An MLC calibration method using a detector array

PURPOSE

The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator.

METHODS

The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaw's leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leaf's position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array.

RESULTS

The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks.

CONCLUSIONS

This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

On the sensitivity of patient-specific IMRT QA to MLC positioning errors

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient-specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of +/-1 mm and +/-2 mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance-to-agreement (DTA) analysis and gamma index with 2%/2 mm and 3%/3 mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and gamma index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2 mm than with 3%/3 mm in general. When using the diode array and 2%/2 mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient-specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient-specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery.

Malignancies in the rheumatoid arthritis abatacept clinical development programme: an epidemiological assessment

Objective:

To provide context for the malignancy experience in the rheumatoid arthritis (RA) abatacept clinical development programme (CDP) by performing comparisons with similar RA patients and the general population.

Methods:

Malignancy outcomes included total malignancy (excluding non-melanoma skin cancer (NMSC)), breast, colorectal, lung cancers and lymphoma. Comparisons were made between the observed incidence in patients within the abatacept CDP and RA patients on disease-modifying antirheumatic drugs (DMARD) identified from five data sources: the population-based British Columbia RA Cohort, the Norfolk Arthritis Register, the National Data Bank for Rheumatic Diseases, the Sweden Early RA Register and the General Practice Research Database. Age and sex-adjusted incidence rates (IR) and standardised incidence ratios (SIR) were used to compare events in the abatacept trials with the RA DMARD cohorts and the general population.

Results:

A total of 4134 RA patients treated with abatacept in seven trials and 41 529 DMARD-treated RA patients in the five observational cohorts was identified for study inclusion. In the abatacept-treated patients, the 51 malignancies (excluding NMSC), seven cases of breast, two cases of colorectal, 13 cases of lung cancer and five cases of lymphoma observed were not greater than the range of expected cases from the five RA cohorts. The SIR comparing RA patients with the general population were consistent with those reported in the literature.

Conclusions:

The IR of total malignancy (excluding NMSC), breast, colorectal, lung cancers and lymphoma in the abatacept CDP were consistent with those in a comparable RA population. These data suggest no new safety signals with respect to malignancies, which will continue to be monitored.

Multileaf collimator characteristics and reliability requirements for IMRT Elekta system

Understanding the characteristics of a multileaf collimator (MLC) system, modeling MLC in a treatment planning system, and maintaining the mechanical accuracy of the linear accelerator gantry head system are important factors in the safe implementation of an intensity-modulated radiotherapy program. We review the characteristics of an Elekta MLC system, discuss the necessary MLC modeling parameters for a treatment planning system, and provide a novel method to establish an MLC leaf position quality assurance program. To perform quality assurance on 40 pairs of individual MLC leaves is a time-consuming and difficult task. In this report, an effective routine MLC quality assurance method based on the field edge of a backup jaw as referenced in conjunction with a diode array as a radiation detector system is discussed. The sensitivity of this test for determining the relative leaf positions was observed to be better than 0.1 mm. The Elekta MLC leaf position accuracy measured with this system has been better than 0.3 mm.

Tomographic physical phantom of the newborn child with real-time dosimetry I. Methods and techniques for construction

A tomographic phantom representing a newborn female patient was constructed using tissue-equivalent materials previously developed at the University of Florida. This phantom was constructed using contoured images from an actual patient data set, a whole-body computed tomography of a newborn cadaver previously described by Nipper et al. [Phys. Med. Biol. 47, 3143-1364 (2002)]. Four types of material are incorporated in the phantom: soft tissue, bone tissue, lung tissue, and air. The phantom was constructed on a slice-by-slice basis with a z-axis resolution of 5 mm, channels for dosimeters (thermoluminescent dosimeter (TLD), metal-oxide-semiconductor field-effect transistor, or gated fiber-optic-coupled dosimeter (GFOC)) were machined into slices prior to assembly, and the slices were then fixed together to form the complete phantom. The phantom will be used in conjunction with an incorporated dosimetry system to calculate individual organ and effective doses delivered to newborn patients during various diagnostic procedures, including, but not limited to, projection radiography and computed tomography. Included in this paper are images detailing the construction process, and images of the completed phantom.

Evidence for genetic modifiers of postnatal lethality in PWS-IC deletion mice

Prader-Willi syndrome (PWS), most notably characterized by infantile hypotonia, short stature and morbid obesity, results from deficiencies in multiple genes that are subject to genomic imprinting. The usefulness of current mouse models of PWS has been limited by postnatal lethality in affected mice. Here, we report the survival of the PWS-imprinting center (IC) deletion mice on a variety of strain backgrounds. Expression analyses of the genes affected in the PWS region suggest that while there is low-level expression from both parental alleles in PWS-IC deletion pups, this expression does not explain their survival on certain strain backgrounds. Rather, the data provide evidence for strain-specific modifier genes that support the survival of PWS-IC deletion mice.

