Monte Carlo calculated detector-specific correction factors for Elekta radiosurgery cones

A radiation field is considered small if its dimension is lower than the range of secondary electrons and the collimating devices partially occlude the source. Different detector types, such as unshielded diodes, diamond detectors, and small-volume ion chambers, are used for small-field measurements. Although the active volumes of these detectors are small, their non-water equivalent materials cause response variations. Herein, we aim to calculate the correction factors for our clinical detectors, EDGE detector (Sun Nuclear), 60017 diode (PTW), and CC01 ion chamber (IBA), for stereotactic radiosurgery cones of diameters of 5-15 mm in an Elekta Synergy linear accelerator using a Monte Carlo simulation. An Elekta Synergy linear accelerator treatment head was simulated using BEAMnrc Monte Carlo code as per the manufacturer specification. All three detectors were simulated as per the manufacturer specification. Three EGSnrc user codes were used for the detector simulation based on the detector geometry. The Monte Carlo model of the treatment head was validated against the measured data for a standard field size of 10 × 10 cm². The off-axis profile, percentage depth dose, and tissue phantom ratioTPR1020were verified in the validation procedure. The measured and Monte Carlo calculated relative output factors (ROFs) were not consistent. In a 5 mm field size, EDGE diode overestimated the ROF by 7.06%, and 60017 diode to 4.611%. In a 7.5 mm field size, the variations were 4.295% and 3.691% for EDGE and 60017 diodes, respectively. CC01 ion chamber under-responded up to 10% because of its low-density active volume. The maximum corrections were obtained in the smallest field size, which were 0.939(0.007), 0.962(0.006), and 1.117(0.008) for EDGE, PTW T60017, and CC01 detectors, respectively. After applying the Monte Carlo calculated correction factor to the measured ROF, it became consistent with the Monte Carlo calculated ROF.

An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry

PURPOSE

An experimental method using the linear portion of the relative film dose-response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.

MATERIALS AND METHODS

The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.

RESULTS

The maximum percentage deviation of PDD_FS with PDD_IC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDD_FS with PDD_IC for electron beams was within 3% for energy range studied of 8-15 MeV. The gamma passing rates of PDD_FS with PDD_IC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDD_FS with PDD_IC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROF_FS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDD_FS and PDD_MC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROF_FS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.

CONCLUSIONS

The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

Radiation dose to dysphagia aspiration-related structures and its effect on swallowing: Comparison of three-dimensional conformal radiotherapy and intensity-modulated radiation therapy plans

CONTEXT

Radiotherapy techniques have improved over years but have also raised concerns over subsequent acute and late effects. One such complication, dysphagia, has led to much interest and optimization in treatment delivery.

AIMS

The aim of this study was to compare radiation dose to dysphagia aspiration related structures (DARS) in intensity modulated radiation therapy (IMRT) and three-dimensional conformal radiotherapy (3DCRT), and assess its relation to post-treatment swallowing status, in patients of carcinoma of the anterior two thirds of tongue.

MATERIALS AND METHODS

Treatment plans of patients treated with IMRT and 3DCRT, from November 2011 to December 2012, were retrieved. Swallowing structures were contoured. Dose volume histograms were generated. Constraint doses were considered based on the statistical derivations. Swallowing status was evaluated based on University of Washington Head- and Neck-related Quality-of-Life questions addressing swallowing.

STATISTICAL ANALYSIS USED

IBM SPSS Statistics 20 Windows (SPSS Inc., Chicago, IL, USA) was used. Mann-Whitney U-test, Spearman's rho correlation coefficient, and receiver operating curves were used.

RESULTS

Definitive statistical and subjective correlations were found to exist between doses of swallowing structures, especially the constrictor muscles, the base of the tongue and larynx. Probable mean dose constraints derived statistically clinically corroborates with the swallowing status of patients. IMRT had statistically significant advantage over 3DCRT, in terms of V30 (P - 0.051), V50 (P - 0.002), V60 (P - 0.002), and D80 (P - 0.023) for swallowing structures taken together.

CONCLUSION

Our study is probably one of the few studies predicting possible mean dose constraints for superior constrictor, the base of the tongue and larynx. Further prospective studies are required to confirm these and to improve the swallowing quality.