estimate the increase in whole-body dose and the likelihood of radiation induced secondary malignancy.

Dosimetry for Quantitative Analysis of the Effects of Low-Dose Ionizing Radiation in Radiation Therapy Patients

We have developed and validated a practical approach to identifying the location on the skin surface that will receive a prespecified biopsy dose (ranging down to 1 cGy) in support of in vivo biological dosimetry in humans. This represents a significant technical challenge since the sites lie on the patient's surface outside the radiation fields. The PEREGRINE Monte Carlo simulation system was used to model radiation dose delivery, and TLDs were used for validation on phantoms and for confirmation during patient treatment. In the developmental studies, the Monte Carlo simulations consistently underestimated the dose at the biopsy site by approximately 15% (of the local dose) for a realistic treatment configuration, most likely due to lack of detail in the simulation of the linear accelerator outside the main beam line. Using a single, thickness-independent correction factor for the clinical calculations, the average of 36 measurements for the predicted 1-cGy point was 0.985 cGy (standard deviation: 0.110 cGy) despite patient breathing motion and other real-world challenges. Since the 10-cGy point is situated in the region of high-dose gradient at the edge of the field, patient motion had a greater effect, and the six measured points averaged 5.90 cGy (standard deviation: 1.01 cGy), a difference that is equivalent to approximately a 6-mm shift on the patient's surface.

Genetic effects of radiotherapy for childhood cancer: gonadal dose reconstruction

PURPOSE

To estimate the doses of radiation to organs of interest during treatment of childhood cancer for use in an epidemiologic study of possible heritable diseases, including birth defects, chromosomal abnormalities, cancer, stillbirth, and neonatal and premature death.

METHODS AND MATERIALS

The study population was composed of more than 25,000 patients with cancer in Denmark and the United States who were survivors of childhood cancer and subsequently had nearly 6,500 children of their own. Radiation therapy records were sought for the survivors who parented offspring who had adverse pregnancy outcomes (>300 offspring), and for a sample of all survivors in a case-cohort design. The records were imaged and centrally abstracted. Water phantom measurements were made to estimate doses for a wide range of treatments. Mathematical phantoms were used to apply measured results to estimate doses to ovaries, uterus, testes, and pituitary for patients ranging in age from newborn to 25 years. Gonadal shielding, ovarian pinning (oophoropexy), and field blocking were taken into account.

RESULTS

Testicular radiation doses ranged from <1 to 700 cGy (median, 7 cGy) and ovarian doses from <1 to >2,500 cGy (median, 13 cGy). Ten percent of the records were incomplete, but sufficient data were available for broad characterizations of gonadal dose. More than 49% of the gonadal doses were >10 cGy and 16% were >100 cGy.

CONCLUSIONS

Sufficient radiation therapy data exist as far back as 1943 to enable computation of gonadal doses administered for curative therapy for childhood cancer. The range of gonadal doses is broad, and for many cancer survivors, is high and just below the threshold for infertility. Accordingly, the epidemiologic study has >90% power to detect a 1.3-fold risk of an adverse pregnancy outcome associated with radiation exposure to the gonads. This study should provide important information on the genetic consequences of radiation exposure to humans.

Film dosimetry in the peripheral region using multiple sensitometric curves

We present a method for applying film dosimetry to the peripheral region utilizing multiple sensitometric curves. There are many instances when the dose to the peripheral region outside the field edges is of clinical and/or research interest. Published peripheral dose data may be insufficient if detailed dose modeling is required, and in those cases measurements must be performed. Film dosimetry is an attractive approach for dose measurement in the peripheral region because it integrates dose, overcoming the low-dose-rate problem, and is time efficient, as it acquires an entire plan of data in a single exposure. However, film response increases at energies below approximately 300 keV. As the scattered photon spectrum changes with distance from the field edge, this increased film sensitivity causes changes in the film response along profiles perpendicular to the field edge. A single sensitometric curve is therefore no longer sufficient for accurate conversion of the optical density to dose. Our new method uses multiple sensitometric curves defined at increasing distances from the field edge. To convert an optical density profile, the dose at each point in the profile is defined as a linear combination of the doses calculated using the two sensitometric curves that bracket the point of interest. A single set of sensitometric curves derived at one field size and source-to-surface distance (SSD) can be applied to density profiles for other field sizes and SSDs. We verified our new method by comparison to ion chamber measurements using three different types of film. Agreement with chamber measurements was within 7%, or less than 2 mm in regions of high gradient, over a wide range of field sizes and SSDs.

[Dose outside the treatment field for external irradiation with high-energy X-ray reduction of the dose of the remaining testis in postoperative irradiation of seminoma]

This study aimed to decrease the radiation dose to the disease-free testis in postoperative irradiation for seminoma patients. We consider the factors influencing the peripheral dose (PD) of 10MV X-ray radiotherapy to be the distance between the caudal edge of the irradiation field and the measuring point, the size of the therapeutic irradiation field, the thickness of the lead shield laid above and lateral to the disease-free testis, and the thickness of the lateral absorber.

