Contribution of CT to quantitative radiation therapy planning

Beams eye view-based photon radiotherapy I

Geographic miss, dosimetric miss (underdosing), and proximity of the tumor to sensitive normal tissues are some of the causes of inadequate radiation dose delivery; this is one of many causes of failure after radiotherapy. In the past decade, computerized tomography (CT)-based treatment planning has helped to overcome some of these problems. Beam's eye view (BEV)-based radiotherapy planning is an improvement over CT-based treatment planning that may further increase the therapeutic ratio. Since January 1988, we have treated 198 patients with BEV-based photon radiotherapy. About 40% of our patients treated with radical radiotherapy undergo BEV-based treatment, and about 70% of patients who undergo planning CT in the treatment position receive BEV-based radiotherapy. Our findings are as follows: (a) routine use of BEV-based RT (BEVRT) is possible in a busy radiation oncology department; (b) BEVRT improves geometric coverage of tumors; (c) BEVRT is extremely useful in the design of oblique portals; (d) time commitments for various members of the RT treatment-planning team are reasonable; (e) BEVRT helps individualize RT technique; (f) preliminary data suggest decreased acute toxicity with the use of BEVRT for prostate cancer patients. Whether these advantages will help to improve the outcome (i.e., improve local control and survival) and/or decrease the long-term toxicity is not yet known.

The objective evaluation of alternative treatment plans: II. Score functions

A series of six patients with adenocarcinoma of the prostate, Stages A2, B1, or B2, were planned for treatment using a four-field box technique at 25 MV. Plans were prepared by three techniques: composite, mid-plane, and conformal. The dose distributions at the central plane and at two planes offset by +/- 2 cm were evaluated by means of score functions which quantify the magnitude of regret for target dose gradient, target over- and under-dose, non-target tissue overdose, and for overdose to the rectum, bladder, and femoral heads. The score functions are normalized to give values in the range from 10 (ideal) to zero (limit of acceptability), with negative values indicating unacceptable deviations from the prescribed dose limits. The scores for off-axis conformal plans were found to be essentially the same as for mid-plane plans on the central plane. However, mid-plane planning was shown to be totally inadequate for off-axis planes, where the average target gradient and underdose scores were reduced by 10 units. Composite planning resulted in adequate target coverage on all planes, but at the expense of unacceptable overdose to non-target tissue. The effect of reducing the posterior beam weight to half that of the other three beams was to reduce the target gradient score by 1.6 +/- 0.5 units and to increase the rectal score by 0.9 +/- 0.3 units.

Localizing the spinal cord in oblique off-cord lung boosts

We present a method to accurately localize the spinal cord in oblique projections on plain radiographs. Utilizing a CT scan done with the patient in the treatment position, a template is generated to localize the spinal cord. The technique involves analyzing successive axial CT slices and locating cord position relative to the beam central axis. The template is then placed on the simulator fiducial plate at the time of verification simulation. Cord position is documented in the "beam's eye view" on the radiograph. Utilizing this technique, our radiation oncologists are more comfortable defining the medial field border in oblique setups. In most cases, this technique will minimize the perceived need to add superfluous spinal cord blocks that compromise tumor dose.

Dose to the cardiac vascular and conduction systems in primary breast irradiation

Using computerized tomography (CT) in which cardiac anatomy was defined, doses delivered to the cardiac compartments, vascular and conduction systems were assessed for various standard techniques of primary breast irradiation. Included in the analysis were 6 MV photon tangents (T) alone, or in conjunction with a separate internal mammary field (IMF). Beams evaluated in the IMF were 6 MV photons, 12 MeV electron beam, and mixed photon/electron beam; Cobalt 60 was also analyzed as an alternate photon beam. Treatment of the IMF with photons, either alone or in combination with electron beam, delivered doses ranging between 30 Gy to 50 Gy to all chambers of the heart, coronary arteries and branches of the conduction system. Complete sparing of the posterior cardiac structures and volume is accomplished with treatment plans using tangents alone or in combination with 12 MeV electron beam irradiation to the IMF. Sparing of the anterior wall of the left ventricle, Bundle of His and left anterior descending coronary artery is also achieved in treatment with tangents and 12 MeV electron beam IMF. Doses to this region with tangents alone ranged from 20 Gy to 45 Gy compared to 0 to 30 Gy with tangents and 12 MeV electron beam IMF. Clinical significance of these findings will be discussed.

