Manickam Ravikumar

Backscattered dose perturbation effects at metallic interfaces irradiated by high-energy X- and gamma-ray therapeutic beams.

Purpose:To analyze backscattered dose enhancements near different metallic interfaces for cobalt-60 (60Co) gamma rays and 6- and 18-MV photon beams.Material and Methods:Measurements were carried out with a PTW thin-window, parallel-plate ionization chamber and an RDM-1F electrometer. Thin sheets of aluminum, mild steel, copper, cadmium and lead were used as inhomogeneities. The chamber was positioned below the inhomogenities with the gantry maintained under the couch.Results:It can be noticed that the backscatter dose factor (BSDF) reaches the saturation value within few millimeters of all inhomogeneities and the thickness at which the saturation value is reached depends on the atomic number of the inhomogeneity. The amount of backscattered radiation was noticed to be greater with lesser-energy photons (60Co) compared to the higherenergy photons. The BSDF varies across the beam when the inhomogeneity is present due to the change in beam quality. The backscattered electrons from lead inhomogeneity have a range in the order of 5–7 mm.Conclusion:Higher atomic number inhomogeneities result in an increase in BSDF, as they have higher scattering cross section for the secondary electrons. The increase in dose was noticed for few millimeters upstream from the metallic inhomogeneity, which suggests that the range of backscattered electrons is very small. Since the factors affecting the BSDF at the interface are energydependent, it is expected that the variation in BSDF will also be sensitive to the beam energy.Ziel:Analyse der Verstärkung der Rückstreudosis an unterschiedlichen metallischen Grenzflächen für Kobalt-60-(60Co-)Gamma-Strahlen sowie für 6- und 18MV-Photonen-Strahlung.Material und Methode:Die Messungen wurden in einer dünnwandigen PTW-Parallelplatten-Ionisationskammer mit einem RDM- 1F-Elektrometer durchgeführt. Dünne Platten aus Aluminium, Weichstahl, Kupfer, Cadmium und Blei dienten als Grenzflächen. Die Ionisationskammer wurde unterhalb der Grenzflächen platziert, wobei sich die Gantry unter der Liegefläche befand.Ergebnisse:Es wurde festgestellt, dass der Rückstreudosisfaktor (BSDF) seinen Sättigungswert innerhalb weniger Millimeter aller Grenzflächenmaterialien erreicht, wobei die Dicke, bei der der Sättigungswert erreicht wird, von der Kernladungszahl des Grenzflächenmaterials abhängt. Weiter wurde festgestellt, dass der Anteil rückgestreuter Strahlung bei weniger energiereichen Photonen (60Co) geringer ausfällt im Vergleich zu höherenergetischen Photonen. Der BSDF variiert in Gegenwart der Grenzfläche innerhalb des Strahlenbündels aufgrund von Veränderungen der Strahlenqualität. Die Elektronen, die von der Blei-Grenzfläche rückgestreut werden, haben eine Reichweite von 5–7 mm.Schlussfolgerung:Grenzflächenmaterialien mit höherer Kernladungszahl bewirken eine Zunahme des BSDF, da sie für die Sekundärelektronen einen höheren Streuquerschnitt aufweisen. Die Dosiszunahme wurde wenige Millimeter vor der metallischen Grenzfläche festgestellt, was auf eine sehr geringe Reichweite der rückgestreuten Elektronen hinweist. Da die Faktoren, die den BSDF an der Grenzfläche beeinflussen, energieabhängig sind, ist zu erwarten, dass die Variabilität des BSDF auch auf die Strahlungsenergie reagiert.

Dosimetric comparison of various optimization techniques for high dose rate brachytherapy of interstitial cervix implants.

HDR brachytherapy treatment planning often involves optimization methods to calculate the dwell times and dwell positions of the radioactive source along specified afterloading catheters. The purpose of this study is to compare the dose distribution obtained with geometric optimization (GO) and volume optimization (VO) combined with isodose reshaping. This is a retrospective study of 10 cervix HDR interstitial brachytherapy implants planned using geometric optimization and treated with a dose of 6 Gy per fraction. Four treatment optimization plans were compared: geometric optimization (GO), volume optimization (VO), geometric optimization followed by isodose reshape (GO_IsoR), and volume optimization followed by isodose reshape (VO_IsoR). Dose volume histogram (DVH) was analyzed and the four plans were evaluated based on the dosimetric parameters: target coverage (V100), conformal index (COIN), homogeneity index (HI), dose nonuniformity ratio (DNR) and natural dose ratio (NDR). Good target coverage by the prescription dose was achieved with GO_IsoR (mean V100 of 88.11%), with 150% and 200% of the target volume receiving 32.0% and 10.4% of prescription dose, respectively. Slightly lower target coverage was achieved with VO_IsoR plans (mean V100 of 86.11%) with a significant reduction in the tumor volume receiving high dose (mean V150 of 28.29% and mean V200 of 7.3%). Conformity and homogeneity were good with VO_IsoR (mean COIN=0.75 and mean HI=0.58) as compared to the other optimization techniques. VO_IsoR plans are superior in sparing the normal structures while also providing better conformity and homogeneity to the target. Clinically acceptable plans can be obtained by isodose reshaping provided the isodose lines are dragged carefully. PACS number: 87.53 Bn