MATERIALS AND METHODS

We measured the scattering radiation dose coming from the accelerator head and that due to irradiation volume. We measured these doses using a testicular phantom as the non-diseased testis.

RESULTS

Scattering radiation from the accelerator head mainly contributes to PD, whereas the larger the size of the irradiation field the more the scattering radiation from the irradiation volume contributed to PD. PD changed more at the surface of the phantom than at its center. PD at the testicular phantom could be reduced to less than 1% of the therapeutic dose when it was situated more than 5cm distant from the caudal limit of the irradiation field, the lead shield above the testicular phantom was 7.5cm thick, and the lateral lead shield was 2mm thick.

CONCLUSIONS

PD is influenced by many factors. It is necessary to clarify the change in PD at the testicular phantom, and it is important to limit the caudal edge of the irradiation field and to lay the lead shield for the attenuation of radiation on the disease-free testis.

Photon scatter kernels for intensity modulating radiation therapy filters

The most important beam property while optimizing photon therapy is the ability to modulate the intensity of the beam. The use of photon absorbers for intensity modulation of beam profiles requires special attention to be paid to the alteration of beam properties due to scatter and spectral changes, in addition to the desired intensity modulation. In this study the influence of photon scatter in high-density filters irradiated with very narrow photon pencil beams was investigated. A simple analytical relation is developed to quantify the contribution by scattered photons. A scatter kernel was derived by convolving the first Compton scatter distribution with an approximate expression for the second-order scattered photons. The calculations were validated experimentally with film dosimetry and also by using Monte Carlo simulations. Results show that the difference in photon scatter estimation by different methods is relatively small when higher order scattering is accounted for. At 6 MV x-rays the agreement is slightly better than that for 18 MV x-rays results. The simple relation presented in this paper can be used to account for the scattered photon contribution in filter optimization codes to deliver biologically or physically optimized intensity modulated treatments.

Dosimetric features of linac head and phantom scattered radiation outside the clinical photon beam: experimental measurements and comparison with treatment planning system calculations

BACKGROUND AND PURPOSE

Dosimetric measurements and treatment planning system (TPS) calculations in the region outside the clinical photon beams have been investigated. The aim was to estimate the calculation accuracy of a specific TPS in areas that are becoming increasingly relevant with the advent of new technologies, such as, for example, intensity modulation radiation therapy.

MATERIALS AND METHODS

Measurements were performed on two different linacs to obtain, separately, the head scatter (electrons and photons), the transmission below the jaws and the phantom scatter outside the primary beam for different photon energies, distances from the field edge and field sizes. Calculations with a commercial TPS (Helax TMS) were then obtained and compared with these measurements.

RESULTS

In general, reasonable agreement between calculations and measurements was obtained (1-2%), especially for photon scattering (head and phantom). Nevertheless, some discrepancies were found in the electron contamination computation, due probably to the approximations and assumptions made in the TPS calculation algorithm.

CONCLUSIONS

The analyzed TPS presented good results, but for some particular clinical cases and moreover for advanced techniques such as intensity modulated radiation therapy, the calculation behaviour with respect to measurements and patient dose delivery should be carefully evaluated.

Peripheral dose from a linear accelerator equipped with multileaf collimation

In radiation therapy, knowledge of the peripheral dose is important when anatomical structures with very low dose tolerances might be involved. Two of the major sources of peripheral dose, leakage from the linac head, and scatter from secondary collimators, depend strongly on the configuration of the linac head and therefore might be affected by the presence of a multileaf collimator (MLC). In this study, peripheral dose was measured at two depths and two field sizes for 6 and 18 MV photons from a linac with a MLC. The MLC was configured both with leaves fully retracted and with leaves positioned at the field edges defined by the secondary collimator jaws. Comparative measurements were also made for 6 MV photons from a linac without MLC. Peripheral dose was determined as a percentage of the central axis dose for the same energy, field size, and depth using diode detectors in solid phantom material. The data for the 6 MV without MLC agreed with those for the beam with MLC leaves retracted. For both energies at all depths and distances from the field edge, configuring the MLC leaves at the field edge yielded a reduction in peripheral dose of 6%-50% compared to MLC leaves fully retracted.

Thyroid dose in children undergoing prophylactic cranial irradiation

PURPOSE

To determine the radiation dose received by the thyroid gland as a result of prophylactic cranial irradiation (PCI) in childhood leukemia and the factors influencing that dose.

METHODS AND MATERIALS

The dose to the thyroid resulting from simulated cranial irradiation with parallel opposed lateral fields of an adult anthropomorphic (ART) phantom with both 6 MV X-rays and Cobalt-60 gamma-rays was measured using thermoluminescent dosimeters (TLDs). The dependence of thyroid dose on the distance of the field from the thyroid and the proportions of thyroid dose from stray radiation (leakage, scatter from jaws, etc.) and tissue scattered radiation were measured. The effects of a shadow tray and shielding blocks were also determined. Calculation of thyroid dose using the Clarkson scatter integration method was performed for 6 MV X-rays to compare with the measured doses. In vivo thyroid dose estimates were made using TLD measurements for three children receiving PCI with 6 MV X-rays.

