Viswanathan Ps

Dosimetry of intracavitary applications in carcinoma of the cervix: rectal dose analysis

The International Commission on Radiation Units and Measurements (ICRU), in its Report-38, has given certain recommendations regarding the specifications of bladder and rectal reference points in the intracavitary treatment of carcinoma of the cervix. Conforming to this report, 182 intracavitary applications performed in stage I and II cervix cases were analyzed. In 113 applications, the maximum rectal dose was found to be in a point on the anteroposterior line drawn through the centre of colpostat sources. However, for our type of applications, the point on the anteroposterior line drawn through the lower end of the uterine tube seldom gets maximum dose. In addition, it was observed that there are other dose points than the ICRU reference point receiving doses close to maximum dose. It was concluded that doses to multiple rectal reference points should be recorded, in addition to the ICRU-defined rectal reference point.

Predictive role of tdf values in late rectal recto-sigmoid complications in irradiation treatment of cervix cancer

Radiation-induced late rectal and recto-sigmoid complications for different doses per fraction were analyzed retrospectively in 203 cases of Stage IIIB carcinoma of the uterine cervix. The patients were treated with a combination of external irradiation and a single intracavitary insertion during January 1979 to December 1983. The external irradiation was randomised to deliver by four different fractionation regimens having dose per fraction of 2 Gy, 3 Gy, 4 Gy and 5.4 Gy. The doses for various fractionations were matched with the daily regimen using the time-dose factors (TDF) model. A single intracavitary insertion delivered a dose of 22 to 24 Gy to point A using Fletcher-Suit applicator. All patients had a minimum follow-up of 30 months. Thirty nine cases of late radiation induced rectal and recto-sigmoid complications were observed. The complication rate of 8.2% for daily treatment regimen delivering 2 Gy per fraction was increased to 33.3% for once weekly treatment regimen delivering 5.4 Gy per fraction (p = 0.041). Despite similar Time-Dose Factors (TDF) values in four different treatment regimens, the complication rate increased significantly in the once weekly regimen. The analysis suggest that the Time-Dose Factors (TDF) values do not predict correctly the late normal tissue reactions for different dose fractionation schedules.

Dosimetric aspects of physical and dynamic wedge of clinac 2100C linear accelerator

AimTo investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation.Material and MethodsA computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer.ResultsA systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for isodoses of both types of wedges.ConclusionThe dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.ZusammenfassungZielEs werden die Keilfilterfaktoren für physikalische und dynamische Keilfilter des Clinac 2100C in Abhängigkeit von Feldgröße und Phantomtiefe untersucht.Material und Methode nDie Dosisprofile der physikalischen Keile werden in einem automatischen Wasserphantom erfaßt, während eine Ionisationskammer von 0,6 cm3 sowohl für die physikalischen als auch die dynamischen Keile in Einsatz kommt. Es ist eine Methode, die es erlaubt, dynamische Keile auch ohne Linear-Array-Detektoren oder Filmdosimetrie zu dosimetrieren.ErgebnisseFür die physikalischen Keile ist eine systematische Abhängigkeit der Keilfaktoren von Keilwinkel und Phantomtiefe, aber nicht von der Feldgröße zu beobachten. Dynamische Keile dagegen zeigen eine starke Abhängigkeit der Keilfaktoren von der Feldgröße, während keine signifikante Abhängigkeit von der Phantomtiefe für verschiedene Keilwinkel zu beobachten ist. Die Feldgrößenabhängigkeit ist nicht systematisch, da die Dynamik der Keile aufgrund segmentierter Tabellen für die Output-Faktoren, die die Feldgrößenunabhängigkeit enthalten, gesteuert wird. Die Handhabung der Daten mit einem kommerziellen Planungssystem wird diskutiert, und es werden die Isodosen beider Keilfilterarten verglichen.SchlußfolgerungDie Isodosenverteilungen unter dynamischen Keilen zeigen viel geradere Flanken als jene unter physikalischen Keilen. Dadurch ergben sich höhere Maxima (hot spots) hinter den dünnen Enden, was bei der Planung sorgfältig bedacht werden muß.

Treatment volume from Total Reference Air Kerma (TRAK) in intracavitory applications and its comparison with ICRU reference volume

PURPOSE A systematic dosimetry study conforming to International Commission on Radiation Units and Measurements Report Number 38, was carried out for, cancer of Cervix Stage I and II cases at Tata Memorial Hospital. METHODS AND MATERIALS The reference Volume dimension were noted for each applicator. Also 3-D dosimetry was performed for selected cases, and actual geometric volume enclosed by various isodose surfaces were obtained. A relation was derived, which gave the volume enclosed by any isodose surface as a function of total reference air kerma of the sources in the applicator. The volume enclosed by reference isodose surface was compared with the product of its dimensions height, width, and thickness. RESULTS The volume enclosed by any isodose surface of an intracavitary application can be easily derived from total reference air kerma of the sources within the applicator. CONCLUSION Ratio of the volume enclosed by reference isodose surface with the product of its dimensions (HWT) could be a predictive parameter for clinical outcome, besides the reference volume specified by the report.

A concave tray with divergent block for desired dose distribution around shielded region in megavoltage radiation therapy

In clinical situations large numbers of rectangular divergent blocks are required at different off-axis positions from central axis to shield sensitive organs. Superiority of divergent blocks over non-divergent blocks on a flat tray lies in improving the dose homogeneity. A method has been developed to minimize the number of divergent shielding blocks by using a concave tray at the collimator. The results of use of a single divergent block are similar to that of the use of several divergent blocks, each to match at different off-axis positions in the radiation beam. This type of single concave tray thus provides a considerable ease of handling divergent blocks in a busy radiotherapy centre.