A comparison of commercial treatment planning systems when calculating dose under shielding blocks.

Several commercial computerized treatment planning systems were compared when calculating dose under shielding blocks in soft tissue and lung tissue. A test case was studied in which a water-equivalent phantom containing a removable cork insert was exposed to 60Co radiation with and without a lead block in the beam. For simplicity, all cross sections were square and only central axis doses were considered. Manufacturers of treatment planning systems were asked to calculate doses for the test case for comparison with corresponding experimental values determined from ionization chamber measurements. When the initial replies seemed to indicate the existence of errors or misinterpretations, the manufacturers were informed and asked to submit revised values. The results for eight systems exhibit wide variations in the doses under the block, with differences of up to 88% between measurements and calculations. Most of the systems did not take into account heterogeneity and shielding simultaneously. While in comparison with the maximum dose the dose differences under the block are not large and their clinical significance may be debated, better accuracy and more of a consensus would be expected when elaborate computer systems dedicated to radiation therapy deal with a relatively simple problem. The results emphasize the need for caution when users implement the options offered by their treatment planning computers.

Radiation qualities of x-ray beams in cooperative clinical trials.

X-ray beams are usually described by "MV" numbers which represent accelerating potentials (AP) and approximations to the maximum energies in the photon spectra. However, these numbers do not uniquely specify the properties of the beams. Current high energy photon dosimetry protocols specify radiation quality in terms of a measured ionization ratio which is equivalent to the ratio of the tissue-maximum ratios at depths 10 cm and 20 cm, for field size 10 cm X 10 cm [TMR)20(10]. For convenience, the American Association of Physicists in Medicine introduced a new parameter, known as the Nominal Accelerating Potential (NAP), which was derived from (TMR)20(10) and features values in MV units that are similar to those of the conventional accelerating potentials. (TMR)20(10) and Nominal Accelerating Potential may be considered to be expressions of the penetrating powers of x-ray beams. We determined (TMR)20(10) and Nominal Accelerating Potential for 460 treatment machines with stated accelerating potentials from 4 MV to 25 MV in the Quality Assurance Review Center's files of machine data from institutions that participate in cooperative clinical trials. The results demonstrate appreciable variability of the two parameters at each stated accelerating potential, with overlapping of adjacent groups of machines. It is concluded that the manufacturers' MV numbers do not reliably identify x-ray beams in terms of their depth dose properties. To promote standardization and consistency of energy specification in clinical trials as well as in general practice, we propose that x-ray beams be designated by their Nominal Accelerating Potential values as an adjunct to the use of (TMR)20(10) in radiation therapy.

Field matching considerations in craniospinal irradiation.

Radiotherapy of the craniospinal axis in leukemia and medulloblastoma patients usually involves parallel-opposed lateral cranial fields adjacent and orthogonal to a posterior spinal field. Most current treatment protocols require rotation of the cranial fields to compensate for the divergence of the spinal field such that the adjacent field edges abut along the match line in the mid-saggital plane. Some departments introduce gaps up to 1-2 cm wide between the matched fields out of concern for overdosing the spinal cord. The behavior of the dose distribution was studied in the junction region of divergence-compensated fields as the separation of their edges varied from 0.5 cm overlap to a gap 1 cm wide. Composite dose profiles and isodose maps were calculated for 60Co and 4 MV photon beams using static and moving junction (feathering) techniques. When the fields are appropriately matched there is no gap between them in the mid-saggital plane and the dose varies smoothly across the junction. Gapping of divergence-compensated fields is detrimental to dose uniformity, producing underdosage in a volume that extends to all depths in the body. With proper localization, there is no way that the commonly accepted critical dose for myelopathy can be exceeded in typical treatments of leukemia patients. It can occur in the treatment of medulloblastoma patients only in the unlikely situation where there is gross overlapping of the adjacent fields. Feathering may be considered as a safety margin against spinal cord damage in medulloblastoma but it is superfluous in leukemia. The importance of treatment machine quality assurance and verification of patient set up geometry are emphasized.

Head-scatter factors in blocked photon fields.

The behavior of the head-scatter factor in shielded 6 and 25 MV X-ray beams from a Philips SL25 linear accelerator was investigated by measuring incident fluences by direct (in-air) and indirect (in-phantom) methods. It was found that perturbations in head-scatter produced by shielding blocks arranged to define a slit-shaped field are considerably less than 1% in unwedged beams, even when 80% of a 20 x 20 cm2 field is shielded. The results are independent of beam energy and orientation of the slit with respect to the collimator jaws. When a 60 degrees wedge is inserted, the head-scatter factor decreases by up to a few percent, depending on slit direction but not on energy. The contributions to head-scatter from the block tray and the shielding blocks are negligible.

The effect of single-dose radiation on cell survival and growth hormone secretion by rat anterior pituitary cells.

Cranial irradiation has been shown to impair growth hormone secretion in children. In this study a cell culture of dispersed rat anterior pituitary cells was exposed to single doses of radiation in the range of 100-1500 rad: Survival curves were obtained for the different anterior pituitary cell lines, and growth hormone secretion was measured in the tissue culture medium. Both survival and growth hormone secretion curves showed an initial shoulder in the range of 0-300 rad, followed by a decline between 300-750 rad. It is concluded that growth hormone secreting acidophilic pituicytes are sensitive to radiation at single doses greater than 300 rad.

Total skin electron irradiation: efficacy in early mycosis fungoides.