Dose delivery accuracy of therapeutic photon and electron beams at low monitor unit settings.

Purpose:Dose delivery accuracy at low monitor units (LMU) was evaluated for photon and electron beams. Knowledge of this study is required for few dosimetric applications and to know the dose delivered to the patient when the treatment is delivered with few monitor units (MU).Material and Methods:Dose measurements were carried out for photon and electron beams with 0.6 cm3 PTW ion chamber in white polystyrene phantom at Dmax with a field size of 10 × 10 cm2 at 100 cm FSD. The relative dose, which is the ratio of dose delivered per MU at the testing to that of the calibration condition, was found out.Results:Significant deviation (+20% to +25%) in dose delivery was noticed for photon and electron beams (+39% to +45%) at LMU settings. Slightly higher inaccuracy in dose delivery was noticed for 6–MV compared to 18–MV photons. The deviation in dose delivery for electron beams was found to be energy–independent and the pattern of variation was similar for all electron energies.Conclusion:The dose delivery accuracy at LMU settings has to be ascertained before implementing conformal and IMRT (intensity– modulated radiotherapy) techniques. When there is dose nonlinearity, the treatment delivered with multiple small MU settings can result in significant error in dose delivery.Ziel:Für Photonen– und Elektronenstrahlung wurde die Abhängigkeit der Output–Faktoren von den Monitoreinheiten gemessen. Relevant sind die Ergebnisse dieser Untersuchung für einige dosimetrische Anwendungen und zur Ermittlung der Referenzdosis, wenn mit wenigen Monitorwerten (MU) bestrahlt wird.Material und Methode:Die Output–Faktoren wurden mit einer 0,6–cm3–Ionisationskammer, PTW, jeweils in Maximumstiefe mit einem weißen Polystyrol–Phantom ermittelt (Feldgröße 10 × 10 cm2, Fokus–Oberflächen–Abstand 100 cm). Das Verhältnis der Output–Faktoren bei verschiedenen Monitoreinheiten zu den Standardwerten bei 200 MU wurde als relative Dosis definiert.Ergebnisse:Bei niedrigen Monitoreinheiten fanden sich signifikante Abweichungen der Output–Faktoren von den Standardwerten (Photonen: 20–25%; Elektronen: 39–45%). Die Abweichungen bei 6–MV–Photonen war geringfügig höher als bei 18 MV, während die Abweichungen bei Elektronen keine Energieabhängigkeit zeigten.Schlussfolgerung:Die Konstanz der Outputfaktoren bzw. deren Abhängigkeit von den Monitorwerten muss vor der Implementierung von IMRT–Techniken sichergestellt werden. Stark abweichende Output–Faktoren können signifikante Abweichungen der Dosis im Patienten zur Folge haben, wenn mehrere Subfelder mit niederen Monitorwerten im Spiel sind.

Dose Measurements in the Build-Up Region for the Photon Beams from Clinac-1800 Dual Energy Medical Linear Accelerator

