EGSNRC Monte Carlo study of the effect of photon energy and field margin in phantoms simulating small lung lesions.

The dose distribution in small lung tumors (coin lesions) is affected by the combined effects of reduced attenuation of photons and extended range of electrons in lung. The increased range of electrons in low-density tissues can lead to loss of field flatness and increased penumbra width, especially at high energies. The EGSNRC Monte Carlo code, together with DOSXYZNRC, a three-dimensional voxel dose calculation module has been used to study the characteristics of the penumbra in the region of the target-lung interfaces for various radiation beam energies, lung densities, target-field edge distances, target size, and depth. The Monte Carlo model was validated by film measurements made in acrylic (simulating a tumor) imbedded in cork (simulating the lung). Beam profiles that are deemed to be acceptable are defined as those in which no point within the planning target volume (target volume plus 1 cm margin) received less than 95% of the dose prescribed to the center of the target. For parallel opposed beams and 2 cm cube target size, 6 MV photons produce superior dose distribution with respect to penumbra at the lateral, anterior, and posterior surfaces and midplane of the simulated target, with a target-field edge distance of 2.5 cm. A lesser target-field edge distance of 2.0 cm is required for 4 MV photons to produce acceptable dose distribution. To achieve equivalent dose distribution with 10 and 18 MV photons, a target-field edge distance of 3.0 and 3.5 cm, respectaively, is required. For a simulated target size of 4 cm cube, a target-field edge distance of 2, 2.5, and 3 cm is required for 6, 10, and 18 MV photons, respectively, to yield acceptable PTV coverage. The effect, which is predominant in determining the target dose, depends on the beam energy, target-field edge distance, lung density, and the depth and size of the target.

The inclusion of capillary distribution in the adiabatic tissue homogeneity model of blood flow.

We have developed a non-invasive imaging tracer kinetic model for blood flow which takes into account the distribution of capillaries in tissue. Each individual capillary is assumed to follow the adiabatic tissue homogeneity model. The main strength of our new model is in its ability to quantify the functional distribution of capillaries by the standard deviation in the time taken by blood to pass through the tissue. We have applied our model to the human prostate and have tested two different types of distribution functions. Both distribution functions yielded very similar predictions for the various model parameters, and in particular for the standard deviation in transit time. Our motivation for developing this model is the fact that the capillary distribution in cancerous tissue is drastically different from in normal tissue. We believe that there is great potential for our model to be used as a prognostic tool in cancer treatment. For example, an accurate knowledge of the distribution in transit times might result in an accurate estimate of the degree of tumour hypoxia, which is crucial to the success of radiation therapy.

The use of associated particle timing based on the D + D reaction for elemental analysis of bulk samples such as the human body.

The use of associated particle timing based on the D + D reaction has been demonstrated for elemental analysis of bulk samples such as the human body. The neutron energy of 2.8 MeV eliminates the background from organic matrices. The nanosecond timing of a HPGe detector renders it possible to identify the spatial origin of the measured gamma radiation limiting the sensitive area to a single pixel. By this technique the background could be reduced by a factor of > or = 1000, but the present set-up has achieved an effective factor only in the range 20-100, due to losses in the generation of timing signals. The very clean gamma-spectra obtained permit the use of high efficiency scintillation detectors. Sensitivities for measuring Al, Ti, and Fe are presented at an extrapolated dose of 10 mSv.

The elemental analysis of the human body using associated particle timing based on the D + D reaction.

Fast (approximately 2.8 MeV) neutrons are generated in the 2H(d,n)3He reaction, and are used for excitation of sample materials via inelastic scattering. Nanosecond timing identifies the spatial origin of the measured gamma radiation, making possible three-dimensional imaging and improved detection sensitivity. Several elements have been measured, and it is estimated that a clinical device, with equivalent dose of 10 mSv, will be capable of detecting the order of 10-100 mg.

Poster - Thurs Eve-42: A revision of the γ-evaluation: Initial interpretation of dose disagreements on γ-vector fields.

Gamma evaluations are a common clinical tool used as a quantitative comparison between dose-distributions, combining both dose difference and distance to agreement criteria. Because gamma evaluations permit rapid analysis of agreement between complex dose distributions, they are often a preferred comparison method for assessing delivery of conformal radiotherapy distributions. Although the comparison provides a useful measure of agreement between distributions when the index is less than one, the scalar gamma value provides little information into the clinical significance or source of disagreements of failing gamma values (i.e., when γ>1). Previously, Stock et al., have presented the gamma angle as an indicator of the relative influence of the distance to agreement versus the dose difference on gamma. We present a modification to the gamma evaluation such that the complete 3D gamma vector information is considered. The predictive nature of each vector component was investigated by simulating various dose disagreements in test distributions. Misalignment tests revealed that the mean gamma vector components indicate the offset direction and relative magnitude for all test distributions. The mean dose component of the gamma vector was prognostic of double Gaussian overdoses and underdoses in a virtual conformal delivery. The response of the vector field depends on properties distinctive to each distribution, such as the local dose gradient. Understanding how these unique properties affect the vector field may permit better diagnosis of dose disagreement sources. Other vector field properties, such as curl and divergence, may yet provide more information for interpreting the cause and significance of γ>1.

