K.M. Ganesh

Spinal cord response to altered fractionation and re-irradiation: radiobiological considerations and role of bioeffect models.

The aim of radiation oncologist is to implement an uncomplicated loco regional control of cancer by radiation therapy. The bioeffect of a physical dose depends on the nature of the tissue, fractionation scheme, dose rate and treatment time. The transformation of absorbed dose into a bioeffect dose is controlled by treatment variables and the radiobiological characteristics of the relevant tissue. Various bioeffect models have been proposed to predict the biological effect of radiotherapy treatments. Dale has proposed extrapolated response dose (ERD) equations for external beam therapy, intracavitary brachytherapy and interstitial brachytherapy. Within the context of the LQ model, the parameter which quantifies the overall biological effect on a given tissue is the biologically effective dose (BED) which is obtained by applying repopulation correction to ERD (Orton). Thames proposed the total effect (TE) concept based on the incomplete repair LQ model which accounts for the biological effect of a fractionated course of radiotherapy. Spinal cord myelitis limits the dose to tumours in the head and neck, thoracic and upper abdominal regions resulting in reduction of tumour control probability. Radiation myelopathy is one of the most devastating complications of clinical radiotherapy. Treatment techniques that are designed to minimize the risk of spinal cord injury are likely to underdose the tumour consequent failure to control the disease. Since radiation myelopathy results in severe and irreversible morbidity, it is important to establish the tolerance dose of the spinal cord. A number of patients have recently been reported to have developed radiation myelopathy following hyperfractionated accelerated radiotherapy. As the survival rates of patients increase, radiation oncologists are more frequently faced with the problem of treatment of late recurrence or second tumours situated within or close to previously treated site. A rationale for taking a decision in treating in such a condition is even more complex than the original condition and requires knowledge of the kinetics of decay of occult injury of the previous treatment. To test the validity of ERD, clinically reported data of altered fractionation to the spinal cord for 7 patients reported by Wong et al, Saunders et al and Bogaert et al, were analysed, ERD values were calculated and compared with compiled clinical literature data of 3233 patients for the incidence of spinal cord myelitis reported by Cohen and Creditor, Wara et al, Abbatucci et al and Jeremic et al for conventional fractionation. ERD values were estimated with alpha/beta of 2.5 Gy for the conventional and altered fractionation data. To test the validity of TE concept for clinical data of re-irradiation tolerance of the spinal cord, the data of the 22 patients compiled by Nieder et al were used. Clinical data compiled from the literature of Cohen and Creditor, Wara et al, Abbatucci et al and Jeremic et al, were used for comparison.

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.

Radiobiological considerations of re-irradiation tolerance of the spinal cord

Abstract Re-irradiation tolerance of the spinal cord depends upon the volume of the spinal cord irradiated, the total dose, the dose per fractional, the elapsed time between the treatments and the region of the spinal cord involved. Clinical data on the retreatment tolerance of the spinal cord are sparse and inconclusive. Radiobiological laboratory evidence has indicated the presence of long term recovery of the spinal cord damage. Fractionation sensitivity during reirradiation is comparable with the first session of radiation treatment. After an initial dose of 45 Gy, 50% recovery has been reported by Schultheiss and Stephens for an elapsed period of two years for re-irradiation considerations.