Aim: Since the skin dose becomes the limiting factor while deciding the tumorcidal dose, the detailed analysis of dose distribution in the build-up region is necessary for high-energy photon beams. In this study the beam characteriwstics affecting the build-up and skin dose for 6- and 18-MV photons are analyzed. Materials and Methods: Measurements were made with 6- and 18-MV photons using a PTW parallel-plate ionization chamber (B23344-036) and a RDM-1F electrometer. Build-up ionization measurements were made with the chamber fitted into a 25 × 25 × 25 cm polystyrene phantom with a fixed SSD of 100 cm. The entrance and buildup dose measurements were made with a polycarbonate and a mesh type metallic shielding tray and a 45° wedge. Exit dose measurements were carried out for the graphite patient supporting assembly table top, 1.0 cm thick piece of wood and the 1.0 cm thick patient supporting perspex base frame for head and neck treatments. Results: It was observed that the dmax decreased slightly with field size as with other accelerators. For both photon energies the surface dose was observed to increase with increase in field size. It was also noticed that the dose in the build-up region increases slightly when the polycarbonate secondary blocking tray is introduced with the increase in surface dose. The data show that the tray pertubation factor (TPF) at surface decreases steadily with tray-surface distance for both photon beams for all field sizes. It was noted that the TPF was more when the polycarbonate tray was introduced at shorter tray-surface distances for both energies. At tray-surface distances above 60 cm the TPF almost remained close to unity for 6-MV photons for all field sizes, whereas the continuous decrease in TPF could be noted for 18-MV photon beams even after the TPF rewached unity. Conclusion: The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons siginificantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.Fragestellung: Die applizierte Dosis im Tumorzielvolumen wird entscheidend von der Hautdosis begrenzt. Daher ist für ultraharte Photonenstrahlung eine detaillierte Analyse der Dosisverteilung notwendig. In dieser Studie werden die Parameter der klinischen Photonenstrahlen, die die Hautdosis und die Aufbauregion bestimmen, für Photonenenergien von 6 MV und 18 MV vorgestellt und analysiert. Material und Methoden: Die Messungen wurden für Photonen der Energie von 6 MV und 18 MV unter Verwendung einer Ionisationskammer der PTW (Typ B 23344-036, Flachkammer) und eines Elektrometers (Typ RDM-1F) durchgeführt. Für die Dosismessung in der Aufbauregion wurde die Ionisationskammer in ein Polystyrenphantom mit einem definierten Volumen von 25 × 25 × 25 cm und einem festen Fokus-Oberflächen-Abstand von 100 cm eingepasst. Die Eintrittsdosis und die Dosis in der Aufbauregion wurden für zwei Blockträger gemessen, die aus Polycarbonat und aus einem Metallgitter bestanden. Die Messung der Austrittsdosis wurde an drei verschiedenen Materialien ausgeführt, bestehend aus einer Graphitunterlage für Patienten, aus einem 1 cm starken Stück Holz und aus einem 1 cm starken Rahmen aus Perspex für Kopf-Hals-Behandlungen. Ergebnisse: Der geometrische Abstand dmax (des Maximums der Tiefendosis) verringerte sich leicht mit der Feldgröße, wie das auch bei anderen Beschleunigern beobachtet wird. Die Oberflächendosis wurde mit wachsender Feldgröße für beide Photonenenergien größer. Weiterhin wurde festgestellt, dass die Dosis in der Aufbauregion und die Oberflächendosis beim Blockträger aus Polycarbonat gegenüber dem Blockträger aus einem Metallgitter und dem offenen Feld leicht anstieg. Die Analyse der Daten zeigt, dass der Trägerstreufaktor (tray perturbation factor, TPF) an der Oberfläche mit zunehmendem Träger-Oberflächen-Abstand für beide Photonenenergien und alle benutzten Feldgrößen kleiner wurde. Der TPF war auch, wiederum für beide Photonenenergien, im Fall des Blockträgers aus Polycarbonat bei geringerem Träger-Oberflächen-Abstand vergleichsweise größer. Für Abstände größer 60 cm blieb der TPF-Wert für 6 MV und für alle benutzten Feldgrößen nahe 1. Im Gegensatz dazuz nahm der TPF bei 18 MV sogar bis unterhalb einem Wert von 1 ab. Schlußfolgerung: Die Zunahme der Oberflächendosis mit zunehmender Feldgröße entsteht für beide Photonenenergien durch die (Compton-)Elektronenstreuung der im Strahlengang liegenden Materialien. Da die Keilfilter vornehmlich gestreute niederenergetische Elektronen absorbieren, nimmt in diesem Fall die relative Oberflächendosis immer Werte unter 1 an. Die relative Zunahme der Dosisverstärkung bei der Graphitunterlage gegenüber der Perspexunterlage deutet darauf hin, dass die Rückstreuung der Elektronen des jeweiligen Mediums proportional zur Ordnungszahl des Mediums ist.

