Output factor measurement in high dose-per-pulse IORT electron beams.

PURPOSE: To assess the capability of different types of detectors to measure relative output factors (OF) at high dose per pulse by comparison with alanine dosimeters, which are independent of dose rate. METHODS: Measurements were made in 9 MeV and 7 MeV electron beams produced by a Novac7 accelerator for intraoperative radiotherapy. Applicators with diameter of 10-7-6-5 and 4 cm were used. The dose per pulse varied from about 30 mGy, for the 10 cm reference applicator, to about 70 mGy, for the 4 cm applicator. Five types of plane-parallel ionization chambers (PTW Advanced Markus, Markus and Roos, IBA PPC40 and PPC05), two types of silicon diodes (PTW 60017 and IBA EFD3G) and a PTW 60019 microDiamond were considered. For the ionization chambers, correction factors for ion recombination effects were determined for each applicator using a modified two-voltage-analysis method that includes the free-electron component. RESULTS: Reference OF values were determined by alanine dosimeters with a standard combined uncertainty of 0.8%. Deviations from the reference OFs were generally within 1.5% for all the detectors, hence within the 95% confidence interval of alanine measurements. Larger deviations of up to about 2% obtained in a few cases are consistent with a 0.7% long-term reproducibility of OF measurements. CONCLUSIONS: Comparison with alanine measurements demonstrated that all the detectors considered in this work can be used to measure OFs in high dose-per-pulse electron beams with an accuracy better than 2%, provided that appropriate corrections for ion recombination effects are applied when using ionization chambers.

Late rectal bleeding after 3D-CRT for prostate cancer: development of a neural-network-based predictive model.

The aim of this study was to develop a model exploiting artificial neural networks (ANNs) to correlate dosimetric and clinical variables with late rectal bleeding in prostate cancer patients undergoing radical radiotherapy and to compare the ANN results with those of a standard logistic regression (LR) analysis. 718 men included in the AIROPROS 0102 trial were analyzed. This multicenter protocol was characterized by the prospective evaluation of rectal toxicity, with a minimum follow-up of 36 months. Radiotherapy doses were between 70 and 80 Gy. Information was recorded for comorbidity, previous abdominal surgery, use of drugs and hormonal therapy. For each patient, a rectal dose-volume histogram (DVH) of the whole treatment was recorded and the equivalent uniform dose (EUD) evaluated as an effective descriptor of the whole DVH. Late rectal bleeding of grade ≥ 2 was considered to define positive events in this study (52 of 718 patients). The overall population was split into training and verification sets, both of which were involved in model instruction, and a test set, used to evaluate the predictive power of the model with independent data. Fourfold cross-validation was also used to provide realistic results for the full dataset. The LR was performed on the same data. Five variables were selected to predict late rectal bleeding: EUD, abdominal surgery, presence of hemorrhoids, use of anticoagulants and androgen deprivation. Following a receiver operating characteristic analysis of the independent test set, the areas under the curves (AUCs) were 0.704 and 0.655 for ANN and LR, respectively. When evaluated with cross-validation, the AUC was 0.714 for ANN and 0.636 for LR, which differed at a significance level of p = 0.03. When a practical discrimination threshold was selected, ANN could classify data with sensitivity and specificity both equal to 68.0%, whereas these values were 61.5% for LR. These data provide reasonable evidence that results obtained with ANNs are superior to those achieved with LR when predicting late radiotherapy-related rectal bleeding. The future introduction of patient-related personal characteristics, such as gene expression profiles, might improve the predictive power of statistical classifiers. More refined morphological aspects of the dose distribution, such as dose surface mapping, might also enhance the overall performance of ANN-based predictive models.

Radiochromic film dosimetry with flatbed scanners: a fast and accurate method for dose calibration and uniformity correction with single film exposure.

Film dosimetry is an attractive tool for dose distribution verification in intensity modulated radiotherapy (IMRT). A critical aspect of radiochromic film dosimetry is the scanner used for the readout of the film: the output needs to be calibrated in dose response and corrected for pixel value and spatial dependent nonuniformity caused by light scattering; these procedures can take a long time. A method for a fast and accurate calibration and uniformity correction for radiochromic film dosimetry is presented: a single film exposure is used to do both calibration and correction. Gafchromic EBT films were read with two flatbed charge coupled device scanners (Epson V750 and 1680Pro). The accuracy of the method is investigated with specific dose patterns and an IMRT beam. The comparisons with a two-dimensional array of ionization chambers using a 18 x 18 cm2 open field and an inverse pyramid dose pattern show an increment in the percentage of points which pass the gamma analysis (tolerance parameters of 3% and 3 mm), passing from 55% and 64% for the 1680Pro and V750 scanners, respectively, to 94% for both scanners for the 18 x 18 open field, and from 76% and 75% to 91% for the inverse pyramid pattern. Application to an IMRT beam also shows better gamma index results, passing from 88% and 86% for the two scanners, respectively, to 94% for both. The number of points and dose range considered for correction and calibration appears to be appropriate for use in IMRT verification. The method showed to be fast and to correct properly the nonuniformity and has been adopted for routine clinical IMRT dose verification.

Monte Carlo investigation of breast intraoperative radiation therapy with metal attenuator plates.