Optimized Dose Distribution of Gammamed Plus Vaginal Cylinders

Endometrial carcinoma is the most common malignancy arising in the female genital tract. Intracavitary vaginal cuff irradiation may be given alone or with external beam irradiation in patients determined to be at risk for locoregional recurrence. Vaginal cylinders are often used to deliver a brachytherapy dose to the vaginal apex and upper vagina or the entire vaginal surface in the management of postoperative endometrial cancer or cervical cancer. The dose distributions of HDR vaginal cylinders must be evaluated carefully, so that clinical experiences with LDR techniques can be used in guiding optimal use of HDR techniques. The aim of this study was to optimize dose distribution for Gammamed plus vaginal cylinders. Placement of dose optimization points was evaluated for its effect on optimized dose distributions. Two different dose optimization point models were used in this study, namely non-apex (dose optimization points only on periphery of cylinder) and apex (dose optimization points on periphery and along the curvature including the apex points). Thirteen dwell positions were used for the HDR dosimetry to obtain a 6-cm active length. Thus 13 optimization points were available at the periphery of the cylinder. The coordinates of the points along the curvature depended on the cylinder diameters and were chosen for each cylinder so that four points were distributed evenly in the curvature portion of the cylinder. Diameter of vaginal cylinders varied from 2.0 to 4.0 cm. Iterative optimization routine was utilized for all optimizations. The effects of various optimization routines (iterative, geometric, equal times) was studied for the 3.0-cm diameter vaginal cylinder. The effect of source travel step size on the optimized dose distributions for vaginal cylinders was also evaluated. All optimizations in this study were carried for dose of 6 Gy at dose optimization points. For both non-apex and apex models of vaginal cylinders, doses for apex point and three dome points were higher for the apex model compared with the non-apex model. Mean doses to the optimization points for both the cylinder models and all the cylinder diameters were 6 Gy, matching with the prescription dose of 6 Gy. Iterative optimization routine resulted in the highest dose to apex point and dome points. The mean dose for optimization point was 6.01 Gy for iterative optimization and was much higher than 5.74 Gy for geometric and equal times routines. Step size of 1 cm gave the highest dose to the apex point. This step size was superior in terms of mean dose to optimization points. Selection of dose optimization points for the derivation of optimized dose distributions for vaginal cylinders affects the dose distributions.

Dosimetric evaluation of Gammamed High Dose Rate intraluminal brachytherapy applicators

Summary Background A survey of the literature on intraluminal brachytherapy reveals that even for a given tumour site, the dose prescribed varies considerably from one centre to another for multiple reasons: the treatment intent, the association with external beam therapy or not, the dose rate, the technique used and the point of dose specification. There is no common language in the literature as to how doses should be recorded and reported. Aim The purpose of this study was to dosimetrically evaluate various intraluminal brachytherapy applicators for the Gammamed high dose rate afterloading system. Materials/Methods Dosimetric evaluation was carried out for 8mm, 10mm, 12mm and 14mm diameter intraluminal applicators available with the Gammamed high dose rate after-loading system. Treatment planning for these applicators was carried out with the Abacus treatment planning system for active source length and 8cm, 10cm and 12cm. All evaluations were carried out for a prescription dose of 5Gy at the reference point of 1cm from the source axis. Reference volume length (RVL), treated volume (TV) and hyperdose sleeve radius (HSR) were noted down from the isodose plans. Iterative, geometric and equal times optimization routines were carried out for all evaluations with step size of 0.5cm. Results The isodose curves showed tapering pattern towards the distal and proximal regions. The reference volume lengths were larger than active source lengths for 8mm and 10mm diameter applicators. Reference volume lengths were smaller than active source lengths for 12mm and 14mm diameter applicators hyperdose sleeve radius decreases with increase in diameter of the applicator. For 14mm diameter applicators, the hyperdose sleeve radius was smaller than the radius of the reference isodose. Iterative optimization routine gave a better average in terms of reference volume length for all four diameter applicators. Conclusions We evaluated the dosimetric parameters for various intraluminal applicators available with the Gammamed high dose rate remote afterloading system. The values of RVL and HSR were within acceptable limits for the four applicators considered in this study.

Dosimetric characterization of optically stimulated luminescence dosimeter with therapeutic photon beams for use in clinical radiotherapy measurements