Poster - Thurs Eve-15: Comparison of Cobalt-60 and 6 MV linac based tomotherapy: A prostate case study.

Previous work reported by us has shown the potential for Cobalt-60 (Co-60) tomotherapy for sites with small separations such as in head and neck site. In this work we extend our investigations by comparing tomotherapy plans for the treatment of a typical prostate cancer obtained for 6 MV and Co-60 beams. Beam collimation was provided by the MIMiC® (NOMOS Corporation, Sewickly, PA) multi-leaf collimator (MLC). Both plans used 21 beam angles, each utilizing the central 10 leaf-pairs of the MLC for intensity modulation. An in-house inverse treatment planning program, based on the active-set conjugate gradient method, was used for dose-volume optimization. BEAMnrc and DOSXYZnrc Monte Carlo simulated beam and dose data, including inhomogeneity corrections, were used to calculate the optimized tomotherapy dose distributions. Prostate, rectum, and external body contours were outlined and dose-volume optimization objectives were set to deliver a minimum of 95% and a maximum of 105% of the 76 Gy dose prescription to the prostate and limiting only 20% of the rectum volume to receive ⩾ 70 Gy. A quantitative analysis of the dose distributions and dose-area histograms show that both Co-60 and 6 MV plans achieve the initial objectives for target (prostate) and organ at risk (rectum). Although the dose to the body and rectum for Co-60 is slightly higher than that for 6 MV, it satisfies the plan objectives based on the clinical dose tolerance. Our results demonstrate that Co-60 based tomotherapy can provide clinically competitive dose distributions for the treatment of prostate cancer.

Anesthetic management in Joubert syndrome.

We review the anesthetic implications of Joubert syndrome and report that spinal anesthesia under intravenous propofol sedation proved satisfactory for repair of an inguinal hernia in a spontaneously ventilating infant with this syndrome. We caution that anatomical peculiarities may complicate performance of a caudal epidural block and suggest consideration of the use of caffeine to ameliorate the apneic episodes seen in early infancy in this syndrome.

Software for the estimation of foetal radiation dose to patients and staff in diagnostic radiology.

Occasionally, it is clinically necessary to perform a radiological examination(s) on a woman who is known to be pregnant or an examination is performed on a woman who subsequently discovers that she was pregnant at the time. In radiological examinations, especially of the lower abdomen and pelvis area, the foetus is directly irradiated. It is therefore important to be able to determine the absorbed dose to the foetus in diagnostic radiology for pregnant patients as well as the foetal dose from occupational exposure of the pregnant worker. The determination of the absorbed dose to the unborn child in diagnostic radiology is of interest as a basis for risk estimates from medical exposure of the pregnant patient and occupational exposure of the pregnant worker. In this paper we describe a simple computer program, FetDose, which calculates the dose to the foetus from both medical and occupational exposures of the pregnant woman. It also calculates the risks of in utero exposure, compares calculated doses with published data in the literature and provides information on the natural spontaneous risks. The program will be a useful tool for the medical and paramedical personnel who are involved with foetal dose (and hence risks) calculations and counselling of pregnant women who may be concerned about in utero exposure of their foetuses.

Minimizing diaphragmatic injury during radio-frequency ablation: efficacy of subphrenic peritoneal saline injection in a porcine model.

PURPOSE: To investigate if a targeted subphrenic peritoneal infusion of normal saline to separate liver from diaphragm before radio-frequency (RF) ablation could minimize or eliminate diaphragmatic injury. MATERIALS AND METHODS: With a 2-cm-diameter, eight-prong RF needle electrode, 37 hepatic dome RF lesions were created in 10 pigs. Seventeen lesions were created before (non-saline group) and 20 lesions after (post-saline group) intraperitoneal infusion of approximately 500 mL of normal saline. Ten non-saline lesions were created deep (centered 1-2 cm from the liver surface) and seven superficially (centered within 1 cm of the capsule). All 20 post-saline lesions were created superficially. Helical enhanced computed tomography was performed 24-48 hours after ablation. The pigs were killed immediately, and the diaphragm was visually inspected and sectioned. Diaphragmatic injury was graded as 0, no injury; 1, injury up to one-third thickness; 2, injury to two-thirds thickness; 3, full-thickness injury. Representative grade 3 injuries and all partial injuries underwent gross and histologic analysis. RESULTS: All 10 deep non-saline RF lesions caused grade 0 injury. All seven superficial non-saline lesions caused grade 3 injury. Of the 20 superficial post-saline lesions, 13 (65%) caused grade 0 injury; four (20%), grade 1; and three (15%), grade 3. The post-saline group caused significantly less diaphragmatic injury (P <.05). CONCLUSION: Intraperitoneal saline infusion may reduce the frequency and severity of diaphragmatic injury when adjacent liver is treated with RF ablation.

Accuracy of nonfocused helical CT for the diagnosis of acute appendicitis: a 5-year review.

