ABSTRACT
Background: Accurate dose measurements are difficult in small fields due to charge particle disequilibrium, partial source occlusion, steep dose gradient, and the finite size of the detector. Aim: The study aims to determine the output factor using various detectors oriented in parallel and perpendicular orientations for three different tertiary collimating systems using 15 MV photon beams. In addition, this study analyzes how the output factor could be affected by different configurations of X and Y jaws above the tertiary collimators. Materials and Methods: Small field output factor measurements were carried out with three detectors for different tertiary collimating systems such as BrainLab stereotactic cones, BrainLab mMLC and Millennium MLC namely. To analyze the effect of jaw position on output factor, measurements have been carried out by positioning the jaws at the edge, 0.25, 0.5, and 1.0 cm away from the tertiary collimated field. Results: The data acquired with 15 MV photon beams show significant differences in output factor obtained with different detectors for all collimating systems. For smaller fields when compared to microDiamond, the SRS diode underestimates the output by up to -1.7% ± 0.8% and -2.1% ± 0.3%, and the pinpoint ion chamber underestimates the output by up to -8.1% ± 1.4% and -11.9% ± 1.9% in their parallel and perpendicular orientation respectively. A large increase in output factor was observed in the small field when the jaw was moved 0.25 cm symmetrically away from the tertiary collimated field. Conclusion: The investigated data on the effect of jaw position inferred that the position of the X and Y jaw highly influences the output factors of the small field. It also confirms that the output factor highly depends on the configuration of X and Y jaw settings, the tertiary collimating system as well as the orientation of the detectors in small fields.
ABSTRACT
BACKGROUND: To assess and compare the local control and toxicities between HDR Intracavitary Brachytherapy with 7.5 Gy and 9 Gy per fraction after EBRT in treatment of carcinoma cervix. METHODOLOGY: A total of 180 patients were randomly assigned to 2 arms. Arm A received HDR intracavitary brachytherapy with a dose of 7.5 Gy per fraction, 1 fraction per week for 3 fractions and Arm B received 9 Gy per fraction, 1 fraction per week for 2 fractions. Patients were evaluated on follow up for assessment of local control and toxicities. RESULTS: The median follow up was 12 months (6-18 months). In arm A 89% of the patient had complete response and 11% had recurrence or metastasis. In arm B 93% of the patient had complete response and 7% had recurrence or metastasis. Grade 2/3 diarrhoea was seen in 4.4% of patients in Arm A and in 7.7% in Arm B. Grade 2/3 proctitis was seen in 3.3% of patients in 7.5 Gy arm and in 6.6% in 9 Gy arm. One patient in each arm had grade 1 haematuria. The overall duration of treatment was significant lower in Arm B compared to Arm A (59 days vs 68 days, p = 0.01). CONCLUSION: The result of this clinical study shows that Intracavitary brachytherapy with a dose of 9 Gy per fraction is non inferior to other schedules in term of local control and does not result in increased toxicity.
ABSTRACT
Background: The experimental determination of relative output factors presents the greatest challenge, especially for small fields with different detectors. The aim of this study is to evaluate the influence of jaw positions on small-field output factors for the fields defined by micro-multileaf collimator and circular cones with different detectors. Materials and Methods: The stereotactic output factors were measured on Primus linear accelerator with BrainLab micro-multileaf collimator (mMLC) and circular cones as add-on tertiary collimators. Square field sizes ranging from 0.6 cm × 0.6 cm to 9.8 cm × 9.8 cm and circular fields of diameter ranging from 1.0 cm to 4.0 cm were defined by mMLC and circular cones, respectively. The influence of jaw position on output factor was assessed for different geometric configurations with three different detectors. Results: The values obtained with PinPoint ion chamber were consistent with microDiamond detector for fields greater than 24 mm × 24 mm, but an underestimation of 23.9% was noticed in 6 mm x 6 mm field size. For the mMLC defined field size of 6 mm × 6 mm, when the X-Y jaw was moved from 8 mm × 8 mm to 80 mm × 80 mm, an increase in the output by a factor of 1.7 was observed with both microDiamond and stereotactic radiosurgery diode, whereas an increase in output by a factor of 1.9 was noticed with PinPoint ion chamber. Conclusion: Output factors obtained with different detectors show high differences in the smallest field size for all collimating systems. This study confirms that the position of X and Y jaw above the tertiary collimator significantly influences the small-field output factor.
