Songül Çavdar Karaçam

The peripheral dose outside the applicator in electron beams of Oncor linear accelerator

In this study, the peripheral dose outside the applicator was measured using electron beams produced by an Oncor linear accelerator and compared with the data of the treatment planning system (TPS). The dose profiles have been measured, by using a water-equivalent slab phantom and a parallel plate ionisation chamber, at 6, 9 and 15 MeV energy levels in 5×5, 10×10, 15×15, 20×20 and 25×25 cm(2) applicators and at 0, 10 and 20° gantry angles; and at the surface, 0.2, 0.5, 1 cm and d(max) depth for each electron energy level. The peripheral dose has been determined with these profiles by normalisation at the field central beam axis (CAX). It has been noticed that, using a 10×10 cm(2) applicator, there is a 1.4 % dose peak on the surface 6 cm away from the field edge where the field CAX is at 100 %, at a gantry angle of 0° with 6 and 9 MeV electron beams; also for the 15 MeV electron beam there is a 2.3 % dose peak. It has been discovered that the peak dose approaches a minimum depending on the increase in depth and reaches 2.5-4 % depending on the growth of the field dimension. At gantry angles of 10 and 20°, 6 and 9 MeV electron beams created small peaks and a maximum dose could be reached at 0.2 and 1 cm depth. Electron beam of 15 MeV did not peak at depths of 0.2 and 1 cm at gantry angles of 10 and 20°. The measured peripheral dose outside the applicators has been compared with the data from a TPSs computer using the Pencil Beam algorithm; it has been stated that dose calculations can be made as far as 3 cm outside the field. In conclusion, the TPS is not sufficient to measure the peripheral dose outside the applicators, and this dose can only be determined by direct measurement.

THE INVESTIGATION OF FETAL DOSES IN MANTLE FIELD IRRADIATION

To determine clinically the fetal dose from irradiation of Hodgkins disease during pregnancy and to quantify the components of fetal dose using phantom measurements. The fetal dose was measured with phantom measurements using thermoluminescent dosemeters (TLDs). Phantom measurements were performed by simulating the treatment conditions on an anthropomorphic phantom. TLDs were placed on the phantom 41, 44, 46.5 and 49.5 cm from the centre of the treatment field. Two TLDs were placed on the surface of the phantom. The estimated total dose to all the TLDs ranged from 8.8 to 13.2 cGy for treatment with (60)Co and from 8.2 to 11.8 cGy for 4 MV photons. It was concluded that the doses in different sections were evaluated to investigate dose changes in different points and depths of fetal tissues in phantom. Precise planning and the use of supplemental fetal shielding may help reduce fetal exposure.

Does rectum and bladder dose vary during the course of image-guided radiotherapy in the postprostatectomy setting?

Aims and Background To assess the variations in actual doses delivered to the rectum and bladder in the course of postprostatectomy radiotherapy using kilovoltagecone-beam computed tomography datasets acquired during image-guided radiotherapy. Methods and Study Design Twenty consecutive patients treated with intensity-modulated or intensity-modulated arc therapy to the prostate bed were retrospectively evaluated. Both the planning tomography and kilovoltage-cone-beam computed tomography were acquired with an empty rectum and a half-full bladder. Target localization was performed on the basis of soft tissue matching using cone-beam computed tomography scans before each treatment fraction. A total of 16 cone-beam computed tomography scans per patient (acquired at the first 5 fractions and twice weekly thereafter) were used for the assessments. The bladder and rectum were recontoured offline on each cone-beam computed tomography scan by a single physician, and the delivered doses were recalculated. The variations in certain dose-volume parameters for the rectum and bladder (BD2cc, RD 2cc, V40%, V50%, V60%, V65%) were analyzed using the paired t test. Results Most of the dose volume variations for rectum and bladder were significantly higher than predicted (P <0.05) for the 320 kilovoltage-cone-beam computed tomography sets, except for the doses received by 2 cc of the bladder and V50 and V60 of the rectum. The dose-volume parameters of the bladder did not meet our criteria of V65 ≤25% and V40 ≤50% in 10% and 20% of the patients, respectively. None of the dose-volume histograms showed rectal V65 ≥17%; however, the rectal V40 ≤35% dose constraint was not met in 11 patients. For all patients, the ANOVA test revealed no significant difference between the variations. Conclusion Actual doses delivered during treatment were found to be higher than predicted, but the majority of calculated bladder and rectal doses remained in the limits of our plan acceptance criteria. Interfraction variability of the rectum and bladder is a major concern in the postprostatectomy radiotherapy setting, even when patients are instructed about rectal and bladder preparation before the radiotherapy course. Image guidance with cone-beam computed tomography at each treatment fraction may offer a viable tool to account for interfraction variations of the rectum and bladder throughout the treatment course.

Depth Dose Characteristics of Electron Beams at Extended SSDS.

The purpec: of this study is to investigate the behaviour of the percent depth dose curves (%DD) and surface doses of electronbeams at extended Source-to Surface Distances (SSDS). A (GE) Saturne 42 linear accelerator was used in this study, which produces dual photon energies of 6 and 15 MV as well as eight electron energies ranging between 4.5 and 21 MeV. The % Depth Dose curves were geneated with water scanning equipment at 6, 9, and 15 MeV for 4x4 cm(2) and 20x20 cm(2) field sizes at SSDS of 100 cm, 108 cm, and 115 cm. According to the measurements from surface to the depth of dose maximum the surface dose increased for all of the electron energies studied at extended SSDS for small field sizes. On the other hand for larger field sizes the surface doses decreased at extended SSDS. It was also observed that the increase in the surface dose diminished as the field size approached to 10x10cm(2) then the surface dose started decreasing at extended SSDS as the field sizes increased. Extended SSDS have no observable effect on the tail portion of the depth dose curves.