Locate: An application of computed tomography in radiation therapy treatment planning with emphasis on tumor localization

Computed tomography can provide precise information for radiation therapy treatment planning. However, inaccuracies in radiation field design may occur when the radiation oncologist attempts to transfer information about tumor location from the transverse plane of the CT scan to the longitudinal plane of the simulation film. This report describes a new computer program, LOCATE, which addresses this problem. The program uses operator generated information from the cross sectional CT images to draw an outline of tumor on AP and lateral longitudinal scanned projection radiographs. The resultant images are useful because they are in the same plane as radiographs obtained on a therapy simulator. The impact of LOCATE on radiation treatment planning for 26 patients is discussed along with several cases in which LOCATE was particularly helpful.

Technical accuracy and clinical efficacy of thoracic computed tomography

The technical accuracy and clinical efficacy of thoracic CAT scans in 302 surgical patients were retrospectively reviewed. We conclude that thoracic CAT scans should be ordered selectively since the technical accuracy of existing radiographic studies in detecting disease is equally high (greater than 90 percent) and the clinical efficacy does not differ substantially (less than 16 percent). The accuracy of CAT scans in staging carcinoma (the extent of the lesion or nodes) is only 58 percent. The technical limitations of these scans include (1) a lack of specificity because of the high incidence of false-positive lymph nodes, (2) a low or unreliable yield from needle aspiration, and (3) unreliable findings due to altered anatomy by previous treatment or associated disease. Surgical treatment regarding operability and resectability should not be solely based on CAT scans. These scans appear to be a useful screening test for chronic vascular disease and localization of obscure thoracic infections.

Computed tomography in lung cancer

Computed-assisted tomography produces a cross-sectional image of the body using x-ray absorption measurements. Density differences are much more apparent than on conventional radiographs, although spatial resolution is not so fine. Lung cancer, can be evaluated accurately with regard to size, location, and whether regional or mediastinal lymph nodes are enlarged. Although enlarged granulomatous nodes generally tend to be dense, histologic specificity is not available. Computed tomography is recommended as a screening technique in lung cancer staging for patients whose routine radiographs are normal but whose primary lesions fulfill criteria for mediastinoscopy. If findings on computed tomography are normal, thoracotomy should then be performed; if enlarged nodes are detected, then a biopsy is needed to exclude operability. This strategy presumes that computed tomography has high sensitivity, although it remains to be proven by further experience whether the false-negative rate of computed tomography is an acceptable 10 to 15 percent. Computed tomography has been shown to be of considerable value in optimizing radiation therapy of lung cancer, and in diagnosing pleural complications of the cancer or its treatment.

Prospects of computed tomography in radiation therapy

Computerized tomography has become an essential element in the staging of tumors and in the localization of the tumor and neighboring normal tissues for treatment planning. It offers the potential for more accurate delivery of higher doses with improved therapeutic ratio, for the identification of and correction for tissue inhomogeneities, and for three-dimensional treatment planning. For treatment planning purposes only minor changes are required in the current generation of scanners. Possibilities for the future include dynamic radiation treatment, combined CT scanners, simulators, and treatment planning computers, and a potential for significant cost saving through improvements in the results of cancer therapy.

Computed tomography in the management of head and neck cancer

Computed tomography is indispensable in determining the deeper extensions of tumors and proper treatment planning as well as in documentation of tumor resolution for lesions arising in the nasal cavity, paranasal sinuses, orbit, and salivary glands. In the hypopharynx and larynx, computed tomography provides information of differing degrees of value according to the clinical stage, site of origin, and usual therapeutic approach for each tissue. Detection of subclinical cervical adenopathy needs further investigation. Computed tomography has not been of particular value for lesions arising in the oral cavity and oropharynx.