RESULTS

Using open, unshielded fields, the thyroid region of the phantom received 1.2-1.4% of the prescribed cranial dose for 6 MV X-rays and 1.5-1.7% for Cobalt-60. For both treatment units, stray radiation accounted for approximately two thirds of the thyroid dose and tissue scatter accounted for the remaining one third. The thyroid dose increased as the field moved closer to the thyroid, with an increasing proportion of the dose due to tissue scatter. Placement of a thyroid shielding block on a shadow tray reduced the thyroid dose by only 20% compared with the open, unshielded setup. Thyroid dose from 6 MV using open fields was affected by the orientation of the collimator. When the inferior field edge was defined by the lower jaw, the dose was reduced by 27% compared with the upper jaw. Good correlation of dose to the thyroid region was obtained between phantom measured doses, in vivo measured doses and calculation of dose using the Clarkson method.

CONCLUSION

For PCI doses of 1800 or 2400 cGy in the adult phantom, the dose to the thyroid was 20-40 cGy (1-2%). For small children this could rise to approximately 5% of the prescribed dose, of which half was due to stray radiation. As the thyroid in children is very sensitive to radiation and the dose-response curve for thyroid tumor induction is linear, attempts to shield the thyroid during cranial irradiation are mandatory. Cobalt-60 units should not be used, as the thyroid dose was higher than using 6 MV X-rays. Collimator orientation and the use of shadow trays and shielding were important factors in determining thyroid dose.

Fetal dose evaluation during breast cancer radiotherapy

PURPOSE

The aim of the work was to estimate the radiation dose delivered to the fetus in a pregnant patient irradiated for breast cancer.

METHODS AND MATERIALS

A 45-year woman was treated for left breast cancer using a 6 MV photon beam with two isocentric opposing tangential unwedged fields. Daily dose was 2.3 Gy at 95% isodose line given by two fields/day, 5 days/week. A total dose of 46 Gy was given in 20 fractions over a 4-week period. Pregnancy confirmed during the second therapeutic week. Treatment lasted between the second and sixth gestation week. Radiation dose to fetus was estimated from in vivo and phantom measurements using thermoluminescence dosimeters and an ionization chamber. In vivo measurements were performed by inserting either a catheter with TL dosimeters or ionization chamber into the patient's rectum. Phantom measurements were performed by simulating the treatment conditions on an anthropomorphic phantom.

RESULTS

TLD measurements (in vivo and phantom) revealed fetal dose to be 0.085% of the tumor dose, corresponding to a cumulative fetal dose of 3.9 cGy for the entire treatment of 46 Gy. Chamber measurements (in vivo and phantom) revealed a fetal dose less than the TLD result: 0.079 and 0.083% of the tumor dose corresponding to cumulative fetal dose of 3.6 cGy and 3.8 cGy for in vivo and phantom measurement, respectively.

CONCLUSIONS

It was concluded that the cumulative dose delivered to the unshielded fetus was 3.9 cGy for a 46 Gy total tumor dose. The estimated fetal dose is low compared to the total tumor dose given due to the early stage of pregnancy, the large distance between fundus-radiation field, and the fact that no wedges and/or lead blocks were used. No deterministic biological effects of radiation on the live-born embryo are expected. The lifetime risk for radiation-induced fatal cancer is higher than the normal incidence, but is considered as inconsequential.

Fetal dose for a patient undergoing mantle field irradiation for Hodgkin's disease

In order to safely treat a 23 weeks pregnant woman for supradiapharagmatic Hodgkin's disease, without compromising the fetus, a custom shielding table was constructed and extensive phantom measurements were performed. For 10 MV photons, the optimal shielding combination was found to consist of a 5 cm thick lead sheet placed onto a 1.25 cm aluminum supporting plate. The structure was placed directly above the phantom, over the region corresponding to the woman's abdomen, without any intervening air gap. By this means the dose to the fetus from machine leakage and collimator scatter was eliminated; the only remaining dose was due to in-phantom scatter. The woman was treated using a mantle field to a dose of 35 Gy in 20 fractions. The accumulated dose to the woman's uterine fundus and to her pubis were monitored with theroluminescent dosimeters. After completion of mantle therapy the doses to the fundus and pubis were 10 and 3 cGy, respectively. The fetal exposure was thus limited to below 10 cGy, within the zone of fetal tolerance. A normal infant was delivered at term.

Influence of modifications in breast irradiation technique on dose outside the treatment volume

PURPOSE

There is increasing interest in potential long-term effects of radiotherapy (RT) in patients treated for breast cancer, particularly those in whom long-term survival can be expected. The purpose of the present study was to determine the effects of treatment techniques, including patient positioning (supine vs. prone) on the absorbed dose in organs at a distance from the treatment volume in breast RT.

