Three-dimensional dose distribution of total body irradiation by a dual source total body irradiator.

This study describes the three-dimensional dosimetric characteristics of total body irradiation by our dedicated irradiation unit, which consists of two modified 4-MV linear accelerators mounted opposite each other, providing a field size of 220 cm x 80 cm at the midplane. Our dose calculation algorithm considers the three-dimensional contour of the patient to evaluate the primary and scatter doses. The data base for the calculation includes tissue-to-maximum ratios measured for the large fields. The lung dose correction was calculated using the methods of Batho or ratio of TMR. The accuracy of the calculated dose distributions was verified by measurements with ionization chambers in a humanoid phantom. We also describe and verified a technique to achieve desirable midline lung doses using lead shields. The flexibility and the accuracy of the planning system offers the potential in optimizing the therapeutic ratio for total body treatments.

Treatment planning for stereotactic radiosurgery of intra-cranial lesions.

Stereotactic radiosurgery of intra-cranial lesions is a treatment modality where a well defined target volume receives a high radiation dose in a single treatment. Our technique delivers this dose using a set of non-coplanar arcs and small circular collimators. We use a standard linear accelerator in our treatments, and the adjustable treatment parameters are: isocenter location, gantry arc rotation interval, couch angle, collimator field size, and dose. The treatment planning phase of the treatment determines these parameters such that the target volume is sufficiently irradiated, and dose to surrounding healthy tissue and critical, dose-limiting structures is minimized. The attachment of a BRW localizing frame to the patient's cranium combined with CT imaging (and optionally MRI or angiography) provides the required accuracy for localizing individual structures in the treatment volume. The treatment is fundamentally 3-dimensional and requires a volumetric assessment of the treatment plan. The selection of treatment arcs relies primarily on geometric constraints and the beam's eye view concept to avoid irradiating critical structures. The assessment of a treatment plan involves isodose distributions throughout the volume and integral dose-volume histograms. We present the essential concepts of our treatment planning approach, and illustrate these in three clinical cases.

Clinical patterns of failure following stereotactic interstitial irradiation for malignant gliomas.

The vast majority of patients treated for malignant gliomas with surgery, conventional radiation therapy, and systemic chemotherapy recur within 2 cm of their original disease site as documented by CT scanning. We have analyzed the clinical patterns of failure in patients treated with stereotactic interstitial irradiation (brachytherapy) for malignant gliomas in order to determine if this modality has altered the recurrence pattern in this disease. Between December 1985 and December 1989, 53 patients with malignant glioma were treated with stereotactic interstitial irradiation using temporary high activity iodine-125. Thirty-three patients were treated as part of a primary treatment protocol that included 5940 cGy external beam prior to implantation. Twenty patients were treated at time of recurrence. The median dose of radiation given at implantation was 5040 cGy for the primary lesions and 5450 cGy for the recurrent lesions. Twenty-two patients have suffered relapse as documented by clinical and radiographic studies. The predominant patterns of failure in these 22 patients were in the margins of the implant volume (8) and distant sites (10) within the CNS (distant ipsilateral or contralateral hemisphere, spinal axis) or extraneural. Thus, marginal and distant recurrences accounted for 82% of the relapses in our patients. We conclude stereotactic interstitial irradiation has changed the recurrence pattern in patients with malignant glioma with true local recurrence no longer being the predominant pattern of failure as is seen with conventional therapy.

Stereotactic radiosurgery of the brain using a standard linear accelerator: a study of early and late effects.