Aim: The modern radiotherapy techniques impose new challenges for dosimetry systems with high precision and accuracy in in vivo and in phantom dosimetric measurements. The knowledge of the basic characterization of a dosimetric system before patient dose verification is crucial. This incites the investigation of the potential use of nanoDot optically stimulated luminescence dosimeter (OSLD) for application in radiotherapy with therapeutic photon beams. Materials and Methods: Measurements were carried out with nanoDot OSLDs to evaluate the dosimetric characteristics such as dose linearity, dependency on field size, dose rate, energy and source-to-surface distance (SSD), reproducibility, fading effect, reader stability, and signal depletion per read out with cobalt-60 (60 Co) beam, 6 and 18 MV therapeutic photon beams. The data acquired with OSLDs were validated with ionization chamber data where applicable. Results: Good dose linearity was observed for doses up to 300 cGy and above which supralinear behavior. The standard uncertainty with field size observed was 1.10% ± 0.4%, 1.09% ± 0.34%, and 1.2% ± 0.26% for 6 MV, 18 MV, and 60 Co beam, respectively. The maximum difference with dose rate was 1.3% ± 0.4% for 6 MV and 1.4% ± 0.4% for 18 MV photon beams. The largest variation in SSD was 1.5% ± 1.2% for 60 Co, 1.5% ± 0.9% for 6 MV, and 1.5% ± 1.3% for 18 MV photon beams. The energy dependence of OSL response at 18 MV and 60 Co with 6 MV beam was 1.5% ± 0.7% and 1.7% ± 0.6%, respectively. In addition, good reproducibility, stability after the decay of transient signal, and predictable fading were observed. Conclusion: The results obtained in this study indicate the efficacy and suitability of nanoDot OSLD for dosimetric measurements in clinical radiotherapy.

Cranio Spinal Irradiation of Medulloblastoma Using High Precision Techniques – A Dosimetric Comparison

Radiotherapy planning, delivery and junction dose verification remain exigent for Cranio Spinal Irradiation (CSI) in medulloblastoma patients. This study aims to evaluate high precision techniques such as Intensity Modulated Radiation Therapy (IMRT), Rapid Arc Therapy (RA) with and without flattening filter (FF) on the basis of dosimetric analysis. Five patients treated with jagged junction Intensity Modulated RadioTherapy (IMRT) using dynamic Multi Leaf Collimators (MLC) were randomly selected for this retrospective study. IMRT, Rapid Arc (RA) plans were simulated in the same CT data set with and without flattening filter. Total dose prescribed was 28.80 Gy in 16 fractions. An evaluation criterion of 98% of PTV receiving 100% of the prescription dose was followed in all plans. Twenty treatment plans with 260 Dose Volume Histograms (DVHs) was created. Dosimetric parameters such as Dmax, Dmin, Dmean, V95%, V107%, CI for PTV and Dmax, Dmean, V80%, V50%, V30%, V10% for Organs At Risk (OAR) were extracted from DVHs. Treatment delivery efficiency was also evaluated for total Beam On Time (BOT). FFF Rapid Arc therapy : 6F_RA) resulted in conformal doses throughout the cranio spinal axis. FF and FFF dynamic IMRT had minimal V107%, 1.23% and 2.88% compared to 49.15 and 66.36 of rapid arc therapy (with and without FF). 6F_IMRT resulted in lesser mean doses to eyes, liver, lungs and kidneys. Heart mean dose was less (3.08 Gy) with 6X_IMRT. Thyroid and esophagus doses could be reduced to about 41.2% and 10% respectively with 6F_RA. The BOT for the treatment techniques were 3.43 min (6X_IMRT), 1.59 min (6F_IMRT), 5min (6X_RA), 4.5 min (6F_RA). Removal of flattening filter in IMRT could improve dose coverage along the caniospinal axis and normal tissue sparing. A reduction of 46.3% BOT could increase treatment efficiency of 6F_IMRT compared to 6X_IMRT. CSI could be simpler since junction doses can be evaded in IMRT and RA techniques.

