J. Van Dyk

Commissioning and quality assurance of treatment planning computers

The process of radiation therapy is complex and involves many steps. At each step, comprehensive quality assurance procedures are required to ensure the safe and accurate delivery of a prescribed radiation dose. This report deals with a comprehensive commissioning and ongoing quality assurance program specifically for treatment planning computers. Detailed guidelines are provided under the following topics: (a) computer program and system documentation and user training, (b) sources of uncertainties and suggested tolerances, (c) initial system checks, (d) repeated system checks, (e) quality assurance through manual procedures, and in vivo dosimetry, and (f) some additional considerations including administration and manpower requirements. In the context of commercial computerized treatment planning systems, uncertainty estimates and achievable criteria of acceptability are presented for: (a) external photon beams, (b) electron beams, (c) brachytherapy, and (d) treatment machine setting calculations. Although these criteria of acceptability appear large, they approach the limit achievable with most of todays treatment planning systems. However, developers of new or improved dose calculation algorithms should strive for the goal recommended by the International Commission of Radiation Units and Measurements of 2% in relative dose accuracy in low dose gradients or 2 mm spatial accuracy in regions with high dose gradients. For brachytherapy, the aim should be 3% accuracy in dose at distances of 0.5 cm or more at any point for any radiation source. Details are provided for initial commissioning tests and follow-up reproducibility tests. The final quality assurance for each patient is to perform an independent manual check of at least one point in the dose distributions, as well as the machine setting calculation. As a check of the overall treatment planning process, in vivo dosimetry should be performed on a select number of patients.

Radiation pneumonitis following large single dose irradiation: A re-evaluation based on absolute dose to lung

Abstract The acute radiation paeumonitis syndrome is a major complication for patients receiving total thoracic irradiation in a large single dose. Previous studies have evaluated the onset of radiation pneumonitis on the basis of radiation doses calculated assuming unit density tissues. In this report, the incidence of radiation pneumonitis is determined as a function of absolute dose to lung. A simple algorithm relating dose correction factor to anterior-posterior patient diameter has been derived using a CT-aided treatment planning system. This algorithm was used to determine, retrospectively, the dose to lung for a group of 303 patients who had been treated with large field irradiation techniques. Of this group, 150 patients had no previous lung disease and had virtually no additional lung irradiation prior or subsequent to their large field treatment. The actuarial incidence of radiation pneaunoniitis versus dose to lung was evaluated using a simplified probit analysis. The resultant best fit sigmoidal complication curve demonstrates the onset of radiation pneumonitis to occur at about 750 red with the 5% actuarial incidence occuring at approximately 820 red. The errors associated with the dose determination procedure as well as the actuarial incidence calculations are considered. The time of onset of radiation paeumonitis occurs between 1 to 7 months after irradiation for 90% of the patients who developed pneumonitis with the peak incidence occurring at 2 to 3 months. No correlation was found between time of onset and the dose to lung over a dose range of 650 to 1250 red.

Idiopathic interstitial pneumonia following bone marrow transplantation: the relationship with total body irradiation

Abstract Interstitial pneumonia is a frequent and often fatal complication of allogenic bone marrow transplantation. Thirty to 40 percent of such cases are of unknown etiology and have been labelled as cases of idiopathic interstitial pneumonia. Idiopathic cases are more commonly associated with the use of total body irradiation; their occurrence appears to be independent of immunosupression or graft versus host disease. Evidence is presented from the literature suggesting that the development of idiopathic interstitial pneumonia is related to the absolute absorbed dose of radiation to lung. The similarity of idiopathic pneumonia to radiation paeumonitis seen in a different clinical setting is described.

MANAGEMENT OF RADIATION ONCOLOGY PATIENTS WITH IMPLANTED CARDIAC PACEMAKERS : REPORT OF AAPM TASK GROUP NO.34

Contemporary cardiac pacemakers can fail from radiation damage at doses as low as 10 gray and can exhibit functional changes at doses as low as 2 gray. A review and discussion of this potential problem is presented and a protocol is offered that suggests that radiation therapy patients with implanted pacemakers be planned so as to limit accumulated dose to the pacemaker to 2 gray. Although certain levels and types of electromagnetic interference can cause pacemaker malfunction, there is evidence that this is not a serious problem around most contemporary radiation therapy equipment.

