Efficacy and safety of Cerebrolysin after futile recanalisation therapy in patients with severe stroke.

INTRODUCTION: Golden standard of acute stroke treatment is recanalisation therapy. However, opening the occluded blood vessel sometimes does not show the expected clinical result or leads to haemorrhagic complications. As neuroinflammation and neurotoxicity play an important role in the pathophysiology of stroke, neuroprotective agents might preserve brain tissue after futile recanalisation. PATIENTS AND METHODS: After recanalisation therapy and not later than 24 h after symptoms onset, patients with initial NIHSS of ≥ 8 were assigned to the investigational and control group. The investigational group received intravenous Cerebrolysin as add-on therapy. The primary objective was to assess the clinical efficacy of Cerebrolysin. The secondary objective was to investigate its effect on haemorrhagic transition and to confirm its safety profile. RESULTS: Baseline characteristics of patients showed no significant differences between the two groups. No difference could be detected between the two groups in the mRS scale though the Cerebrolysin group showed descriptive superiority over the control group. We found a statistically significant difference considering haemorrhagic transition and mortality rate in favour of the Cerebrolysin group. DISCUSSION: The multimodal neurotrophic agent Cerebrolysin holds promise to impact on the late consequences of a reperfusion syndrome. Its influence on reducing neuroinflammation, promoting neuronal cell viability and neurogenesis as well as the stabilising effect on the blood-brain barrier suggests a protective effect on the neurovascular unit even when no recanalisation occurs. We confirmed the excellent safety profile of Cerebrolysin. CONCLUSION: Cerebrolysin as add-on therapy might be beneficial and safe for patients with acute stroke in terms of lowering risk for haemorrhagic complications after recanalisation therapy.

A simple method for evaluating stereotactic accuracy of Magnetic Resonance Imaging and Computed Tomography Imaging in Frame Based Radiosurgery.

PURPOSE: To design a simple and reproducible method with minimum uncertainty for evaluating the stereotactic accuracy of Computed Tomography imaging and Magnetic Reso-nance Imaging and to compare the results. METHODS: 3D phantom has been designed with 1 mm diameter targets in eight sectors for stereotactic MR imaging and CT imaging. The phantom was connected to Leksell stereotactic frame and the images obtained are exported to treatment planning system. Leksell stereotactic coordinates (X, Y, Z) of the holes are determined in each slice and mean maximum errors were calculated along with 3D vector distances from center of stereotactic coordinate system (100,100,100) to five known Targets. The target volumes are calculated independently for each image data sets. RESULTS: The mean of maximum absolute error estimated for MR images from the Siemens Magnetom vision MR unit were 0.46 mm (X-Axis), 1.66 mm (Y-Axis) and 2.11 mm (Z-Axis). Mean of absolute maximum error estimated using CT images from the Philips Brilliance 16 CT scanner were 0.29mm (X-axis), 0.51mm (Y-Axis) and 0.90mm (Z-Axis). 3D vector calculations were 0.38 mm for CT and 0.72 mm for MRI. No significant variation is noticed in volume calculations from both image data sets. CONCLUSION: This study showed that accuracy and quality of stereotactic CT- imaging and MR-imaging are limited by localization devices and their designs in frame based Radiosurgery. This method is simple, direct and economical with good resolution and high reproducibility. KEYWORDS: Stereotactic accuracy, Computed Tomography, Magnetic Resonance Imaging, frame based Radiosurgery.

Trends in oesophagus and Stomach cancer incidence in Bangalore, India.

