[Estimation of collimator scatter factor, S(c), of rectangular fields by a saturation model].

We estimated collimator scatter factor, S(c), of symmetric rectangular fields of any size by applying a two-component scatter model to measured in-air output data in width and length directions of measured rectangles. The in-air output was measured for symmetric rectangles with combined width and length sizes of 7 x 7 and 6 x 6 using 10 MV and 4 MV X-rays of Varian's Clinac 2100 C/D, respectively. The model consists of scatter components from the primary collimator and flattening filter and from the collimator jaws: the former shows a saturation curve and the latter increases linearly with enlarging field size. This model was fitted to the measured dataset firstly in the width and secondly in the length directions of rectangles; then, by compiling interpolated matrix data, the S(c) table of symmetric rectangles was constructed. In addition, using this S(c) table, values of S(c) were calculated for a few asymmetric rectangles by Day's method, and were in good agreement with measured values. Therefore, we think that our method is practical and precise for constructing the S(c) table of symmetric rectangles from measured data, and that using this table, the S(c) of any asymmetric rectangles may be calculated.

[Estimation of collimator scatter factor, S(c), of small field sizes using long-SCD method and two saturation models].

To estimate the collimator scatter factor, S(c) of small field sizes in which a mini-phantom cannot be fully included at the nominal treatment distance (NTD=100 cm), we measured the in-air output of 4 MV and 10 MV X-rays of a Varian's Clinac 2100 C/D using a mini-phantom at NTD and at a long source-to-chamber distance (SCD=200 cm) with field-size defined at the isocenter down to 4.6 x 4.6 cm(2) and 2.3 x 2.3 cm(2), respectively. We then compared the fitted curve to the NTD dataset by a cumulative exponential distribution model with that by a cumulative Gaussian distribution (error function) model containing a zero-field extrapolated term derived from the long SCD dataset. The results showed that the zero-field extensions of two fitted curves coincided for a 4 MV X-ray, but a large discrepancy was seen between them for a 10 MV X-ray. Therefore, the S(c) of small field sizes not measurable using a mini-phantom at the NTD can be well estimated by applying the cumulative exponential model to the NTD dataset in the case of a 4 MV X-ray beam filtrated with a cone-shaped flattener. However, to estimate the S(c) of such small field sizes in the case of a 10 MV X-ray beam filtrated with a bell-shaped flattener, we consider it preferable to also measure in-air output at a long SCD and to apply the cumulative Gaussian model as described here.

Long-term results of chemoradiotherapy for locally advanced esophageal cancer, using daily low-dose 5-fluorouracil and cis-diammine-dichloro-platinum (CDDP).

BACKGROUND: We evaluated the efficacy and toxicity of radiation combined with daily, low-dose protracted chemotherapy for locally advanced esophageal cancer. METHODS: We analyzed data for 68 patients with locally advanced esophageal cancer, including 18 surgical candidates. Standard fractionation (total dose range, 60 to 70 Gy) was used for radiotherapy. The chemotherapy consisted of a daily 5-fluorouracil dose of 250 mg/m2, with a cis-diammine-dichloro-platinum dose of 3 mg/m2 administered on radiotherapy days. RESULTS: Sixty-four patients (94%) received at least 60 Gy. Grade 3 acute hematological toxicity was observed in 13 (19%) patients; there was no grade 4 hematological toxicity. Complete response, partial response, no change, and progressive disease were obtained in 22, 35, 7, and 4 patients, respectively. Minimum follow-up for surviving patients was 45 months. Locoregional progression-free rates at 3 and 5 years were 47% and 47%. Four patients died of late cardiac toxicity; the primary site for all 4 patients was the middle thoracic esophagus. Overall survival rates at 2, 3, and 5 years were 40%, 32%, and 20%. The 3- and 5-year survival rates in patients with T2-3M0 disease were 43% and 27%, and the rates were 24% and 15% in patients with T4/M1. CONCLUSION: Given the large proportion of patients in this study with inoperable disease (roughly three quarters), our treatment seemed to provide equivalent efficacy and less hematological toxicity than standard-dose chemoradiotherapy.

[Approximation of collimator scatter factor Sc by a saturation model].

To more easily estimate accurate values of collimator scatter facor, S(c), we suggest a two-component saturation model that accounts for scatter from the primary collimator and flattening filter and from the collimator jaws. This model, which assumes an exponential distribution of scatter intensity, was tested by in-air measurements using a mini-phantom for 4 MV and 10 MV X-rays of a Clinac 2100 C/D linear accelerator. The results showed a good fit of this model to our measured data (R(2)>0.9993). When the measured value was divided into the primary collimator/flattening filter component and the collimator jaw component, as expected, the former component showed a rapid and full saturation curve with increased field size, while the latter showed an almost linearly increasing curve. Therefore, we think that this saturation model is useful for the estimation of S(c) and is applicable to monitor unit calculation for an asymmetric field.

Multivariate analysis of dissemination relapse of medulloblastoma and estimation of its time parameter for craniospinal irradiation.

PURPOSE: To examine in retrospect prognostic factors influencing meningeal dissemination relapse of medulloblastoma (MB) and to estimate time parameter gamma/alpha of the biologically effective dose for fractionated craniospinal irradiation (CSI). MATERIALS AND METHODS: Fifty-eight patients with MB who had been treated at our six hospitals from 1980 to 1990, were analyzed by the proportional hazards model consisting of radiation factors of both CSI and local irradiation (LI), sequential CSI time-lag, and eight non-radiation factors (gender, age, performance status, T-stage, dissemination score, extent of resection, and use of chemotherapy and immunotherapy). The gamma/alpha for CSI was estimated by the profile likelihood method using the maximum value of conditionally calculated time-incorporated biologically effective dose, tBEDmax, of the field treated with the least dose. RESULTS: Dissemination relapse was seen in 23 (40%) patients. Nineteen disseminations occurred within four years, and the cumulative dissemination-free rate was 64% at five years. The site of initial dissemination relapse was both cranial and spinal in 13 patients (57%). Dissemination relapse was accompanied with local failure in 43% (10/23) of patients, and four of them were seen later than four years. In the multivariate analysis, significant prognostic factors were dissemination score (p=0.0008) and total dose of CSI (p=0.018). The estimate of gamma/alpha for CSI was about 0.2 Gy/day in BED units. In another multivariate analysis including the best-fitted tBEDmax, significant prognostic factors were dissemination score and the tBEDmax of both CSI (p=0.021) and LI (p=0.024). CONCLUSION: This analysis indicated that the dissemination score, total dose of CSI, and tBEDmax of both CSI and LI were significantly prognostic for dissemination relapse of MB. The estimate of gamma/alpha for CSI was smaller than that derived from our previous analysis for LI. However, in order to estimate the time factor for CSI more precisely, a larger group of patients treated with concurrent CSI is needed.