Lung dose corrections for 6‐ and 15‐MV x rays

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Lung density as measured by computerized tomography: implications for radiotherapy

Accurate dose calculations for radiotherapy planning require a detailed knowledge of the internal anatomy of the patient both in terms of geometry and density. Computed tomography (CT) is presently the best means of providing these data. Fifty-eight patients who had scans of the thorax for radiotherapy planning were studied. The CT numbers were converted to relative electron densities and average lung densities were obtained for every patient. A linear correlation of lung density with age was found with the mean lung density of 0.35 at an age 5 years and 0.19 at an age of 80. The effect of scanning the patient under full inspiration, full expiration or normal shallow breathing conditions was analyzed. At the age of 5 years the expiration and inspiration average lung densities were 0.36 and 0.20, while at the age of 80 years they were 0.22 and 0.16, respectively. Respiratory volume changes were linearly correlated with changes in relative electron density. Differences in lung density between expiration and inspiration scans were found to demonstrate a similar trend with age as the relationship between vital capacity and age. The dosimetric and the possible clinical implications of lung density measurements for radiotherapy are considered. In particular, dose calculations were performed using scans taken under a number of different respiratory conditions. Doses calculated for a single cobalt-60 beam can differ by more than 25% when comparing a full inspiration scan to a normal breathing scan. A similar comparison for a parallel pair distribution on the lung yields a difference of about 3% while a typical three field technique for treating cancer of the esophagus shows a difference of nearly 10%.

Myelopathy following hyperfractionated accelerated radiotherapy for anaplastic thyroid carcinoma

From 1975 to 1982, 32 patients with a diagnosis of anaplastic carcinoma of the thyroid were entered into a protocol of hyperfractionated accelerated radiotherapy. The tumor dose was 30-45 Gy at 1 Gy per fraction given 4 times a day at 3-h intervals. The results were disappointing with a median survival of less than 6 months. Two patients developed radiation myelopathy at 8 and 13 months, total spinal cord dose being 39.9 and 48.3 Gy, respectively. The risk of spinal cord damage was much higher than expected. The possible radiobiological causes and clinical implications are discussed.

Quantitative measurement of changes in human lung density following irradiation.

In a recent clinical study, we have shown that computed tomography (CT) can provide a good qualitative endpoint for the presence of radiation-induced pulmonary damage. However, CT data are also potentially useful as a quantitative measure of lung damage. Changes in lung density were quantified by comparing CT images taken before and after radiotherapy for 54 patients. Lung densities were assessed separately in the irradiated and nonirradiated regions using both the average regional densities and density distributions. For the 36 patients demonstrating visible post-irradiation lung damage on CT, the mean relative increase in average lung density was 0.20 +/- 0.10 in the irradiated regions. The mean change in the nonirradiated regions was 0.02 +/- 0.09 for the same group. For 18 patients without visible damage, the mean relative changes were 0.00 +/- 0.09 and -0.05 +/- 0.07 for the irradiated and nonirradiated regions, respectively. The results suggest that an increase in average lung density of 5% or greater is associated with the visible detection of pulmonary damage. The dose-response relationship based on this quantitative criterion was comparable to that derived using the qualitative endpoint.

Evaluation of isoeffect formulae for predicting radiation-induced lung damage

An experiment has been performed in which fractionated irradiation was given to the whole thorax of Sprague-Dawley rats with schedules chosen so that doses per fraction and overall treatment time were changed independently. Damage was monitored by lethality. The data have been analyzed to yield dose per fraction and time parameters using multiple non-linear regression analysis. The results show that a linear-quadratic cell survival formula, extended to include an exponential time component to account for proliferation or slow repair during the treatment, can predict isoeffective doses to within 7% accuracy over a wide range of times (3.5-49 days) and doses per fraction (1.8-10.2 Gy). Other isoeffect formulae based on the linear-quadratic and empirical power law functions were also evaluated. A linear-quadratic formula with a time dependent alpha parameter fitted the data particularly well. This result suggests an alternative underlying mechanism and requires further investigation.

Post-irradiation lung density changes measured by computerized tomography.

To investigate the possibility of using computerized tomography (CT) as a prognostic indicator of radiation induced lung toxicity, a series of CT scans were performed on one patient. These scans suggested an increase in lung density at day 73 after an upper half body irradiation. Because of the difficulty in lung density follow-up in patients irradiated for palliation, further studies were performed with LAF1 mice. Serial scans were taken on three groups of mice: (1) control group, (2) irradiated to 11.0 Gy and (3) irradiated to 14.0 Gy. Lung density increases between 10 and 15% were observed in the two irradiated groups. The time course was dependent on dose with an earlier onset for the high dose group (14 weeks) than for the low dose group (24 weeks). These density changes were observed only a few weeks prior to the death of the animals, indicating that small animals such as mice will not likely be useful for assessing CT scanning as an early predictor of radiation damage to lungs.