BACKGROUND: During recent decades, an increase in the incidence of certain oesophago-gastric cancer has been reported in some countries and in India. This study sought to analyze oesophageal and gastric cancer incidence trends in Bangalore by sex and morphology for the period 1982-2007. PATIENTS AND METHODS: Oesophageal and gastric cancer cases were drawn from Bangalore population-based cancer registry locating in Kidwai memorial Institute of Oncology started in 1982 under national cancer Registry Programme funded by Indian Council of Medical Research. Time trends in sex- and age-standardized cancer incidence rates were analyzed by site and histology over the study period, using relative change. RESULTS: Age-standardised oesophageal cancer incidence rates increased in males, in females failed to register a significant trend over the study period. Overall, gastric cancer decreased from 9.81 and 5.48 rates per 100 000 person-years in 1982-86 to 9.45 and 5.25 in 2002-07, among men and women, respectively. Where as oesophageal adenocarcinomas increased sharply in both sex, among men, oesophageal squamous cell cancer rates increased steadily from the mid-1982s onwards a bit decline was observed from 1997, the same trend observed in females. The gastric cancer decreased over the study period. There was a marked decrease in the incidence of oesophago-gastric cancer presenting with unknown and unspecified morphology reported. KEYWORDS: adenocarcinoma, Oesophageal and stomach, incidence, age specific rate, age adjusted rate, population-based registry, trends.

Dosimetric comparison between bone marrow sparing intensity-modulated radiation therapy and conventional techniques in the treatment of cervical cancer: a retrospective study.

PURPOSE: To investigate enlargement of prostate volume by edema during brachytherapy seed implantation and develop a nomogram model to calculate air-kerma strength (AKS) required for implantation of the enlarged transient prostatic volume. MATERIALS AND METHODS: The prostate volume was measured prior and after seed implantation using trans-rectal ultrasound imaging in the operating room to obtain volume enlargement. A nomogram model was developed that calculates AKS required for implantation of the enlarged transient prostate volume with optimal dose coverage. RESULTS: The measured prostate enlargement in this study was up to 60% of the initial volume. The effective prostatic volume enlargement was calculated for three isotopes: 125I, 103Pd and 131Cs. The effective volume enlargement for 125I implants was relatively small (< 10%) because of its long half-life. For 103Pd and 131Cs with short half-lives, additional AKS up to 20% and 30%, respectively, might be required to provide appropriate dose coverage of possible enlarged prostatic volumes. CONCLUSIONS: Prostate volume enlargement should be considered to obtain optimal dose coverage particularly for short half-life isotopes such as 131Cs and 103Pd. The nomogram model developed in this work provides the AKS required for implants with a wide range of prostatic volume enlargements (5-100%) for three isotopes. KEYWORDS: prostate brachytherapy, nomogram, airkerma strength, edema, volume enlargement.

Impact of cutout off axis on electron beam dosimetric parameters.

Dosimetric changes caused by the positional uncertainty of centering a small electron cutout to the machine central axis (CAX) of the linear accelerator (linac) were investigated. Six circular cutouts with 4 cm diameter were made with their centres shifted off by 0, 2, 4, 6, 8 and 10 mm from the machine CAX. The 6 x 6 cm(2) electron applicator was used for the measurement. The percentage depth doses (PDDs) were measured at the Machine CAX and also with respect to cutout centre for 6, 9, 12, 16 and 20 MeV electron beams. The in-line and cross-line profiles were measured at the depth of maximum dose (R100). The relative output factor (ROF) was measured at the reference depth. All the measurements were made at nominal source to surface distance (100 cm SSD) as well as at extended SSDs (100, 102, 106 and 110 cm). When the cutout centre was shifted away from the machine CAX for low energy beams the depth of 100% dose (R(100)), the depth of 90% dose (R(90)) and the depth of 80% dose (R(80)) had no significant change. For higher energies (>9 MeV) there was a reduction in these dosimetric parameters. The isodose coverage of the in-line and cross-line profile was reduced when the cutout centre was shifted away from the machine CAX. At extended SSDs the dosimetric changes are only because of geometric divergence of the beam and not by the positional uncertainty of the cutout. It is important for the radiation oncologist, dosimetrist, therapist and physicist to note such dosimetric changes while using the electron beam to the patients.

Comparison of homogeneity indices for quantitative evaluation of dose homogeneity for IMRT treatments of head and neck cancers.