METHODS AND MATERIALS

Dose distribution was studied in a Rando-Alderson phantom, modified with a simulated left breast of tissue-equivalent material. RT delivery was studied using 60Co and 6 MV x-ray beams, as well as electrons and a 192Ir source for tumor bed boost RT. Doses were measured in several organs and tissues of interest using LiF thermoluminescent dosimeters. Tangential breast RT was simulated using both supine and prone positioning.

RESULTS

Peripheral doses generally decreased approximately exponentially with distance from the edge of the treatment field. Peripheral doses in various target organs were significantly higher for supine than for prone tangential breast RT (for 50 Gy prescribed dose): 0.50 Gy vs. 0.25 Gy for the upper abdomen, 0.05 Gy vs. 0.02 Gy for pelvic organs, 0.17 Gy vs. 0.08 Gy for active bone marrow, and 0.47 Gy vs. 0.12 Gy for ipsilateral lung (discounting lung in primary beam).

CONCLUSIONS

The present study suggests that peripheral doses in several organs and tissues of interest can be reduced by 40 to 75% by prone tangential breast RT. These results may have implications for future strategies in the treatment of screen-detected breast cancer.

Effects of beam spoiler on radiation dose for head and neck irradiation with 10-MV photon beam

PURPOSE

To determine the effects of a lucite beam spoiler on the dose distribution to points inside and outside the primary beam for head and neck irradiation with a 10-MV photon beam.

METHODS AND MATERIALS

Build-up and depth-dose measurements were performed with a parallel-plate ionization chamber for 5 x 5, 10 x 10, and 15 x 15-cm field sizes using lucite spoilers with two different thicknesses at two different lucite-to-skin distances (LSD) for a 10-MV x-ray beam. Corrections were applied to account for finite chamber size. Beam profiles and isodose curves were obtained at several depths using film dosimetry. Beam uniformity was determined from uniformity indices. Peripheral doses (PD) were measured at the surface and at 1.5- and 2.5-cm depths using film dosimetry and a parallel-plate ionization chamber. Measurement points were positioned at the edge of a 10 x 10-cm field and at distances extending to 5.0 cm away. The treatment planning data for the 10-MV x-ray beam were modified to account for the effects of the beam spoiler when treating head and neck patients.

RESULTS

The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10-MV x-ray beam and a 1.2-cm-thick lucite at 15 cm LSD, a build-up dose similar to a 6-MV x-ray beam was achieved. The beam uniformity was altered at shallow depths. The peripheral dose was enhanced particularly at the surface and at the points close to the beam edge. The effects of the beam spoiler on beam profile and PD were reduced with increasing depths.

CONCLUSION

The lucite spoiler allowed use of a 10-MV x-ray beam for head and neck treatment by yielding a build-up dose similar to that of a 6-MV x-ray beam while maintaining skin sparing. The increase in PD was at superficial depths and was reduced at points away from the edge; therefore, it is clinically nonsignificant. Spoiling the 10-MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treatment volume underdose for regions near the skin surface.

A simple and generally applicable method to estimate the peripheral dose in radiation teletherapy with high energy x-rays or gamma radiation

PURPOSE

Many articles have been published on the measurement of the dose to points outside the primary beam, often called the peripheral dose (PD), for instance, to the gonads, for specific treatment machines and/or techniques. We investigated the possibilities for developing a generalized method based on the data from several publications.

METHODS AND MATERIALS

The data from several publications were recalculated for a reference situation, then averaged, and the frequency distributions around the mean were determined. Published data were available for 60Co, 4, 6, 8, and 10 MV, and 18 to 25 MV for a large variety of treatment machines. Furthermore, an analysis of possible corrections for depth dependence, field elongation, irregularly shaped fields, wedges, and shielding blocks was carried out.

RESULTS

The frequency distributions of all published PD values for square fields for photon energies of 4 MV to 25 MV showed a standard deviation of 33%. The PD values of 60Co are significantly different with a standard deviation of 25%. A difference in the leakage radiation between cobalt machines and linear accelerators can possibly explain this difference, especially for large distances, where leakage radiation predominates. Taking the uncertainty of the risk factors into consideration, we conclude that the use of average values is justified. Although statistically not significant, the peripheral dose appears to be dependent on photon energy with a minimum around 6 MV.

CONCLUSIONS

It is possible to estimate the peripheral dose for photon energies of 4 MV to 25 MV with an accuracy of +/- 33%; for 60Co, the accuracy is even better. The variation of the PD between different treatment machines is so small that it is justified to use average PD values, irrespective of the treatment machine.