OBJECTIVE: The clinical usefulness of routine, nonfocused helical CT was evaluated in diagnosing acute appendicitis or providing an alternative diagnosis in patients presenting to the emergency department with acute lower abdominal pain. MATERIALS AND METHODS: We reviewed CT reports and clinical records of 650 consecutive adult patients who presented between January 1996 and December 2000 with right lower quadrant pain or lower abdominal pain and clinical findings suggestive of appendicitis. Helical CT was performed with oral contrast material in 610 cases (93.8%) and IV contrast in 572 cases (88.0%). Both vascular and enteric contrast media were administered in 544 cases (83.7%). Rectal contrast material was administered in 52 cases (8.0%). The abdomen was helically scanned from the dome of the diaphragm to the iliac crests with a collimation of 7 mm, from the iliac crests to the acetabular roof at a 5-mm collimation, from the acetabular roof to the symphysis pubis with a collimation of 5-10 mm. The surgical or clinical record was used for follow-up. RESULTS: Of the 650 patients, 552 (84.9%) had adequate clinical follow-up. There were 137 true-positive, eight false-positive, five false-negative, and 402 true-negative cases. The sensitivity, specificity, and accuracy of nonfocused helical CT were 96.5%, 98.0%, 97.6%, respectively. The positive and negative predictive values were 94.5% and 98.8%, respectively. In patients without acute appendicitis, CT suggested an alternative diagnosis, which clinically explained the patient's acute abdominal pain in 266 patients (66.2%). CONCLUSION: Nonfocused helical CT was highly accurate in diagnosing acute appendicitis or suggesting an alternative diagnosis in patients with acute lower abdominal pain or right lower quadrant pain.

Patient gender-related performance of nonfocused helical computed tomography in the diagnosis of acute appendicitis.

OBJECTIVE: To determine the impact of patient gender on the performance of helical computed tomography (CT) in the diagnosis of acute appendicitis. MATERIALS AND METHODS: From January 1, 1996 to December 31, 2000, 650 consecutive nonfocused helical abdominal CT scans were performed in adult patients presenting with acute lower abdominal pain. In general, most patients received both intravenous and oral contrast with 5-mm scan collimation through the lower abdomen and pelvis; details regarding technique and overall accuracy have been published previously. We subanalyzed results with respect to patient gender-related differences, especially in false-positive and false-negative cases. A chi2 analysis was performed to determine if significant gender-related differences were present in major or minor CT diagnostic criteria for acute appendicitis, extra-appendiceal findings, sensitivity, specificity, and accuracy. RESULTS: Of the 650 patients, 552 had adequate clinical follow-up. The sensitivity for diagnosing acute appendicitis was 100% (65/65) in men and 93.6% (74/79) in women (P < 0.05); specificity was 96.2% (130/135) in men and 98.9% (272/275) in women (P > 0.05); and accuracy was 97.5% (193/198) in men and 97.6% in women (346/354) (P > 0.05). A thickened appendix and periappendiceal stranding were seen in 92.1% of men and 84.5% of women (P = 0.15) All 5 false-negative results were in thin women. In patients without acute appendicitis, CT was able to provide a relevant alternative diagnosis in 183 of 272 women (67.3%) and 81 of 130 men (62.3%) (P > 0.05). CONCLUSION: Nonfocused helical CT was highly accurate for diagnosing acute appendicitis in both men and women, although there was a slight but significant decrease in sensitivity in thin women.

Effect of vessel size on creation of hepatic radiofrequency lesions in pigs: assessment of the "heat sink" effect.

OBJECTIVE: The effect of vessels and their size on radiofrequency lesion creation in the liver was evaluated with respect to potential for vascular injury and perfusion-mediated "heat sink" effect. SUBJECTS AND METHODS: Radiofrequency lesions targeted to tissue adjacent to a variety of vessels were created in vivo in the liver of 10 Yorkshire pigs. Postablation contrast-enhanced CT and then histopathologic analysis of the vessels and lesions were performed after sacrifice of the pigs. Degree of vascular injury and viability of perivascular hepatocytes were recorded and tabulated according to vessel size for both CT and histologic data sets. RESULTS: At CT, 42 (95%) of 44 veins greater than 3 mm remained patent, and four (20%) of 20 veins less than 3 mm were occluded. Heat sink effect, indicated by invagination of enhancing tissue between vessel and radiofrequency lesion, was observed in 32 (73%) of 44 veins greater than 3 mm and in zero of 20 veins less than 3 mm. On histopathology, 111 (100%) of 111 vessels less than 3 mm showed at least partial vessel wall injury, characterized by endothelial cell necrosis and luminal thrombus. In 24 vessels greater than 3 mm, the extent of vessel wall injury decreased with increasing vessel diameter. Viable perivascular tissue indicative of heat sink effect was identified in 12 of 24 veins greater than 3 mm, increasing to seven of seven veins greater than 5 mm. None of 96 vessels less than 2 mm and three of 111 vessels less than 3 mm showed any heat sink effect. CONCLUSION: There appears to be a narrow transition zone for hepatic vessels at 2-4 mm, beyond which the heat sink effect was seen consistently and substantial vascular injury was rare.