The purpose of this study was to evaluate different homogeneity indices for IMRT of head and neck cancers and to suggest the best representative homogeneity index for quantitative measure of dose homogeneity. In this study 3 different homogeneity indices were evaluated for 22 head and neck cancer patients receiving dynamic IMRT treatments on Clinac-DHX linear accelerator with 6 MV photon beam. IMRT planning was carried out with Helios software on Eclipse treatment planning system. H index, HI index and S index proposed by Yoon (2007) were calculated for these patients. The H-index, HI index and S index values varied between 1.024 to 1.112, 4.03 to 16.9 and 0.94 to 3.43 respectively. H index values for patient 5 and 10 are identical (H index-1.06) though different in DVH distributions but the S index values for these patients are different (1.36 and 2.01). Similarly the HI index for the patient 11 and 16 are identical in spite of different DVH distributions but S index values are different for patient 11 and 16. Since the S index represents the whole DVH curve unlike the conventional indices which depends on dose at a point, it is the better method to quantify the dose homogeneity. These results indicate that H and HI indices do not provide the accurate dose homogeneity information, but the S indices uniquely provide quantitative information about the dose homogeneity.

Peripheral dose from a dual energy linear accelerator equipped with tertiary multileaf collimators and enhanced dynamic wedge.

Peripheral dose (PD) or the dose outside the geometrical boundaries of the radiation field is of clinical importance when anatomical structures with low dose tolerances might be involved(1). It is the aim of this study is to estimate the PD on linear accelerators on different wedge systems without multileaf collimator (MLC). Measurements were performed on a dual energy linear accelerator equipped with tertiary MLC and enhanced dynamic wedge (EDW). Measurements were made using an ionization chamber embedded in a Radiation Field Analyser (RFA-300) with the secondary collimator and MLC setting of 5x5, 10x10, 15x15, and 20x20 cm2, and with the MLC fully retracted. The effects of SSD on PD were measured at three SSDs of 90, 100, and 110 cm for the irradiation fields of 5x5, 10x10, 15x15, and 20x20 cm2 and the effects of the three different wedges (Upper wedge, Lower Wedge and Enhanced Dynamic Wedge) on PD were measured for 45° wedges with field size of 15x15 cm2. Data were taken from 3 cm to 24 cm away from the field edge. Results show that due to tertiary MLC, PD can be reduced by means of a factor of two to three at certain distance from the edge of the field compared with TG-36 data. In between the wedges, the PD was less for the EDW when compared with the upper and lower physical wedges. We conclude that the reduction in PD is significant in reducing or eliminating the need for external peripheral shielding to reduce the dose on affected critical organs.

IMRT implementation and patient specific dose verification with film and ion chamber array detectors.

Implementation of Intensity Modulation Radiotherapy (IMRT) and patient dose verification was carried out with film and I'mariXX using linear accelerator with 120-leaf Millennium dynamic multileaf collimator (dMLC). The basic mechanical and electrical commissioning and quality assurance tests of linear accelerator were carried out. The leaf position accuracy and leaf position repeatability checks were performed for static MLC positions. Picket fence test and garden fence test were performed to check the stability of the dMLC and the reproducibility of the gap between leaves. The radiation checks were performed to verify the position accuracy of MLCs in the collimator system. The dMLC dosimetric checks like output stability, average leaf transmission and dosimetric leaf separation were also investigated. The variation of output with gravitation at different gantry angles was found to be within 0.9 %. The measured average leaf transmission for 6 MV was 1.6 % and 1.8% for 18 MV beam. The dosimetric leaf separation was found to be 2.2 mm and 2.3 mm for 6 MV and 18 MV beams. In order to check the consistency of the stability and the precision of the dMLC, it is necessary to carryout regular weekly and monthly checks. The dynalog files analysis for Garden fence, leaf gap width and step wedge test patterns carried out weekly were in good agreement. Pretreatment verification was performed for 50 patients with ion chamber and I'mariXX device. The variations of calculated absolute dose for all treatment fields with the ion chamber measurement were within the acceptable criterion. Treatment Planning System (TPS) calculated dose distribution pattern was comparable with the I'mariXX measured dose distribution pattern. Out of 50 patients for which the comparison was made, 36 patients were agreed with the gamma pixel match of >95% and 14 patients were with the gamma pixel match of 90-95% with the criteria of 3% delta dose (DD) and 3 mm distance-to-agreement (DTA). Commissioning and quality assurance of dMLC for IMRT application requires considerable time and effort. Many dosimetric characteristics need to be assessed carefully failing which the delivered dose will be significantly different from the planned dose. In addition to the issues discussed above we feel that individual MU check is necessary before the treatment is delivered.