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws

Dosimetric investigation of dual energy photon beams with assymmetric collimator jaws Many modern linear accelerators are equipped with asymmetric collimators or jaws that can be moved independently. Asymmetric jaws have got many clinical applications in radiation therapy. In the present study, the dosimetric characteristics of asymmetric collimators from our linear accelerator with 6 and 18 MV X-rays were carried out. The field size factors (FSF) and half value layer (HVL) were measured in a water phantom using 0.6 cc Farmer chamber for symmetric and asymmetric fields for both 6 and 18 MV X-rays. Measurements of beam penumbra, percentage depth dose (PDD), cross beam profiles and calculated isodose curves were measured by RFA 300 for both asymmetric and symmetric fields. The FSF were found to agree with in 3% for symmetric and asymmetric fields. The HVL in water was found to be 15.8 cm and 14.4 cm for 6 MV photons and 26 cm and 22.9 cm for 18 MV photons at the central axis and at 20 cm off the central axis. At 30 cm depth the percentage depth dose for symmetric and asymmetric fields were found to differ as high as 6% for 6 MV and 4% for 18 MV fields. No observable difference in penumbra was noticed for symmetric and asymmetric fields of same dimensions. The constrictions of isodose curves at the edge nearer to central axis were noticed for asymmetrically placed fields. The observed differences could be due to the passage of primary beam through differential thickness of the flattening filter which alters the beam quality.

Measurement of Transit Time of a Gammamed-Plus Remote Afterloading High Dose Rate Brachytherapy Source

Measurement of Transit Time of a Gammamed-Plus Remote Afterloading High Dose Rate Brachytherapy Source Accurate measurement of transit time of the HDR brachytherapy source of a remote after-loading unit is necessary to calculate the total radiation dose given to the treatment volume. Presently, most of the HDR brachytherapy treatment planning systems neglect the transit time in the computation of dose. The aim of this investigation is to use a well type ionization chamber to measure the transit time during the source movement between two dwell positions. As well type ionization chamber and a precision electrometer (manufacturer CD instruments, Bangalore) were used to measure the charge generated during the movement of the Ir-192 source of a Gammamed HDR brachytherapy unit with an interstitial needle. Effective transit time and effective speed were determined on the basis of methodology described by Sahoo [2]. Corrections were done on the basis of relative sensitivity values for varaious dwell position in the ionization chamber. In the present study the variation of effective speed with interdwell distance was minimal as compared with that of Sahoo [2]. The effective transit times were 0.129, 0.182, 0.301, 0.402, 0.701, and 0.993 seconds for 1, 2, 4, 6, 8 and 10 cm interdwell separations respectively. The effective transit times in the present study were higher than those of Sahoo [2]. Software modification accounting for the dynamic dose should be incorporated into all HDR planning systems. Such an improvement would enhance the safety and accuracy of HDR brachytherapy.

Radiobiological quality of high dose rate interstitial brachytherapy treatments of carcinoma of the cervix

Summary Aim The aim of the present study was to apply the Figure of Merit (FOM) concept to High Dose Rate (HDR) interstitial brachytherapy treatments delivered for carcinoma of the cervix at the Kidwai Memorial Institute of Oncology, Bangalore, in order to evaluate the radiobiological quality of the implants. Materials/Methods Twenty seven patients received external beam radiotherapy, as 50 Gy in 25 fractions over five weeks, followed by interstitial brachytherapy by the HDR template technique on a Gammamed HDR machine. Doses were delivered 5mm from the periphery of the implant along the transverse plane. Points in the bladder and rectum, receiving the maximum dose rate, were used for the evaluation of total dose received by these critical organs. BED and FOM values were evaluated for all patients. Results Two patients had a rectal FOM value greater than 10. A FOM bladder value of higher than 10 was observed in six patients. Only in one patient were FOM values for both the rectum and bladder found to be above 10. The relationship between FOM values for the rectum and the bladder indicated that most FOM values were clustered near the initial portion of the scattergram obtained. For one patient the rectal FOM value was above 10, while the FOM for the bladder was significantly smaller. In another patient, having a high FOM value for the bladder, the FOM value for the rectum was also above the critical value of 10. Conclusions Good physical optimization can lead to good radiobiological optimization, as observed in our study. Radiobiology can not improve a bad quality implant. HDR interstitial brachytherapy, which gives better geometrical sparing for critical normal organs such as the rectum and bladder, should be the treatment of choice for patients with carcinoma of the cervix. In optimizing treatment for a patient, the maximizing of FOM should be supplemented by a comparison of reference BED values and new treatments for the tumour, rectum and bladder.