A health perception score predicts cardiac events in patients with heart failure: results from the IMPRESS trial

BACKGROUND

New York Heart Association (NYHA) class and treadmill exercise test variables are widely used for estimating prognosis and measuring the outcomes of treatment in patients with heart failure, but they do not take patients' perceptions into account.

METHODS AND RESULTS

Five hundred forty-five patients enrolled in a multicenter 24-week comparison of the effects of omapatrilat and lisinopril on functional capacity in patients with heart failure reported a visual analog scale (VAS) score of their overall health perception at week 12 of the study. A total of 27 first events, defined as death or worsening heart failure (hospitalization, emergency room visit, or study discontinuation), occurred in the subsequent 12 weeks. The mean (+/-SD) health perception scores were 0.43 +/- 0.31 and 0.68 +/- 0.20 in patients with and without events, respectively (P =.0006). The risk ratio (RR) for an event associated with a decile change in the health perception score was 0.74 (95% confidence interval [CI], 0.61-0.88; P =.001). The RR was unaltered by adjustment for demographic variables, treadmill time, and NYHA functional class. Although the week 12 NYHA functional class was predictive of events (RR = 2.1; 95% CI, 1.2-4.6; P =.04), treadmill time was not (RR = 0.87; 95% CI, 0.73-1.03; P = 0.11).

CONCLUSIONS

A patient-reported measure of perceived health predicts events in patients with heart failure.

Assigning time-linked exposure status to controls in unmatched case-control studies: alcohol use and nearly lethal suicide attempts

In case-control studies, determination of alcohol consumption by cases immediately prior to the injury event is often conceptually straightforward. However, determination of consumption status by controls is difficult because they lack a reference point, especially when cases and controls are not individually matched. We describe a method of assigning alcohol consumption status to controls using a 24-hour drinking history, the distribution in time of case events, and the random assignment of a specific time period to each control subject. This methodology offers a practical approach for determining alcohol consumption status among control subjects immediately prior to a case event, when controls lack a reference point and have not been individually matched to cases. Published in 2001 by John Wiley & Sons, Ltd.

Tuberculosis in hemodialysis patients in New Jersey: a statewide study

OBJECTIVE

To study the incidence of tuberculosis (TB), tuberculin skin testing (TST) practices, and infection control practices at outpatient hemodialysis centers.

DESIGN

Mail surveys performed in December 1994 and 1995.

MAIN OUTCOME MEASURES

The numbers of patients with incident active TB during 1994 and 1995, TST policies during 1994, and TB infection control policies in 1994.

SETTING

All outpatient dialysis centers in New Jersey.

PATIENTS OR PARTICIPANTS

Healthcare workers and patients in dialysis centers in New Jersey.

RESULTS

Of 47 centers, 41 provided information on TST and TB infection control policies and practices. TSTs were performed on newly hired healthcare workers at all 41 centers and on established workers at 39 centers. In contrast, only 1 center reported performing TSTs on hemodialysis patients; 5 other centers reported screening of patients for TB using chest radiographs. Active TB was reported in 3 of 4,550 chronic hemodialysis patients in 1994 (rate, 66/100,000 patient-years) and in 4 of 4,831 patients in 1995 (rate, 83/100,000 patient-years). Both rates were several times higher than the rate in the New Jersey general population during this period (10.7-10.8/100,000).

CONCLUSION

Although based on small numbers of patients with TB, we found a relatively high incidence of TB among hemodialysis patients in New Jersey. Most centers reported performing TSTs on workers but not on patients. These results suggest the need for improved TB screening and infection control precautions at outpatient dialysis centers.

Safety of irbesartan in the treatment of mild to moderate systemic hypertension

Nine multicenter, randomized, placebo-controlled studies were conducted to evaluate the safety and tolerability of the angiotensin II subtype 1 receptor blocker (AT1 blocker) irbesartan for the treatment of mild to moderate hypertension. After a 4- to 5-week placebo lead-in phase, patients were randomized to 4 to 12 weeks of double-blind therapy with either placebo (n = 641) or irbesartan (n = 1,965) at doses of 1 to 900 mg orally. All doses of irbesartan were well tolerated with no evidence of dose-related adverse effects. Across the full recommended clinical dose range, although not statistically significantly different, irbesartan use was associated with a lower incidence of adverse events, serious adverse events, and discontinuations due to adverse events compared with placebo. No clinically significant or unexpected changes in laboratory analyses were observed. Withdrawal of irbesartan therapy did not result in rebound hypertension or clinically important adverse events. Thus, irbesartan use in hypertensive patients was associated with a placebo-like safety and tolerability profile.

Selective embolisation of intrahepatic aneurysms

The radiological techniques that can be used to selectively embolise small arteries have improved markedly in the past few years. This article discusses the use of transcatheter embolisation in the management of three patients with aneurysms involving the intrahepatic arterial tree.