Recurrent optic neuromyelitis with endocrinopathies: a new syndrome or just a coincidence?

The spectrum of optic neuromyelitis (ONM) ranges from monophasic or recurrent idiopathic forms of the disease, to ONM associated with autoimmune disorders. A distinct form of the disease, called recurrent ONM with endocrinopathies, characterized by spinal cord involvement (cavitations with syringomyeloid sensory disturbance), rapid evolution to blindness and paraplegia, characteristic cerebrospinal fluid (CSF) findings, and association with hypothalamus-pituitary dysfunction, has recently been described. The first case of ONM with endocrinopathies in a female Caucasian from Europe is presented, supporting the existence of this syndrome as a separate entity.

Forward scatter dose effect at metallic interfaces irradiated by X and gamma ray therapy beams.

AIM: In this study forward scattering effects near different metallic interfaces are measured for Co-60 gamma and 6 and 18 MV photon beams. The studied effects are the transport of secondary electrons across the metallic interface and the scattering of photons by the metallic inhomogeneity. MATERIALS AND METHODS: All measurements were carried out with a PTW thin-window, parallel plate ionisation chamber (B 23344-036) and an RDM-1F electrometer with digital readout. Thin sheets of aluminium, mild steel, copper, cadmium and lead were used as inhomogeneities. The inhomogeneities were placed between the polystyrene phantom and the front window of the chamber which was maintained at 100 cm SSD. RESULTS: It was noticed that for a high energy photon beam (18 MV) the forward scatter dose factor (FSDF) increases rapidly as the thickness of the metallic inhomogeneity increases. For low energy photons, there is a sharp initial decrease of the FSDF until a minimum value is reached followed by a slow increase with increasing thickness of the inhomogeneity. It was also noted that the FSDF variation at off-axis distances has slightly more slope compared with the ionization ratio (IR) curves for both 6 MV and 18 MV photons. However, the variation in slope is prominent for 18 MV compared with 6 MV photon beam. CONCLUSION: The sharp dose decrease observed downstream of a metallic inhomogeneity at relatively low photon energies (Co-60, 6 MV) is attributed to the internal scattering of secondary electrons within the metal. The dose enhancement observed for high energy photon beams is attributed to the domination of the pair production process, increasing with atomic number. Since FSDF is dependent on the photon beam spectra, it can be used as a measure of beam quality across the beam.

Factors influencing the evaluation of an article in cancer literature.

Casual Interpretation of results in a given article has several pitfalls. In order to assess the strengths and limitations of a given article several factors need to be considered. This review article is intended to list, classify and describe briefly the factors that should be considered in the evaluation of an article. The three primary criteria which help in proper assessment of an article are (1) Accurate and adequate description of material and methods; (2) Data on the total sample size and elimination criteria; (3) Statistical methodology employed.

Role of electron beam treatment in postoperative management of carcinoma of the breast.