Between February 1986 and December 1988, 44 patients were treated with stereotactic radiosurgery using a standard linear accelerator. Twenty one patients were treated for cerebrovascular abnormalities and 23 patients were treated for intracranial tumors. Fifteen of the 23 patients treated for intracranial tumors had received previous radiotherapy. The range of doses given by radiosurgery was 1000-2500 cGy. Nausea and vomiting occurred in seven patients within six hours of treatment. The incidence and symptoms were correlated with the dose of radiation to the vomiting center (area postrema) with the median dose to the postrema in symptomatic patients being 618 cGy compared to a range of less than 5 to 184 cGy in the remaining 36 asymptomatic patients. Temporary alopecia occurred in a single patient who received 400 cGy to the scalp. Alopecia did not occur in the remaining 43 patients who received from less than 5 to 175 cGy. Two patients treated for arteriovenous malformations developed an enhancing lesion on CT scanning (one with cerebral edema) on follow-up CT scanning six and twenty-eight months following radiosurgery. The location of these enhancing lesions corresponded to the volumes treated. In one patient, the enhancing pattern and edema disappeared within 18 months of treatment and no neurological deficits developed. Aphasia occurred in one patient treated for a recurrent glioma two hours following treatment to the left temporal lobe and cleared within 12 h of radiosurgery. One patient with an arteriovenous malformation of the pons developed weakness of the contralateral arm and leg six weeks following treatment and this has slowly resolved over the last 12 months. In conclusion, the complications to date have been self-limited and appear to be directly related to the dose and area of brain treated. Prior radiation therapy has not been associated with increased risk of complication in patients treated with radiosurgery for recurrent tumors to date.

Can simulation measurements be used to predict the irradiated lung volume in the tangential fields in patients treated for breast cancer?

A simple method of estimating the amount of lung irradiated in patients with breast cancer would be of use in minimizing lung complications. To determine whether simple measurements taken at the time of simulation can be used to predict the lung volume in the radiation field, we performed CT scans as part of treatment planning in 40 cases undergoing radiotherapy for breast cancer. Parameters measured from simulator films included: (a) the perpendicular distance from the posterior tangential field edge to the posterior part of the anterior chest wall at the center of the field (CLD); (b) the maximum perpendicular distance from the posterior tangential field edge to the posterior part of the anterior chest wall (MLD); and (c) the length of lung (L) as measured at the posterior tangential field edge on the simulator film. CT scans of the chest were performed with the patient in the treatment position with 1 cm slice intervals, covering lung apex to base. The ipsilateral total lung area and the lung area included within the treatment port were calculated for each CT scan slice, multiplied by the slice thickness, and then integrated over all CT scan slices to give the volumes. The best predictor of the percent of ipsilateral lung volume treated by the tangential fields was the CLD. Employing linear regression analysis, a coefficient of determination r2 = 0.799 was calculated between CLD and percent treated ipsilateral lung volume on CT scan. In comparison, the coefficients for the other parameters were r2 = 0.784 for the MLD, r2 = 0.071 for L, and r2 = 0.690 for CLD x L. A CLD of 1.5 cm predicted that about 6% of the ipsilateral lung would be included in the tangential field, a CLD of 2.5 cm about 16%, and a CLD of 3.5 cm about 26% of the ipsilateral lung, with a mean 90% prediction interval of +/- 7.1% of ipsilateral lung volume. We conclude that the CLD measured at the time of simulation provides a reasonable estimate of the percent of the ipsilateral lung treated by the tangential fields. This information may be of value in evaluating the likelihood of pulmonary complications from such treatment and in minimizing toxicity.

Three-dimensional photon dose distributions with and without lung corrections for tangential breast intact treatments.

The influence of lung volume and photon energy on the 3-dimensional dose distribution for patients treated by intact breast irradiation is not well established. To investigate this issue, we studied the 3-dimensional dose distributions calculated for an 'average' breast phantom for 60Co, 4 MV, 6 MV, and 8 MV photon beams. For the homogeneous breast, areas of high dose ('hot spots') lie along the periphery of the breast near the posterior plane and near the apex of the breast. The highest dose occurs at the inferior margin of the breast tissue, and this may exceed 125% of the target dose for lower photon energies. The magnitude of these 'hot spots' decreases for higher energy photons. When lung correction is included in the dose calculation, the doses to areas at the left and right margin of the lung volume increase. The magnitude of the increase depends on energy and the patient anatomy. For the 'average' breast phantom (lung density 0.31 g/cm3), the correction factors are between 1.03 to 1.06 depending on the energy used. Higher energy is associated with lower correction factors. Both the ratio-of-TMR and the Batho lung correction methods can predict these corrections within a few percent. The range of depths of the 100% isodose from the skin surface, measured along the perpendicular to the tangent of the skin surface, were also energy dependent. The range was 0.1-0.4 cm for 60Co and 0.5-1.4 cm for 8 MV. We conclude that the use of higher energy photons in the range used here provides lower value of the 'hot spots' compared to lower energy photons, but this needs to be balanced against a possible disadvantage in decreased dose delivered to the skin and superficial portion of the breast.