Collimator-related radiation dose for different cobalt machines and linear accelerators

PURPOSE

In previous publications (12, 13) measurements are described of the dose outside the primary beam (the peripheral dose (PD)) for 60Co gamma radiation to 25 MV photons. Comparison with data published by other investigators for different treatment machines, showed good agreement. This can only be explained when the contribution to the PD of radiation leaking through and scattering from the collimator does not differ considerably between treatment machines from different manufactures, and it is the purpose of this article to investigate whether this assumption is valid.

METHODS AND MATERIALS

A request was sent out to all radiotherapy departments in The Netherlands and one in Belgium to measure the dose outside the primary beam for as many machines as possible. The following geometry was given: field sizes of 10 x 10 cm2 and 20 x 20 cm2 at distances of 30 and 50 cm, for collimator angles 0 degrees and 90 degrees at the standard source surface distance. This, therefore, resulted in a dataset of eight measurements per photon energy.

RESULTS

Data were collected for four cobalt machines and 37 linear accelerators, from seven different manufacturers. All together 56 datasets were collected for 12 different photon energies. Although the variation of the leakage radiation dose is small, there can be differences of about 50% in the collimator scatter dose between collimator angles of 0 degrees and 90 degrees, depending on the collimator design or on the design of the flattening filter. For dual energy machines with a large gap between the low and the high energy, the values for the high energy are higher by about 40%. Old cobalt machines show higher leakage radiation dose than modern ones.

CONCLUSION

Although there is no large variation in leakage radiation dose between different makes of accelerators, some show higher collimator scatter dose than others. The magnitude of the leakage radiation dose is well within regulatory limits. As the PD not only consists of a contribution from collimator-related radiation, but also of patient scatter, the differences are negligible when estimating the peripheral dose for an individual patient.

Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes

PURPOSE

To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques.

METHODS AND MATERIALS

A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration. In vivo dose measurements on 11 patients undergoing tangential breast radiation therapy with 6 MV photons were performed. Medial and lateral field entrance and exit doses were measured and compared with the expected values from the treatment planning system. Scatter doses to the contralateral breast for three breast setup techniques were measured and documented as a function of distance from the field edge and various beam modifiers commonly used in breast radiation therapy.

RESULTS

The diodes used in this study exhibited excellent linearity, dose reproducibility, and minimal anisotropy. The in-phantom measurements resulted in dose accuracy within +/- 1.5%. Dose measurements on patients resulted in standard deviations of 1.2 and 2.3% for the medial entrance and exit doses and 1.7 and 2.2% for the lateral entrance and exit doses, respectively. In patients, the scatter doses to the opposite breast at a 5 cm perpendicular distance from the medial field edge resulted in cumulative scatter doses of 2.47 to 5.30 Gy from the tangential fields and an additional 0.50 Gy from the supraclavicular or axillary field, if included.

CONCLUSION

Quantitative verification of the prescribed daily dose is important in breast radiation therapy to ensure precision in patient setup and accuracy in dose delivery. Diodes provide a convenient way of real-time patient dose verification and are easy to use by the therapists.

Testicular doses in definitive radiation therapy for localized prostate cancer

PURPOSE

To measure the dose received by the unshielded testes during a conventional course of 18 MV photon radiotherapy for localized prostate cancer and to identify the factors influencing it.

METHODS AND MATERIALS

For each of four patients, a wax block containing thermoluminescent chips was attached to the posterior aspect of the scrotum in close proximity to the testes on each day of treatment during a full course of radical radiotherapy, and dose measurements were obtained. The distances between the thermoluminescent chips and the beam edge were verified by measurement from port films. The accuracy of the in vivo measurements and the factors influencing the testis dose were studied using a phantom arrangement. Six factors were considered: (a) the relative contributions from primary and scattered radiation, (b) the amount of buildup required for the thermoluminescent chips that monitored testis dose, (c) the position of the testes within the scrotum, (d) field size, (e) distance from the field lower border, and (f) the effect of port films.

RESULTS

Median daily doses to the testes in four patients ranged from 5 to 7 cGy. Daily doses for the four patients ranged from 4 to 14 cGy. The total dose to the testes over the full course of therapy ranged from 1.8 to 2.4 Gy. The daily dose depended primarily on the distance from the field lower border. This was increased by approximately 2.5 cGy when a 6 MV port film was taken. The relative contributions from primary and scattered radiation were found to be similar. Dose measurements at the posterior aspect of the scrotum overestimated the testis dose by approximately 15%.

CONCLUSION

The most important factors influencing the dose received by the testis are the distance from the testes to the field lower border and the occasion of a port film. A knowledge of the number of port films and the average distance from the field lower border to the testes allows a reasonable prediction of testes dose without daily measurement.

Calculation and measurement of the dose at points outside the primary beam for photon energies of 6, 10, and 23 MV

PURPOSE

A generalized model is developed to calculate the dose at any point in the body outside the primary beam, using experimentally determined data.

METHODS AND MATERIALS

The contribution of leakage radiation, radiation scattered from the collimator, from the floor, and in the patient was measured for 6, 10, and 23 MV photons.

