Göran K. Svensson

Analysis of cosmetic results following primary radiation therapy for stages I and II carcinoma of the breast.

Abstract In 31 cases of Stages I or II carcinoma of the breast treated by primary radiation therapy, the cosmetic results were analyzed with regard to the details of treatment. Three principal treatment factors were identified which influenced the cosmetic outcome: (1) the extent and location of the biopsy procedure, (2) the time/dose factors of the radiation therapy and (3) the technique of the radiation therapy. Cosmetic results were lessened when the biopsy procedure included a wide resection of adjacent breast tissue or when the biopsy scar was obvious. Increasing doses of external beam radiation were associated with greater degrees of retraction and fibrosis of the treated breast. All 6 patients who received 6000 rad by external beam had significant retraction and fibrosis while patients who received 5000 rad rarely showed significant changes. Local boost doses by interstitial implantation did not diminish the cosmetic outcome. All patients were treated using supervoltage equipment without bolus and skin changes secondary to treatment were infrequent. Seventeen patients developed localized areas of fibrosis and skin changes at the matchline between adjacent radiation fields. Recommendations are made for improved cosmetic results based on these findings.

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.

Measurements of dose from secondary radiation outside a treatment field

Radiation dose to organs outside the radiotherapy treatment field can be significant and therefore is of clinical interest. We have made measurements of dose at distances up to 70 cm from the central axes of 5 X 5, 15 X 15 and 25 X 25 cm radiation fields of 300 kVp, 4 MV and 8 MV X rays, and 60Co gamma rays, at the surface and at depths in water of 5 and 10 cm. Contributions to the total secondary radiation dose from water scatter, machine (collimator) scatter and leakage radiation have been separated. We have found that the component of dose from water scatter can be described by a simple exponential function of distance from the central axis of the radiation field for all energies and field sizes. Machine scatter contributes 20 to 40% of the total secondary dose depending on machine, field size and distance from the field. Leakage radiation contributes very little dose, but becomes the dominant component at distances beyond 60 cm from the central axis. Estimates of the risk of second tumors in long term survivors indicate a small incremental increase above the natural incidence rate based on information from the 1980 BEIR Committee report.

Three-field technique for breast irradiation using tangential field corner blocks

A further modification of the three-field technique for the radiotherapy of the breast has been developed. Two isocentric opposing tangential fields encompass the breast, chest wall, and may include the internal mammary lymph nodes. The third, an anterior field, encompasses the axilla and supraclavicular areas. As with our previously reported techniques, the objectives of the present modification is to make the posterior edges of the tangential fields coplanar and to match the cephalad geometric edges of the tangential fields to the caudad geometric edge of the supraclavicular field. A half-beam block is used to shield the caudad half of the anterior field, thus producing a vertical transverse plane to which the tangential fields are matched. Small corner blocks are used on the cephalad edges of the tangential fields to produce the vertical edge necessary for matching to the anterior field. It is essential that the match between the tangential fields and the anterior field be geometrically correct to ensure both local control of disease and good cosmetic results. Two advantages of the present technique are the ease with which it can be carried out and the precision of the match plane without the use of cumbersome mechanical accessories.

A modified three-field technique for breast treatment

Radiation therapy of breast cancer commonly uses three high-energy photon fields. The supraclavicular field and the two tangential fields are often matched on the skin. An analysis of the dose distribution in the matchline region shows a dose of up to 200% of the prescribed dose over the width of 8 mm at 3 cm depth. This is a result of the beam divergence and the complex geometry. A modification of the treatment technique has reduced the dose to less than 140% of the prescribed dose over approximately 4 mm. The modified technique uses a shielding block which covers the caudad half of the supraclavicular field and makes the caudad edge vertical. The cephalad edges of the two tangential fields are defined by a shielding block, which hangs vertically from the treatment head regardless of gantry and collimator position. This block obstructs a small portion of the radiation field and defines a vertical field edge. These three vertical field edges are brought into coincidence by adjustment of couch turntable angle and longitudinal couch position. The nearly perfect geometric match in three dimensions achieved in this manner further improves the cosmetic results after radiation treatment of breast cancer.

Quality assurance in stereotactic radiosurgery using a standard linear accelerator

Methods have recently been developed for using standard linear accelerators to perform stereotactic radiosurgery. The accuracy necessary to perform this procedure requires an intensive quality assurance program to encompass all aspects of dose calibration and mechanical integrity of the treatment unit, the treatment planning process, and treatment delivery. The programs developed at the Joint Center for Radiation Therapy (JCRT) include testing of the linear accelerator and the stereotactic system, cross checking of the treatment planning process, and a quality assurance check list of the treatment delivery procedure. This report outlines in detail the quality assurance program currently in use at the JCRT.

Head scatter data for several linear accelerators (4-18 MV).

Measurements were made in air on the central axis of radiation beams from linear accelerators operating in the energy range from 4 to 18 MV, to determine the magnitude and source of head scattered radiation. Machines of several manufacturers were studied. The data indicate that, except for one unique collimator design, head scatter originates primarily in the flattening filter and is relatively independent of energy and machine.

Dose optimization with computer-controlled gantry rotation, collimator motion and dose-rate variation

The applications of a computer-controlled radiation therapy system to optimize dose distributions in two dimensions are explored. This study is limited to a target volume with constant cross-section along an axis parallel to the long axis of the patient. The machine components that are continuously varied during treatment are the dose rate, the gantry angle, and the four independent collimator jaws, two of which can cross the beam centerline. Basic control strategies, treatment planning and delivery techniques are illustrated with clinical examples. We conclude that the computer-controlled radiation therapy system can easily and reliably deliver dose distributions which are significantly better than those produced by conventional multiple-field techniques.

A computer-controlled radiation therapy machine for pelvic and para-aortic nodal areas

Abstract A computer-controlled radiation therapy technique has been developed to treat cancer of the uterine cervix that has extended to the pelvic and pare-aortic lymph nodes. During five longitudinal scans with a 4 cm wide 8 MV X ray beam, conformation of the high dose region to the target volume was achieved primarily by varying the other field dimensions. Treatment planning and dose calculation were performed in three dimensions. Conventional two-dimensional planning in a series of transverse planes through the patient was combined with a mathematical normalization of dose in the longitudinal direction to attain uniform dose throughout the target volume and to fulfill criteria for protection of critical organs. The resulting dose distributions were compared, in detail, with those resulting from conventional treatment techniques. It was concluded that the computer-controlled therapy scheme offered the potential of reducing the probability of complications, for the same target dose, by lowering the doses to small bowel, duodenum, kidneys, liver and spinal cord.

Three-field technique for breast treatment using a rotatable half-beam block☆☆☆

A new three-field technique for the radiation therapy of breast cancer has been developed. Two opposing tangential fields treat the breast, chest wall, and ipsilateral internal mammary lymph nodes. The third, an anterior field, treats the axilla and supraclavicular area. The objective of the technique is to make the posterior geometric edges of the tangential fields coplanar, and also to make their cephalad geometric edges and the caudad geometric edge of the supraclavicular field coplanar. To effect the matching of fields, the technique utilizes a rotatable half-beam shielding block. To simplify the simulation procedure, an exact mathematical solution is given for the required treatment set-up parameters.