The aim of post operative radiotherapy to chest wall and drainage areas in advanced breast cancer is to sterilize the possible microscopic disease. Fifty cases of post surgical breast cancers were evaluated in a prospective study of chest wall radiation therapy by electrons. Fifty women of histologically proved post operative invasive breast cancers underwent simple mastectomy with axillary dissection followed by post-operative radiotherapy (Electron) to chest wall +/- drainage area. The chest wall and IMC was treated by a direct electron beam (9/12 MeV and 12 MeV) at fixed angle. 6 MV X-ray beam was used to treat axillary apex and supraclavicular area. In some cases, supplement RT was given by 6 MV X-rays through small posterior axillary portal. The energy of electron beam was 9 MeV or 12 MeV depending on the thickness of treated area. The supraclavicular and axillary lymphatic regions were treated, by direct custom shaped anterior portal. The dose delivered was 50 Gy over 5 weeks at 2 Gy per fraction. Adjuvant chemotherapy was given to patients who were pre-menopausal with histologically proved axillary node involvement as Cyclophosphamide, Methotrexate and 5-Fluorouracil (CMF) after radiotherapy. Post menopausal patients received adjuvant tamoxifen 10 mg.bid. The overall time varied from 26 to 87 days with a median of 41.5 days (SD 11.7). The followup period varied from 4-32 months with a mean of 22 months (SD 6.4). All the fifty patients developed dense pigmentation towards the end of radiation therapy, along with patchy dry desquamation. Moist desquamation occurred in 6 patients. The median OTT in the patients who developed moist desquamation was 34.5 days. The moist reaction healed over 10-15 days after completion of radiotherapy. Lung Fibrosis (Apical lobe) occurred in four patients. The fibrosis was evident at a median of 12 months of follow up. Lymphoedema of the ipsilateral arm occurred in six patients. None of 50 patients developed late sequalae like Cardiac effects, Rib fracture, Brachial Plexopathy, Shoulder joint impairement and Soft tissue effects. Local control in this study was 96% (48 patients). The two year actuarial overall survival was 80%. In early stages survival was significantly better compared to late stages. The actuarial two year DFS was 72%. In early stages DFS was significantly better compared to late stages.

Prognostic factors for spinal cord myelitis--an analysis of compiled literature data.

The effectiveness of radiotherapy in the treatment of malignant tumors in the head and neck, thorasic and upper abdominal regions is frequently limited by the tolerance of the spinal cord. Therefore knowledge of the factors that influence the tolerance of the spinal cord to radiation is of the upmost important. Safe limits for irradiation of the spinal cord determined in a number of clinical studies is on the conservative side from the point of view of tumor control probability. The bioeffect of a physical dose takes into account the treatment variables and the radiobiological characteristics of the relevant tissue; hence deciding spinal cord tolerance on the basis of bioeffect models would be better approach. The aim of the present study was to analyze the relationship of the prognostic factors with percentage incidence of spinal cord myelitis and to arrive at a spinal cord tolerance bioeffect dose TDF and ERD for optimum incidence of radiation myelitis.

Serum tumor marker CYFRA 21-1 in the diagnostics of NSCLC lung cancer.

Cytokeratins are epithelial markers whose expression is not lost during malignant transformation. The level of soluble cytokeratin fragment 19 was measured with an enzyme immunoassay method developed by Boehringer Mannheim (Enzymum-test CYFRA 21-1) in the serum of 200 male and 50 female patients with NSCLC (Non Small Cell Lung Cancer) lung cancer (120 planocellulare and 80 adenocarcinoma in males; 22 planocellulare and 28 adenocarcinoma in females). The comparative group comprised 50 young healthy males and 50 females without any clinical proof for malignancy or any other lung disease. The aim of this investigation was to find out if any possible statistical difference exists in the serum level of CYFRA 21-1 between patients with lung cancer and healthy controls, and also between different types of lung cancers. The mean value of serum CYFRA 21-1 in NSCLC (6.25 ng/ml) was significantly higher than in healthy controls (1.26 ng/ml) (p < 0.001). Sensitivity for CYFRA 21-1 (using 3.3 ng/ml, a cut-off value corresponding to a 98% specificity for healthy controls) in NSCLC was 60.5%. Positive CYFRA 21-1 levels were significantly higher in patient with carcinoma planocellulare (66.2%) than in adenocarcinoma (52.1%). CYFRA 21-1 levels were significantly different between squamous cell carcinoma (6.52 ng/ml) and adenocarcinoma (5.86 ng/ml) (p < 0.05). Our results indicate that CYFRA 21-1 may be a useful tumor marker in NSCLC, especially in carcinoma planocellulare. CYFRA 21-1 may also be useful in identification of the preoperative stages of diseases and the postoperative monitoring of NSCLC.