RESULTS

The radiation scattered in the patient and the radiation scattered from the collimator exhibit a strong dependence on field size and distance and are predominant only at short distances. At larger distances the leakage radiation makes the largest contribution. With appropriate factors, correcting for patient dimensions and field shape, the accuracy is better than +/- 50%.

CONCLUSION

It is possible to calculate the contribution of radiation scattered in the patient at any point of the body outside the primary beam for photons with an energy up to 23 MV. At large distances this contribution increases with energy. Radiation leaking through, and scattering from the collimator has to be measured for each individual treatment machine.

Scattered radiation from linear accelerator and cobalt-60 collimator jaws

PURPOSE

Solid state diodes and/or thermoluminescent dosimeters (TLDs) are often used to measure scattered radiation doses to critical organs immediately adjacent to radiation field sites. The energy-dependent response of these commonly used in vivo dosimeters sometimes makes the interpretation of measured values uncertain. This study investigates scattered radiation arising from the collimator jaws of linear accelerators and the treatment head of a cobalt-60 teletherapy unit.

METHODS AND MATERIALS

A thin window Markus-type parallel-plate ionization chamber placed in a polystyrene phantom was employed to document the magnitude, energy composition, and sources of scattered radiation at surfaces near radiation fields. Measurements were taken both with and without additional phantom material covering the ionization chamber, as well as with various distances between the ionization chamber and edges of the radiation fields tested.

RESULTS

Data was collected, analyzed and compared for treatment units produced by different manufacturers. It was found that the magnitude of scattered radiation to surfaces immediately adjacent to radiation fields ranged from 1% to 15% of the maximum dose along the beam central axis. These values showed a strong dependence upon distance from the edge of the radiation field, beam energy, collimator setting (field size), and the presence of externally mounted accessories. Teletherapy unit differences due to manufacturing firm origins were found to only slightly affect scattered radiation magnitude, while the orientation of upper and lower collimator jaws had absolutely no effect.

CONCLUSIONS

Percent depth dose curves of scattered radiation were obtained and analyzed. The shapes of these depth dose curves suggest the presence of complex energy spectra from secondary electrons and scattered x-rays. Because of the presence of these complex energy spectra in areas immediately adjacent to radiation fields, caution should be observed when interpreting patient doses near radiation fields, if dose values have been measured in vivo using thermoluminescent dosimeters (TLDs) or solid state diodes. Many of these on-patient dosimetry devices are strongly energy dependent and may demonstrate large over- or under-responses in areas dominated by scattered radiation. The results of this study, thus, suggest that ionization chambers are preferred for determination of scattered radiation doses in such regions.

Estimation of peripheral dose from two linacs: Mevatron MX6700 and Mevatron KDS

An empirical calculation method for high-energy beam peripheral dose estimation is described. The peripheral dose has been measured for a Siemens Mevatron MX6700 (6 MV) and a Siemens Mevatron KDS (6 and 18 MV) linear accelerators. The dose distribution is parameterized for each beam energy as a function of depth, distance from the edge of the field, and field size. A simple algorithm has been developed for dose calculation up to 100 cm from the field central axes. Predictions by this algorithm are compared with measurements in an Alderson phantom.

Doses to radiation sensitive organs and structures located outside the radiotherapeutic target volume for four treatment situations

This study documents dosage to radiation sensitive organs/structures located outside the radiotherapeutic target volume for four treatment situations: (a) head and neck, (b) brain (pituitary and temporal lobe), (c) breast and (d) pelvis. Clinically relevant treatment fields were simulated on a tissue-equivalent anthropomorphic phantom and subsequently irradiated with Cobalt-60 gamma rays, 6- and 18-MV x-ray beams. Thermoluminescent dosimeters and diodes were used to measure absorbed dose. The head and neck treatment resulted in significant doses of radiation to the lens and thyroid gland. The total treatment lens dose (300-400 cGy) could be cataractogenic while measured thyroid doses (1000-8000 cGy) have the potential of causing chemical hypothyroidism, thyroid neoplasms, Graves' disease and hyperparathyroidism. Total treatment retinal (400-700cGy) and pituitary (460-1000 cGy) doses are below that considered capable of producing chronic disease. The pituitary treatment studied consisted of various size parallel opposed lateral and vertex fields (4 x 4 through 8 x 8 cm). The lens dose (40-200 cGy) with all field sizes is below those of clinical concern. Parotid doses (130-1200 cGy) and thyroid doses (350-600 cGy) are in a range where temporary xerostomia (parotid) and thyroid neoplasia development are a reasonable possibility. The retinal dose (4000 cGy) from the largest field size (8 x 8 cm2) is in the range where retinopathy has been reported. The left temporal lobe treatment also used parallel opposed lateral and vertex fields (7 x 7 and 10 x 10 cm). Doses to the pituitary gland (5200-6200 cGy), both parotids (200-6900 cGy), left lens (200-300 cGy) and left retina (1700-4500 cGy) are capable of causing significant future clinical problems. Right-sided structures received insignificant doses. Secondary malignancies could result from measured total treatment thyroid doses (670-980 cGy). Analysis of three breast/chest wall and regional nodal irradiation techniques demonstrated a 25-50% decrease in secondary lung dose with use of independent collimation compared to use of custom alloy blocking material. However, it is unlikely that a reduction in secondary dose of this magnitude would reduce the risk of treatment sequellae. In four-field "box" pelvic irradiation, secondary testes dose may result in temporary (clamshell shield) or permanent azoospermia, but is unlikely to impair androgen production.