Stereotactic radiosurgery for intracranial arteriovenous malformations using a standard linear accelerator.

We have previously described the development of a technique which utilizes a standard linear accelerator to provide stereotactic, limited field radiation. The radiation is delivered using a modified and carefully calibrated 6 MV linear accelerator. Precise target localization and patient immobilization is achieved using a Brown-Roberts-Wells (BRW) stereotactic head frame which is in place during angiography, CT scanning, and treatment. Seventeen arteriovenous malformations (AVMs) have been treated in 16 patients from February 1986 to July 1988. Single doses of 1500-2500 cGy were delivered using multiple non-coplanar arcs with small, sharp edged x-ray beams to lesions less than 2.7 cm in greatest diameter. The dose distribution from this technique has a very rapid dropoff of dose beyond the target volume. Doses were prescribed at the periphery of the AVMs, typically to the 80-90% isodose line. Eleven of 16 patients have been followed by repeat angiography at least 1 year following treatment. Five of 11 have had complete obliteration of their AVM in 1 year and an additional three patients have achieved complete obliteration by 24 months. There have been no incidences of rebleeding or serious complications in any patient. We conclude that stereotactic radiosurgery using a standard linear accelerator is an effective and safe technique in the treatment of intracranial AVMs and the results compare favorably to the more expensive and elaborate systems that are currently available for stereotactic treatments.

The acute onset of nausea and vomiting following stereotactic radiosurgery: correlation with total dose to area postrema.

From 1986 to 1988, 44 patients have been treated for tumors or vascular lesions with stereotactic radiosurgery using a modified standard linear accelerator. In seven patients, nausea and vomiting occurred within 6 hours after the completion of radiosurgery. One of these patients with nausea and occasional vomiting pretreatment had exacerbation several hours after treatment, in spite of droperidol and prochlorperazine prophylaxis. Nausea and vomiting in the other six patients was self-limited and was completely resolved by 12 hours from onset. None of these six patients suffered from nausea and vomiting before treatment. This was directly correlated with the total dose to the vomiting center in the floor of the fourth ventricle (area postrema). The median dose to the vomiting center in the seven patients was 618 cGy (range 275-1257). The final patient in the series received 1088 cGy to the area postrema after droperidol and dexamethasone prophylaxis without developing nausea or vomiting. In the remaining 36 patients who received from less than 5 to 184 cGy to area postrema, nausea and vomiting did not occur. We recommend that patients treated with large fractions of radiation by radiosurgery in this area be premedicated appropriately.

Radiosurgery for arteriovenous malformations of the brain using a standard linear accelerator: rationale and technique.

We have recently initiated a program for irradiating small, unresectable arteriovenous malformations (AVM's) in the brain. The treatments are delivered using a modified and carefully calibrated 6 MV linac. We are using high, single doses (15 to 25 Gy) with a goal of sclerosing the vessels and preventing hemorrhages. This technique, radiosurgery, is somewhat controversial in the radiotherapy community. Since the treatment is given in a single sitting, rather than in the more conventional pattern of multiple small daily fractions, there is some concern about late radiation damage to the normal brain tissue. However an extensive review of the literature leads us to the conclusion that if a technique is used that keeps the volume irradiated to high dose small, radiosurgery is a safe and efficacious treatment for small (less than 2.5 cm) AVM's. To decrease the risk of necrosis of normal brain tissue, it is important to confine the high dose region as tightly as possible to the target volume. Precise target localization and patient immobilization is achieved using a stereotactic head frame which is used during angiography, CT scanning, and during the radiation treatment. This minimizes the margin of safety that must be added to the target volume for errors in localization and set-up. The treatment is delivered using multiple noncoplanar arcs, with small, sharp edged X ray beams, and with the center of the AVM at isocenter. This produces a rapid dropoff of dose beyond the target volume. Early results in our first few patients are encouraging.