Analysis of the quantitative dermatoglyphics of the digito-palmar complex in patients with multiple sclerosis.

Recent studies on the etiopathogenesis of multiple sclerosis (MS) all point out that there is a polygenetical predisposition for this illness. The so called "MS Trait" determines the reactivity of the immunological system upon ecological factors. The development of the glyphological science and the study of the characteristics of the digito-palmar dermatoglyphic complex (for which it was established that they are polygenetically determined characteristics) all enable a better insight into the genetic development during early embriogenesis. The aim of this study was to estimate certain differences in the dermatoglyphics of digito-palmar complexes between the group with multiple sclerosis and the comparable, phenotypically healthy groups of both sexes. This study is based on the analysis of 18 quantitative characteristics of the digito-palmar complex in 125 patients with multiple sclerosis (41 males and 84 females) in comparison to a group of 400 phenotypically healthy patients (200 males and 200 females). The conducted analysis pointed towards a statistically significant decrease of the number of digital and palmar ridges, as well as with lower values of atd angles in a group of MS patients of both sexes. The main discriminators were the characteristic palmar dermatoglyphics with the possibility that the discriminate analysis classifies over 80% of the examinees which exceeds the statistical significance. The results of this study suggest a possible discrimination of patients with MS and the phenotypically health population through the analysis of the dermatoglyphic status, and therefore the possibility that multiple sclerosis is genetically predisposed disease.

Extrapolated response dose as a potential tool in radiotherapy and chemo-radiotherapy of head and neck cancers.

An analysis of head and neck cancer patients treated by radiotherapy (RT) alone (114 patients) and by chemo-radiotherapy (RT + CT) (115 patients) was carried out; the doses varied from 40-77 Gy and 35-71 Gy in RT and RT + CT groups respectively. The chemotherapy (CT) (induction/concurrent) drugs used were 5-FU, cisplatin, methotrexate either single or in combination. Extrapolated response dose values were evaluated with alpha/beta values of 10, 2.5 and 6 Gy for acute, late complications and tumour response, respectively. Dose enhancement factor (DEF) and Therapeutic gain factor (TGF) values were evaluated on the basis of ERD for patients receiving 5-FU RTCT (72 patients). ERD vs late complication rate and response rate curves were drawn for RT, RT + CT (< 7 cycles), RT + CT (> 6 cycles) and RT + CT (cumulative). DEF values for response rate were 0.95, 0.95 and 0.82 for the three RT + CT groups respectively. Similarly DEF values for late complication rate were evaluated as 0.87, 0.93 and 0.88. TGF values for RT + CT were 1.09, 1.02 and 0.93. TGF values indicated lack of significant influence of CT on clinical outcome. The correlation of ERD with late complication, response and status at last follow up (NED) was statistically significant for both groups (P < 0.01). ERD did not correlated with acute complication in RT group (P > 0.01). From the present analysis, in RT + CT treatments of head and neck cancers, an ERD value of 69 Gy is suggested as the limit for an acceptable 5% late complication rate.

Many component model and its parameters to fractionated irradiation.