Sequelae of lateral ovarian transposition in irradiated cervical cancer patients

Lateral ovarian transposition (LOT) is a useful technique for preserving ovarian function in "high-risk" premenopausal Stage I cervix cancer patients who undergo hysterectomy and subsequent postoperative whole pelvic radiation therapy. From 1978 to 1988, 38 FIGO Stage I cervical cancer patients underwent LOT as part of their initial operative procedure and 14 of these patients (37%) subsequently received pelvic radiation therapy (LOT + RT) because of pathological findings such as metastatic pelvic lymph node involvement or positive surgical margins (13 patients) or recurrent disease (1 patient). Ten (71%) of the 14 (LOT + RT) patients have maintained ovarian function with a median follow-up of 35 months. Preservation of ovarian function was directly related to the estimated scatter dose to the ovaries. For patients whose estimated ovarian dose was 300 cGy or less, only 1 of 9 patients (11%) underwent menopause, whereas 3 of 5 patients (60%) became menopausal if the ovarian dose was more than 300 cGy. The placement of the ovaries was also crucial for preservation of ovarian function, with 100% of the patients developing menopause if the ovaries were placed below the iliac crest. A major side effect of LOT was the development of symptomatic ovarian cysts in 7 (18%) of the 38 Stage I patients who underwent LOT. In the 24 patients who underwent LOT alone without RT, the incidence of symptomatic ovarian cysts was 25% compared to only 7% of the patients who underwent LOT + RT, although this difference was not statistically significant (p = .18).

Immunologic changes after loco-regional radiotherapy and fractionated total body irradiation (TBI) in mice

The immunologic effects of fractionated irradiation to both hind limbs and the tail of adult (2.5-3 months old) male Balb/c mice were investigated. A dose of 34 Gy given in 17 fractions of 2 Gy, 1 fraction per day, 5 days per week, was delivered with a 60Co source. A significant decrease of the total splenocyte count (29% of control value) and of the PHA(phytohemagglutinin)-induced proliferation of T cells (22% of control value) was found immediately after irradiation. Both parameters normalized within 30 days after irradiation. Immediately after irradiation, the MLC (mixed lymphocyte culture) was supranormal (126% of control value), dropped to 45% 1 week later, and normalized within 1 month after radiotherapy. The NK (natural killer) activity was significantly decreased only the first week after loco-regional irradiation, while the LAK (lymphokine activated killer) activity was not altered at all. The percentage of goat-anti-mouse+ cells (mainly B lymphocytes) was not changed immediately after loco-regional irradiation, but rose to supranormal values (175% of control level) 3 months after irradiation. A persistent decrease of the percentage and the absolute numbers of the Lyt2+ cells (= CD8+ cells, suppressor/cytotoxic phenotype) was observed up to 3 months after irradiation, while the percentage of L3T4+ cells (= CD4+ cells, helper phenotype) remained normal for the total follow-up. No differences in allogeneic skin graft survival could be demonstrated between irradiated and control animals. The observed immunological effects could not be explained by the scatter irradiation to the whole body as total body irradiation (TBI) administered in a dose and dose rate similar to the scatter dose did not result in persistent immunologic changes. No dose-rate effect could be demonstrated in a low dose fractionated total body irradiation schedule. A total body irradiation similar to the scatter dose in humans did not result in significant immunologic changes.

Comparison of contralateral breast doses from 1/2 beam block and isocentric treatment techniques for patients treated with primary breast irradiation with 60CO

The risk of carcinogenesis in breast tissue subject to low to moderate radiation doses may be of concern to the clinical radiotherapist. With earlier diagnosis, more women, and especially younger women, are electing breast preservation radiation therapy. During therapy, tissue outside the treatment field is exposed to leakage and scattered radiation. Such exposure could lead to significant doses of radiation resulting in carcinogenesis. Therefore, reduction of contralateral breast dose may be an important factor to consider when selecting a treatment technique. This study measured dose in the contralateral breast on 15 patients treated with 60Co gamma rays with the source to skin distance (SSD), 1/2 beam block, and the source to axis distance (SAD), no 1/2 beam block, techniques. Thermoluminescent dosimeters (TLD), with 0.5 cm of superflab used as build-up material, were placed on the contralateral breast to measure dose from the medial tangential beam and from the lateral tangential beam. Dose measurements were done on patients in the treatment position and do not represent phantom or formula calculation of dose to the opposite breast. Our results indicated that total opposite breast dose ranged from 325-650 cGy for SSD treatments as opposed to 200-450 cGy for SAD treatments, in patients receiving a total prescribed dose of 5,040 cGy in 28 fractions to the involved breast. This paper points out that a simple solution for reduction of opposite breast dose for patients treated with 60Co may be utilization of the modified SAD treatment technique.