The three-dimensional localization of internal mammary lymph nodes by radionuclide lymphoscintigraphy.

In breast cancer patients, radiation therapy planning must account for individual anatomy to ensure optimal coverage of tumor and internal mammary nodes. To achieve this, three-dimensional radionuclide lymphoscintigraphy (RNLS) was performed in 167 patients by obtaining two images of the nodes using a 30-degree slant hole collimator rotated 180 degrees between images. Analysis of 768 nodes (mean 4.6/patient) visualized from the level of rib 1 through interspace 5 was performed. The number of nodes seen was not a function of patient age. Cross-communication to the contralateral node chain occurred in 13.8% of cases. Eighty-two percent of nodes were located near the first three ribs and interspaces; 23% were located beyond 3.0 cm from the mid-sternal line. At the level of the radiation beam match line (second rib or interspace), 4.5% of nodes were deeper than 3.0 cm. From rib 3 through interspace 5, 3.9% were deeper than 3.0 cm. Using an idealized tangential field, at least one node would have been missed in 16.2% of patients. Three-dimensional RNLS allows definition and localization of normal sized nodes and ensures that radiation therapy portals can be tailored for each individual under treatment.

Three-dimensional internal mammary lymphoscintigraphy: implications for radiation therapy treatment planning for breast carcinoma.

Conservative surgery combined with radiation therapy for the treatment of early breast carcinoma has been shown to achieve both a high rate of local tumor control and good cosmetic results with a minimum of complications. Whether the internal mammary lymph nodes (IMNs) should be included in the treatment volume is a topic of considerable controversy. Radionuclide internal mammary node lymphoscintigraphy (IMN-LS) can locate these nodes in three dimensions. We have analyzed the results of IMN-LS in 167 patients imaged at the Dana-Farber Cancer Institute and treated at the Joint Center for Radiation Therapy between 1977 and 1980. The location of the IMNs was found variable from patient to patient. At least one IMN was not included within tangential fields arbitrarily arranged to have a medial entrance point 3.0 cm across the midline in 17% of evaluable patients. However, 48% and 66% of patients had IMNs that could be adequately treated with fields positioned only 1.0 cm or 2.0 cm across midline, respectively. We conclude that when treatment of the IMNs is warranted, IMN-LS not only assures their complete coverage in the majority of patients but also may help reduce the amount of heart and lung irradiated.

Scatter dose for wedged fields.

The behaviour of scatter dose in 4 and 8 MV wedged x-ray beams has been studied by calculating scatter-to-primary dose ratios (SPR) and comparing these with SPR for non-wedged beams. On the central axis the SPR for wedged and non-wedged beams differ only by a few per cent, a difference which increases slightly with wedge angle and field size. In other points within the field the differences are larger but generally less than 3% of the total dose on the central axis at the same depth. The product rule for points that do not lie in a principal plane is valid within the same limits as for non-wedged beams.

Measurements of dose distributions in small beams of 6 MV x-rays.

Dose distributions produced by small circular beams of 6 MV x-rays have been measured using ionisation chambers of small active volume. Specific quantities measured include tissue maximum ratios (TMR), total scatter correction factors (St), collimator scatter correction factors (Sc) and off-axis ratios (OAR). Field sizes ranged from 12.5 to 30 mm diameter, and were defined by machined auxiliary collimators with the movable jaws set for a 4 cm x 4 cm field size. Due to the lack of complete lateral electronic equilibrium for these small fields, the accuracy of the measurements was also investigated. This was accomplished by studying dose response as a function of detector size. Uncertainties of 2.5% were observed for the central axis dose in the 12.5 mm field when measuring with an ionisation chamber with a diameter of 3.5 mm. The total scatter correction factor exhibits a strong field size dependence for fields below 20 mm diameter, while the collimator scatter correction factor is constant and is defined by the setting of the movable jaws. Off-axis ratio measurements show larger dose gradients at the beam edges than those achieved with conventional collimator systems. Corrected profiles measured with an ionisation chamber are compared with measurements made with photographic film and LiF thermoluminescent dosemeters.

Stereotaxic localization of intracranial targets.