Electron beam characteristics at extended source-to-surface distances for irregular cut-outs

Electron beam therapy is widely used in the management of cancers. The rapid dose fall-off and the short range of an electron beam enable the treatment of lesions close to the surface, while sparing the underlying tissues. In an extended source-to-surface (SSD) treatment with irregular field sizes defined by cerrobend cutouts, underdosage of the lateral tissue may occur due to reduced beam flatness and uniformity. To study the changes in the beam characteristics, the depth dose, beam profile, and isodose distributions were measured at different SSDs for regular 10 × 10 cm2 and 15 × 15 cm2 cone, and for irregular cutouts of field size 6.5 × 9 cm2 and 11.5 × 15 cm2 for beam energies ranging from 6 to 20 MeV. The PDD, beam flatness, symmetry and uniformity index were compared. For lower energy (6 MeV), there was no change in the depth of maximum dose (R100) as SSD increased, but for higher energy (20 MeV), the R100 depth increased from 2 cm to 3 cm as SSD increased. This shows that as SSD increases there is an increase in the depth of the maximum dose for higher energy beams. There is a +7 mm shift in the R100 depth when compared with regular and irregular field sizes. The symmetry was found to be within limits for all the field sizes as the treatment distance extended as per International Electro technical Commision (IEC) protocol. There was a loss of beam flatness for irregular fields and it was more pronounced for lower energies as compared with higher energies, so that the clinically useful isodose level (80% and 90%) width decreases with increase in SSD. This suggests that target coverage at extended SSD with irregular cut-outs may be inadequate unless relatively large fields are used.

Measurement of backscattered dose at metallic interfaces using high energy electron beams

Summary Background The amount of backscattered electrons depends on the thickness of the backscattering metal. The electron backscatter increases with the increase in thickness of the metal until a saturation level is reached and thereafter no change in scatter enhancement is noticed. Aim Electron backscatter effects at metallic interfaces were analysed in this study. High energy electron beams ranging from 6 to 20 MeV were used. Materials/Methods Measurements were carried out with a PTW thin-window parallel plate ionization chamber and an RDM-1F electrometer. Thin sheets of aluminium, copper and lead were used as inhomogeneities. The chamber was positioned below the inhomogeneities with the gantry maintained under the couch. Results The electron backscatter factor (EBSF) increases with increase in energy for aluminium, copper and lead. With low atomic number materials EBSF increases with increase in scatterer thickness and for lead it attains saturation within a few millimetres. Conclusions The information from this study could be useful in predicting the increase in dose at the metal-tissue interface due to electron backscatter.

Dosimetric comparison of high dose rate brachytherapy and intensity-modulated radiation therapy for cervical carcinoma.

Intracavitary brachytherapy is an integral part of radiotherapy for locally advanced gynecologic malignancies. A dosimetric intercomparison of high dose rate intracavitary brachytherapy (HDR_BT) and intensity-modulated radiotherapy in cervical carcinoma has been made in the present study. CT scan images of 10 patients treated with HDR_BT were used for this study. A sliding-window IMRT (IMRT_SW) and step-and-shoot IMRT plans were generated using 6-MV X-rays. The cumulative dose volume histograms of target, bladder, rectum and normal tissue were analyzed for both techniques and dose distributions were compared. It was seen that the pear-shaped dose distribution characteristic of intracavitary brachytherapy with sharp dose fall-off outside the target could be achieved with IMRT. The integral dose to planning target volume was significantly higher with HDR_BT in comparison with IMRT. Significant differences between the two techniques were seen for doses to 1 cc and 2 cc of rectum, while the differences in 1 cc and 2 cc doses to bladder were not significant. The integral doses to the nontarget critical and normal structures were smaller with HDR_BT and with IMRT. It is concluded that IMRT can be the choice of treatment in case of non-availability of HDR brachytherapy facilities or when noninvasive treatments are preferred