The rare, indolent, but lethal malignancy, mycosis fungoides (MF), is amenable to durable remissions if treated topically at an early stage with nitrogen mustard, PUVA, or radiotherapy. A modification of conventional therapeutic irradiation which utilizes electron beams rather than photons, has been in use since 1951. This method, termed total skin electron irradiation (TSEI), has achieved consistently good CR rates (95-100%) at a variety of centres in the U.S.A., England, France, and Italy, despite troublesome differences in staging systems. In northern Israel we have treated 37 MF patients with TSEI during the past 13 years. All 21 of our early stage patients achieved CR, which is no longer regarded as an unusual result. However, most workers in the field acknowledge that issues of optimal dosing and curative potency remain unresolved.

Factors affecting sperm motility. III. Influence of visible light and other electromagnetic radiations on human sperm velocity and survival.

Specimens of semen from fertile and infertile patients were exposed to different electromagnetic radiations, including visible light, ultraviolet (UV) light, x-rays, and high-frequency radio waves. Sperm motility was analyzed before, during, and after irradiation by the multiple exposure photography (MEP) method. No significant difference was found between controls and specimens exposed to various doses of visible and UV light and x-rays either immediately or several hours after exposure. In contrast to spermatozoa of other species that were reported to be adversely affected by visible and UV light and x-rays, human spermatozoa seem to be highly resistant to similar doses of these radiations. A deleterious influence was observed when high-frequency radio waves were applied to human spermatozoa. This may be attributed to an intracellular diathermic effect. The informative value of this study in relation to routine semen analyses and experimental studies in the physiology and comparative anatomy of spermatozoa is discussed.

Equation for scatter-air ratios for megavoltage radiotherapy calculations.

Scatter-air ratios are tabulated quantities which are based on measurement and which are used to calculate the dose to points in the body of a patient exposed to radiation fields of arbitrary shape. For computing purposes it is convenient to represent these quantities by an equation. We have found a nine-parameter empirical function which expresses scatter-air ratio data for 60Co gamma rays and for 4- and 6-MV x rays. To test the equation it was used to calculate tissue-air ratios for a range of rectangular fields. The results are in good agreement with published values.

Head-scatter factors and effective x-ray source positions in a 25-MV linear accelerator.

The behavior of the effective source position and the correction factor associated with the collimator opening (head-scatter factor) were investigated for the 6- and 25-MV x-ray beams of a linear accelerator. The primary photon fluence was measured in air for square field sizes from 5 x 5 cm to 40 x 40 cm at distances from the nominal source of 80 to 140 cm, for open and wedged fields (wedge angle 60 deg). An inverse-square analysis shows that, for open fields, the effective source position of the accelerator is about the same (approximately 1 cm downstream) at 6 and 25 MV, for all field sizes. For the wedged fields, the effective source position depends on field size and ranges from about 2 to 4 cm. The head-scatter correction factors for given collimator settings were found to be essentially independent of distance at both energies.

Head-scatter factors in rectangular photon fields.

Head-scatter factors were measured for a range of field sizes of rectangular shape from a linear accelerator that provides 6 and 25 MV photon beams. For a given field length and width, exchange of the inner and outer collimator pairs produces differences in the head-scatter factor of up to about 2% and 3% in open and wedged fields, respectively. Dependence on elongation deviates by up to 2% from that given by the equivalent squares. Such effects can easily be taken into account by using head-scatter data that have been directly measured in rectangular fields or by applying empirical corrections to values for square or equivalent square fields.

Equivalent squares of irregular photon fields.

The tables of equivalent fields published by the British Journal of Radiology (BJR) are intended for calculation of depth-dose functions in rectangular photon fields. We have investigated the validity of the equivalent-field concept for fields of arbitrary shape over a range of photon energies, field sizes and depths. We show that the empirical scatter-radius function (Day function) used to generate the equivalent-field tables is a good approximation to the average over energy of normalized scatter-air ratios extracted from BJR beam data for depths up to 10 cm. However, this function tends to diverge from the data as depth increases. Accuracy can be improved by making the Day function depend on depth. Equivalent squares, determined by sector integration of the original or modified Day functions, are suitable for megavoltage photon-beam dose calculations at central-axis and off-axis points in irregular as well as rectangular fields.

The Clatterbridge high-energy neutron facility: dosimetry intercomparisons.

Dosimetry intercomparisons have been performed between the Clatterbridge high-energy neutron facility and the following institutions, all employing beams with similar neutron energies: Université Catholique de Louvain, Belgium; University of Washington, Seattle, USA; MD Anderson Hospital, Houston, USA; and Fermi National Accelerator Laboratory, Batavia, USA. The purpose of the intercomparison was to provide a basis for the exchange of dose-response data and to facilitate the involvement of Clatterbridge in collaborative clinical trials. Tissue-equivalent ionization chambers were used by the participants in each intercomparison to compare measurements of total (neutron plus gamma) absorbed dose in the host institution's neutron beam, following calibration of the chambers in a reference photon beam. The effects of differences in exposure standards, chamber responses in the neutron beams and protocol-dependent dosimetry factors were all investigated. It was concluded that the overall difference in the measurement of absorbed dose relative to that determined by the Clatterbridge group was less than 2%.

Experience in the treatment of localized carcinoma of the prostate by definitive external irradiation.

One hundred and sixteen patients with stages A2, B, and C histologically proven adenocarcinoma of the prostate were treated by definitive radiation. They received first whole pelvic irradiation, 46 Gy in 23 treatments, followed by a prostatic "boost" of 20 Gy in 10 fractions. Five-year survival rates varied between 39 and 84%. Prophylactic hormonal manipulation and transurethral resection of the prostate (TURP) did not influence survival figures. Complications of radiotherapy were in an acceptable range. It is concluded that external beam irradiation is an effective, simple, and usually well-tolerated treatment modality for localized carcinoma of the prostate. Stage and degree of differentiation and response of the primary tumour to radiation are predictors of patient outcome. There is no advantage in prophylactic hormonal manipulation, and TURP does not influence survival according to our experience.