Dosimetric evaluation of Gammamed High Dose Rate intraluminal brachytherapy applicators

BackgroundA 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.AimThe purpose of this study was to dosimetrically evaluate various intraluminal brachytherapy applicators for the Gammamed high dose rate afterloading system.Materials/MethodsDosimetric 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.ResultsThe 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.ConclusionsWe 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

Analysis of physical parameters and determination of inflection point for Flattening Filter Free beams in medical linear accelerator

BackgroundMedical Linear accelerators manufactured without flattening filters are increasing popular in recent days. The removal of flattening filter results in increased dose rate, reduced mean energy, reduction in head leakage and lateral scattering, which have shown advantageous when used for special treatment procedures.AimThis study aims to analyze physical parameters of FFF beams and to determine the inflection point for standardizing the beam flatness and penumbra.Materials and methodsThe beam profiles and depth dose patterns were measured using Radiation Field Analyzer (RFA) with 0.13[[ce:hsp sp="0.25"/]]cc cylindrical ion chamber. The beam energy characteristics, head scatter factor (Sc) were obtained for 6FFF and 10FFF beams and compared with 6[[ce:hsp sp="0.25"/]]MV and 10[[ce:hsp sp="0.25"/]]MV photons, respectively. The symmetry and stability of unflattened regions were also analyzed. In addition, the study proposes a simple physical concept for obtaining inflection point for FFF beams and results were compared using the Akima spline interpolation method. The inflection point was used to determine the field size and penumbra of FFF beams.ResultsThe Sc varied from 0.922 to 1.044 for 6FFF and from 0.913 to 1.044 for 10FFF with field sizes from 3[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]3[[ce:hsp sp="0.25"/]]cm to 40[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]40[[ce:hsp sp="0.25"/]]cm which is much less than FF beams. The obtained value of field size and penumbra for both simple physical concept and Akima spline interpolation methods is within the ±1.0[[ce:hsp sp="0.25"/]]mm for the field size and ±2[[ce:hsp sp="0.25"/]]mm penumbra. The results indicate that FFF beams reduce Sc compared with FF beams due to the absence of a flattening filter.ConclusionThe proposed simple method to find field size and penumbra using inflection point can be accepted as it is closely approximated to mathematical results. Stability of these parameters was ascertained by repeated measurements and the study indicates good stability for FFF beam similar to that of FF beams

Influence of segment width on plan quality for volumetric modulated arc based stereotactic body radiotherapy

AimTo study the influence of segment width on plan quality for volumetric modulated arc based stereotactic body radiotherapy.BackgroundThe redundancy of modulation for regularly shaped small volume tumors results in creation of many small segments and an increase of monitor units, with a consequent prolongation of treatment and uncertainty in treatment delivery.Materials and methodsSix cases each in lung, abdomen and liver were taken for the study. For each case, three VMAT SBRT plans were generated with different penalties on minimum segment width of 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm. A comparison was made on the metrics of dose volume histogram, dosimetric indices, monitor units (MUs) and delivery accuracy.ResultsThe mean reduction of total MUs when compared with 0.5[[ce:hsp sp="0.25"/]]cm plan was observed as 12.7[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]6.0% and 17.5[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]7.2% for 1.0[[ce:hsp sp="0.25"/]]cm and 1.5[[ce:hsp sp="0.25"/]]cm of minimum segment width, respectively. The p value showed a significant degradation in dosimetric indices for 1.5[[ce:hsp sp="0.25"/]]cm plans when compared with 0.5[[ce:hsp sp="0.25"/]]cm and 1.0[[ce:hsp sp="0.25"/]]cm plans. The average deviation of measured dose with TPS calculated was 3.0[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]1.1%, 2.1[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]0.84% and 1.8[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]0.9% for 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm, respectively. The calculated gamma index with pass criteria of 2% dose difference and 2[[ce:hsp sp="0.25"/]]mm distance to agreement was 95.9[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]2.8%, 96.5[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]2.6% and 97.8[[ce:hsp sp="0.25"/]]±[[ce:hsp sp="0.25"/]]1.6% as calculated for 0.5, 1.0 and 1.5[[ce:hsp sp="0.25"/]]cm of penalties, respectively. In view of the trade off between delivery efficiency and plan quality, 1[[ce:hsp sp="0.25"/]]cm minimum segment width plans showed an improvement.ConclusionsVMAT SBRT plans with increased optimal value of minimum segment width showed better plan quality and delivery efficiency for stereotactic body radiotherapy