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 mor 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 energy (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.Ziel: Messung der Vorwärtsstreueffekte von Co-60-Gamma- sowie 6- und 18-MV-Photonenstrahlen in der Umgebung unterschiedlicher Metallgrenzflächen. Untersuchung des Transports von Sekundärelektronen durch die Metallgrenzfläche und die Streuung von Photonen infolge der Metallinhomogenität. Material und Methode: Alle Messungen wurden mit einer PTW-Parallelplatten-Ionisationskammer (B 23344-036) und einem RDM-1F-Elektrometer mit Digitalanzeige durchgeführt. Es wurden als Grenzflächen dünne Platten aus Aluminium, Weicheisen, Kupfer, Kadmium und Blei eingesetzt. Die Metallplatten wurden platziert zwischen dem Polystyrolphantom und der Vorderwand der Ionisationskammer, die sich in einem Fokus-Haut-Abstand von 100 cm befand. Ergebnisse: Für 18-MV-Photonenstrahlen wurde ein mit der Dicke der Metallplatte stark zunehmender Vorwärtsstreudosisfaktor (FSDF) festgestellt. Bei Photonenstrahlen niedrigerer Energie kam es initial zu einem steilen Abfallen des Vorwärtsstreudosisfaktors bis zu einem Minimalwert, der mit zunehmender Stärke der Metallplatte langsam wieder anstieg. Außerdem zeigten sich sowohl für 6- als auch für 18-MV-Photonen bei lateralem Achsabstand (off-axis) für die FSDF-Variabilität geringfügig steilere Kurven im Vergleich zu dem Effekt im Zentralstrahl. Dabei war die Veränderung der Kurvensteigung ausgeprägter bei 18-MV- als beim 6-MV-Photonenstrahl. Schlussfolgerung: Der starke Abfall der Vorwärtsstreudosis infolge einer Metallgrenzfläche bei relativ niedriger Photonenenergie (Co-60, 6 MV) wird der Streuung von Sekundärelektronen innerhalb des Metalls zugeschrieben. Die beobachtete Steigung der Dosis bei hoher Photonenenergie wird mit der Zunahme des Paarbildungseffekts erklärt, der proportional zur Kernladungszahl steigt. Da der Vorwärtsstreudosisfaktor von den Photonenspektren abhängt, kann er als Maß der Photonenstrahlqualität in Abhängigkeit vom Divergenzwinkel betrachtet werden.

Analysis of small field percent depth dose and profiles: Comparison of measurements with various detectors and effects of detector orientation with different jaw settings

The advent of modern technologies in radiotherapy poses an increased challenge in the determination of dosimetric parameters of small fields that exhibit a high degree of uncertainty. Percent depth dose and beam profiles were acquired using different detectors in two different orientations. The parameters such as relative surface dose (DS), depth of dose maximum (Dmax), percentage dose at 10 cm (D10), penumbral width, flatness, and symmetry were evaluated with different detectors. The dosimetric data were acquired for fields defined by jaws alone, multileaf collimator (MLC) alone, and by MLC while the jaws were positioned at 0, 0.25, 0.5, and 1.0 cm away from MLC leaf-end using a Varian linear accelerator with 6 MV photon beam. The accuracy in the measurement of dosimetric parameters with various detectors for three different field definitions was evaluated. The relative DS(38.1%) with photon field diode in parallel orientation was higher than electron field diode (EFD) (27.9%) values for 1 cm ×1 cm field. An overestimation of 5.7% and 8.6% in D10depth were observed for 1 cm ×1 cm field with RK ion chamber in parallel and perpendicular orientation, respectively, for the fields defined by MLC while jaw positioned at the edge of the field when compared to EFD values in parallel orientation. For this field definition, the in-plane penumbral widths obtained with ion chamber in parallel and perpendicular orientation were 3.9 mm, 5.6 mm for 1 cm ×1 cm field, respectively. Among all detectors used in the study, the unshielded diodes were found to be an appropriate choice of detector for the measurement of beam parameters in small fields.

Implementation and validation of a commercial portal dosimetry software for intensity-modulated radiation therapy pre-treatment verification.

Electronic portal imaging devices (EPIDs) are extensively used for obtaining dosimetric information of pre-treatment field verification and in-vivo dosimetry for intensity-modulated radiotherapy (IMRT). In the present study, we have implemented the newly developed portal dosimetry software using independent dose prediction algorithm EPIDose™ and evaluated this new tool for the pre-treatment IMRT plan quality assurance of Whole Pelvis with Simultaneous Integrated Boost (WP-SIB-IMRT) of prostate cases by comparing with routine two-dimensional (2D) array detector system (MapCHECK™). We have investigated 104 split fields using γ -distributions in terms of predefined γ frequency parameters. The mean γ values are found to be 0.42 (SD: 0.06) and 0.44 (SD: 0.06) for the EPIDose and MapCHECK™, respectively. The average γ∆ for EPIDose and MapCHECK™ are found as 0.51 (SD: 0.06) and 0.53 (SD: 0.07), respectively. Furthermore, the percentage of points with γ < 1, γ < 1.5, and γ > 2 are 97.4%, 99.3%, and 0.56%, respectively for EPIDose and 96.4%, 99.0% and 0.62% for MapCHECK™. Based on our results obtained with EPIDose and strong agreement with MapCHECK™, we may conclude that the EPIDose portal dosimetry system has been successfully implemented and validated with our routine 2D array detector