We report on a useful clinical method for precisely locating intracranial targets. Utilizing the BRW system, the technique is currently used in stereotaxic irradiation of arteriovenous malformations. An intracranial localizer box, with four radio-opaque markers on each face, surrounds the patient's head and is attached to the BRW Head Ring. Two localization films are required. One film includes the target and the eight anterior and posterior markers, whereas the other film includes the target and the eight right and left markers. There are no constraints that the films be orthogonal or parallel to the box faces, only that the target and radio-opaque markers appear on the films. In addition, knowledge of the source-image and source-target distances are not required. Analysis of the projected target and radio-opaque markers gives both the target location and magnification. Simulation with the BRW Phantom Base demonstrates that point targets can be located with respect to the BRW system to within 0.3 mm and magnification determined to within 0.5%.

A multiplicative correction factor for tissue heterogeneities.

A new multiplicative correction factor for tissue heterogeneities based on an exact expression for homogeneous non-unit-density media is proposed. O'Connor's density scaling theorem is invoked to evaluate the medium-specific tissue-maximum ratios used in the exact formulation. Monte Carlo calculations were performed at energies ranging from 60Co to 15 MV to test the technique for single- and multiple-slab geometries as well as for the clinical benchmark problem described recently by Orton et al consisting of a six-field lung plan. Within the level of statistical uncertainty (less than 2%) in the Monte Carlo calculations, the new correction factor agrees with the calculated data for single slabs of non-unit-density material. For the multiply layered geometries, the correction factor accurately represents the data beyond the build-up region at any interface, but is less reliable close to a boundary. For high energies, however, it is more accurate in the build-up region than other commonly used correction techniques such as the ratio-of-TMR or Batho methods. For all energies considered, the new correction factor agrees with the data measured by Orton et al to within 1.5%. It is more accurate than other correction factors considered by Orton et al at high energies, and is competitive with the Batho and equivalent TAR methods at low energies.

Scatter dose summation for irregular fields: speed and accuracy study.

Using program IRREG as a standard, we have compared speed and accuracy of several algorithms that calculate the scatter dose in an irregular field. All the algorithms, in some manner, decompose the irregular field into component triangles and obtain the scatter dose as the sum of the contributions from those triangles. Two of the algorithms replace each such component triangle with a sector of a certain "effective radius": in one case the average radius of the triangle, in the other the radius of the sector having the same area as the component triangle. A third algorithm decomposes each triangle further into two right triangles and utilizes the precalculated "equivalent radius" of each, to find the scatter contribution. For points near the center of a square field, all the methods compare favorably in accuracy to program IRREG, with less than a 1% error in total dose and with approximately a factor of 3-5 savings in computation time. Even for extreme rectangular fields (2 cm X 30 cm), the methods using the average radius and the equivalent right triangles agree to within 2% in total dose and approximately a factor of 3-4 savings in computation time.

Progress in 3-D treatment planning for photon beam therapy.

The purpose of this report is to study the feasibility of improving dose distributions using non-coplanar photon beams from a linear accelerator. Non-coplanar beams may enter the patient in any arbitrary configuration. This type of treatment technique requires a three-dimensional (3-D) planning system. Clinical examples are used to illustrate the general problems in 3-D treatment planning, and the potential improvement over coplanar beam treatments. Features of a treatment planning system for 3-D planning are discussed.

Effective wedge angles with a universal wedge.

Some recently designed x-ray-producing accelerators are equipped with a single built-in wedge, and different 'effective' wedge angles are obtained by combining an open (unwedged) and a wedged field in the appropriate proportions. This paper describes a technique for determining these proportions from measured isodose distributions of the two component fields. Our data for the Philips SL/75 6 MV accelerator are compared with two existing theoretical models. One model, in which the beams are weighted by the ratio of the tangents of the effective and nominal wedge angles, agrees with the data to within 3 degrees over the range of effective wedge angles and square field sizes examined. The second and simpler model, in which the beams are weighted by the ratio of the wedge angles directly, results in errors of as much as 11 degrees. It is shown that both of these models are approximations to an exact theoretical solution which may be formulated in terms of one free parameter. This parameter may be interpreted physically as the ratio of the slopes of the central-axis depth-dose curves for the open and wedged fields.