A fairly good correspondence is found between the curves of 2 sets of data for brain, heart and rectum. Hence the model may, therefore, be used to interpolate clinical data to provide an estimate of NTCP for these organs for any altered fractionated treatment schedule.

This study was designed to investigate the effect of prostatic edema on various dosimetric quality indices following transperineal permanent 131Cs seed implant. Thirty-one patients with early prostate cancer, who received 131Cs permanent seed implant, were included in this study. Each patient received a prescribed dose of 115 Gy from the implant. Transrectal ultrasound (U.S.) was used to measure the preimplant prostate volume and pre- and postneedle implant volumes, and postimplant CT images were used to obtain postimplant prostate volumes at days 0, 14, and 28 for all patients. The magnitude of edema was determined by comparing the preneedle and postimplant prostate volumes, which was used to compute the half life of the edema using the least-squares method. Dose volume histograms were generated for each set of volumes to determine the percentage of the prostate volume that received a dose equal to or greater than the prescribed dose to compute the quality index (V100) and fractional D90 (FD90). There were no statistically significant differences between the postneedle and postimplant (day 0) volumes obtained by U.S. and CT scanned images (student's t-test p=0.56). The mean half life of the edema was found to be (9.72 +/- 8.31) days (mean +/- 1 SD), ranging from 3.64 to 34.48 days. The mean values of V100 and FD90 from preimplant plan to postimplant plan at day 0 were decreased by 8.0% and 6.3%, respectively. On the other hand, the mean values of V100 and FD90 increased with increasing postimplant time and attained optimal values when postimplant volume reached the original volume of the prostate. The short half life 131Cs radioactive source delivered about 85% of the prescribed dose before the prostate reached its original volume. Therefore, improvement in V100 and FD90 due to edema decay does not improve the physical dose delivery to the prostate. It is important to note that at the time of 131Cs implant, the effect of edema must be accounted for when defining the seed positions. Implants performed based only on the guidance of a preimplant volume study would result in poor dosimetric results for 131Cs implants.

No abstract

This study evaluates changes in the dosimetric characteristics of a Varian Millennium 80‐leaf multileaf collimator (MLC) in a radiation field. In this study, dose rate, scatter factor, percentage depth dose, surface dose and dose in the buildup region, beam profile, flatness and symmetry, and penumbra width measurements were made for 6‐MV and 15‐MV photon beams. Analysis of widths between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement shows a significant difference between the fields defined by MLC and/or jaws and MLC (zero gap) and the fields defined by jaws only. The position of the MLC leaves in the radiation field also significantly affects scatter factors. A new relationship has, therefore, been established between the scatter factors and the position of the MLC, which will indeed be useful in the dose calculation for irregular fields. Penumbra widths increase with field size and were higher for fields defined by jaws and/or MLC than jaws and MLC (zero gap) by 1.5 mm to 4.2 mm and 3.8 mm to 5.0 mm, for 6‐MV, and 1.5 mm to 2.4 mm and 3.0 mm to 5.6 mm, for 15‐MV, at 20% to 80% and 10% to 90% levels, respectively. The surface dose and the dose in the buildup region were smaller for fields defined by jaws and MLC (zero gap) than the fields defined by jaws and/or MLC for both photon energies. No significant differences were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness and symmetry for both energies. The results of this study suggest that while one collects linear accelerator beam data with a MLC, the effects of the positions of the MLC leaves play an important role in dosimetric characteristics of 3D conformal radiation therapy as well as intensity‐modulated radiotherapy.PACS number: 87.53.Dq

No abstract