Adult respiratory distress syndrome after limited thoracic radiotherapy

The authors report three patients who developed fatal respiratory failure after limited or unilateral thoracic radiation therapy for neoplasms. The respiratory failure was characteristic of the adult respiratory distress syndrome (ARDS) with refractory hypoxemia and diffuse bilateral infiltrates including areas of lung well outside of the radiation ports. No patient received drugs known to cause lung injury, and cardiogenic edema and infections were excluded. At autopsy the lungs exhibited interstitial fibrosis, Type II alveolar cell hyperplasia, inflammatory infiltrates, and squamous metaplasia. The etiology of the pulmonary injury is unclear.

Measurements of dose from secondary radiation outside a treatment field: effects of wedges and blocks

Radiation dose outside the radiotherapy treatment field can be significant and therefore is of clinical interest in estimating organ doses. In a previous paper we reported the results of measurements made using unmodified radiation fields. We have extended this study to include the effects of wedge filters and blocks. For a given dose on the central axis of a radiation field, wedges can cause a factor of 2 to 4 increase in dose at any point outside the field compared with the dose when no wedge is used. Adding blocks to a treatment field can cause an increase in dose at points outside the field, but the effect is much smaller than the effect of a wedge, and generally less than a factor of 2. From the results of these measurements, doses to selected organs outside the field for specified treatment geometries were estimated, and the potential for reducing these organ doses by additional shielding was assessed.

Peripheral dose to the testes: the design and clinical use of a practical and effective gonadal shield

A simple and practical gonadal shield has been developed for use near megavoltage radiation fields. The lead shield encloses only the testes, allowing its use with nearly any radiation field that does not include the testes. The dose to the testes with and without the shield has been measured extensively both in phantoms and on patients. The gonadal shield allows a 3 to 10-fold reduction in dose to the testes depending primarily on the distance from the field edge to the gonads. When the shield is used, the gonadal dose is always less than 1% of the patient's prescription dose. Based on our patient studies of testicular injury following conventionally-fractionated irradiation, a dose of less than 50 cGy (1% of a typical 5000 cGy treatment regimen) should preserve normal testicular function.

Dose to the contralateral breast due to primary breast irradiation

The radiation dose received by the contralateral breast during primary breast irradiation is of concern because breast tissue is subject to cancer induction from low to moderate doses of radiation. In this paper the dose to the opposite breast has been studied in detail for common breast treatment techniques. Measurements have been made on 16 patients, a water phantom, a polystyrene phantom with cork inserts to simulate lung tissue, and a body-shaped phantom with wax breasts. Thermoluminescent dosimeters (TLD), ion chambers, diodes, and film have been used in the various configurations. The patient measurements have shown that there is a wide variation in the opposite breast dose received by patients, even when all are treated with, for example, tangential fields alone. Addition of more radiation fields, such as supraclavicular/axillary and internal mammary fields, may increase the dose to the opposite breast for a particular patient. Variations in the details of the technique such as what wedges are used, the use of blocks, and the orientation of the field edges are all important to the final dose received by the patient's contralateral breast. With the phantom measurements, it has been possible to determine the contributions to the opposite breast dose from each of the relevant factors. This makes it possible to explain the wide variation in patient dose measurements, and to make some relatively simple recommendations that will allow the reduction of the dose to the opposite breast from several hundred cGy to about 50 cGy for a typical treatment course dose of 5000 cGy.

The RBE of the leakage radiation from the Hiletron neutron therapy unit

The RBE of the leakage radiation from the Hiletron 14.7 MeV neutron therapy unit has been measured using three sensitive biological systems in mice, which differ markedly in their radiobiological characteristics. These systems comprise type A spermatogonia and bone marrow stem cells, which are affected insignificantly by dose rate, and pigment abnormalities in hair follicles which are affected markedly by dose rate. For mice irradiated at 10 cm depth in a water phantom, the leakage radiation up to 40 cm from the beam axis was virtually as effective as the primary beam for the latter two biological systems, and for spermatogonia in mice when irradiated in air. At this distance, the total dose rate was about 0.2 cGy (rad) per minute (3% of that in the primary beam), and the gamma-ray component was about 70%. This equal effectiveness of the total dose for all three systems was considered fortuitous, and it implied high RBE values for equal effect with the small neutron component at far distances. Considering published data on RBE versus neutron energy, the evidence suggested either a positive interaction of neutron and gamma-ray components in killing bone marrow stem cells when the neutron component was less than 40% of the total dose, or an increased efficiency of neutrons when delivered at very low dose rates. However the components were additive in killing spermatogonia.