PURPOSE: In this study, a many component model has been formulated to address the contribution of various bio-molecules in radiation induced biological damage and this model is fitted to radiation isoeffect data to evaluate the parameter alpha/beta. MATERIAL AND METHODS: Every living cell contains a number of organic and inorganic molecules which may act as targets to lead radiation induced damage. During irradiation, many kinds of physical, chemical and biological products are produced which are due to active participation of aforesaid molecules. The many component model, hereinafter called linear-quadratic-cubic (LQC) model, is an attempt to provide a better mathematical formulation which could demonstrate the contribution of various kinds of biomolecules in biological effect of radiation. This model has only one independent parameter alpha/beta and a number of dependent parameters which are the derivatives of alpha/beta. RESULTS: Three methods, for testing the fit of the proposed model and deriving the values of alpha/beta, are employed using multifractionation isoeffect data. These methods, i.e. (logDm-logDn) versus (dn-dm) plot, SFe-plot and logD versus d plot, provide their respective values of alpha/beta as 12.50 Gy, 12.51 Gy and 12.54 Gy for spleen. 7.41 Gy, 7.79 Gy and 7.42 Gy for kidney, 15.87 Gy, 15.09 Gy and 14.69 Gy for colon with 3 h interval, 12.50 Gy, 12.84 Gy and 12.45 Gy for colon with 12 h interval and 8.47 Gy, 8.84 Gy and 8.51 Gy for colon with 24 h interval. Since other parameters of the model are the derivatives of alpha/beta therefore can be derived using the values of alpha/beta. DISCUSSION: Graphical representation of afore said sets of data show that the plotted points have a certain amount of scattering which is not at the wider scale and can be well or less well described by a straight line. CONCLUSION: This model eliminates the shortcomings of LQ model and provides a satisfactory explanation of various radiobiological processes occurring during and after irradiation.

Physical characteristics of photon and electron beams from a dual energy linear accelerator.

Clinically pertinent data of the photon beams of nominal energy 6 MV and 18 MV from a dual photon energy linear accelerator are measured using a radiation field analyser with semiconductor diodes and ionisation chambers. Percentage depth dose values are compared with available data from other linacs and BJR-17. Measurements made in the buildup region using a parallel plate chamber show a marked increase in the surface dose and dmax shifts by 3 mm and 14 mm for 6 MV and 18 MV photons, respectively as the field size is increased from 4 x 4 cm2 to the maximum. Variations of wedge angles with energy and field size are also determined up to wedge widths of 15 cm for all the four available wedge angles. Isodose plots of both the energies are plotted using semiconductor diodes in a RFA-3 system. Output factors, wedge transmission factors, and shielding tray factors were also measured in clear polystyrene phantom for both the photon energies at the depths of maximum ionisation. Beam characteristics of the electron beams of energies 6, 9, 12, 16, and 20 MeV produced by the Clinac-1800 have also been studied. The characteristics include percentage depth dose, isodose distribution, depth of maximum dose, surface dose, photon contamination, uniformity index, and penumbra. Most of the measurements were carried out using semiconductor detectors, whereas small volume ionization chambers and a plane-parallel chamber were kept as standards for comparison. Isodose distributions were drawn from the film densitometry method. Range-energy parameters are obtained from the observed depth dose data. These parameters vary from machine to machine and must be ascertained for individual units. The parameters differ to a considerable extent from their theoretically predicted values but generally follow the trend, experimentally observed by others, for similar types of units.

Dosimetric accuracy of a dual photon energy linac at low monitor setting for various pulse repetition frequencies.

Accuracy of dose delivery at low monitor unit setting is studied for a dual photon energy linear accelerator. Dose delivered per MU is found to be constant for both the photon beams for MU settings above 30. For lower MUs there is definite deviation from the calibrated value and the error is found to be increasing as fewer MUs are set for dose delivery. This dose/MU ratio at low MU setting is found to be dose-rate dependent, showing an increasing trend with pulse repetition frequency (PRF). Also, the dosimetric ratio is observed to be mode dependent; its value for an 18 MV beam is almost double that observed in the case of a 6 MV beam at very low MU setting. The magnitude of this error should be determined for each energy so that appropriate corrections can be applied if very low MUs are to be used.