In intraoperative electron radiation therapy for breast cancer, attenuation plates are commonly used to protect organs at risk. These plates can be made of different materials, and the correct material (or combination of materials) has to be chosen in order to achieve the desired attenuation, while avoiding excessive backscattered radiation. The Monte Carlo method (BEAMnrcMP and DOSXYZnrcMP) has been used to characterize the electron beam generated by the setup (composed of a nondedicated linac and an applicator), and to simulate the percent depth dose (PDD) for plates of different materials. The beam has been characterized for nominal energies of 9 and 12 MeV. Several differently composed plates have been investigated: it was found, as expected, that the use of a plate presenting to the electron beam a high-Z material (i.e., lead) has to be avoided because of excessive backscatter (up to 52% compared to the PDD without plate). On the other hand, the use of a single low-Z material (i.e., aluminum) in the plate can lead to an insufficient attenuation of the beam. The two-layer plate (6 mm of Al plus 3 mm of Cu) used in S. Chiara Hospital has been found to attenuate the beam almost completely for both considered energies, causing negligible backscatter radiation. The spectrum at various depth and at the tissue-plate interface has also been investigated.

Fitting late rectal bleeding data using different NTCP models: results from an Italian multi-centric study (AIROPROS0101).

BACKGROUND AND PURPOSE: Recent investigations demonstrated a significant correlation between rectal dose-volume patterns and late rectal toxicity. The reduction of the DVH to a value expressing the probability of complication would be suitable. To fit different normal tissue complication probability (NTCP) models to clinical outcome on late rectal bleeding after external beam radiotherapy (RT) for prostate cancer. PATIENTS AND METHODS: Rectal dose-volume histograms of the rectum (DVH) and clinical records of 547 prostate cancer patients (pts) pooled from five institutions previously collected and analyzed were considered. All patients were treated in supine position with 3 or 4-field techniques: 123 patients received an ICRU dose between 64 and 70 Gy, 255 patients between 70 and 74 Gy and 169 patients between 74 and 79.2 Gy; 457/547 patients were treated with conformal RT and 203/547 underwent radical prostatectomy before RT. Minimum follow-up was 18 months. Patients were considered as bleeders if showing grade 2/3 late bleeding (slightly modified RTOG/EORTC scoring system) within 18 months after the end of RT. Four NTCP models were considered: (a) the Lyman model with DVH reduced to the equivalent uniform dose (LEUD, coincident with the classical Lyman-Kutcher-Burman, LKB, model), (b) logistic with DVH reduced to EUD (LOGEUD), (c) Poisson coupled to EUD reduction scheme and (d) relative seriality (RS). The parameters for the different models were fit to the patient data using a maximum likelihood analysis. The 68% confidence intervals (CI) of each parameter were also derived. RESULTS: Forty six out of five hundred and forty seven patients experienced grade 2/3 late bleeding: 38/46 developed rectal bleeding within 18 months and were then considered as bleeders The risk of rectal bleeding can be well calculated with a 'smooth' function of EUD (with a seriality parameter n equal to 0.23 (CI 0.05), best fit result). Using LEUD the relationship between EUD and NTCP can be described with a TD50 of 81.9 Gy (CI 1.8 Gy) and a steepness parameter m of 0.19 (CI 0.01); when using LOGEUD, TD50 is 82.2 Gy and k is 7.85. Best fit parameters for RS are s=0.49, gamma=1.69, TD50=83.1 Gy. Qualitative as well as quantitative comparisons (chi-squared statistics, P=0.005) show that the models fit the observed complication rates very well. The results found in the overall population were substantially confirmed in the subgroup of radically treated patients (LEUD: n=0.24 m=0.14 TD50=75.8 Gy). If considering just the grade 3 bleeders (n=9) the best fit is found in correspondence of a n-value around 0.06, suggesting that for severe bleeding the rectum is more serial. CONCLUSIONS: Different NTCP models fit quite accurately the considered clinical data. The results are consistent with a rectum 'less serial' than previously reported investigations when considering grade 2 bleeding while a more serial behaviour was found for severe bleeding. EUD may be considered as a robust and simple parameter correlated with the risk of late rectal bleeding.

[Radiotherapy of T1N0 neoplasms of the glottis. Analysis of the parameters that influence local control]. / La radioterapia nelle neoplasie T1N0 della glottide. Analisi di alcuni parametri che influenzano il controllo locale.

Treatment and tumor-related parameters were reviewed in 176 patients with T1N0 carcinoma of the glottic larynx submitted to primary radiation therapy from 1980 to 1992. Our aim was to analyze local control and treatment-related toxicity. Over-all local control rates at 10 years were 88.3% with irradiation alone and 94.5% after salvage surgery (larynx preservation: 91%). Verrucous histology was a negative factor affecting local control and anterior commissure involvement exhibited only a negative trend but had no statistical significance. Among treatment-related factors, local control was 76.5% after split-course and 91.1% after continuous-course irradiation (p < 0.05). With continuous-course irradiation, the total dose influenced local control only for single of 2 Gy (local control rates were 69% with 60 Gy and 93% at > or = 64 Gy; p < 0.05), but not for single doses of 2.25 Gy and total doses ranging 56.25-65.25 Gy (local control failed in 55 patients). Early and late complications did not increase with single doses > or = 2.25 Gy. Our current policy in T1N0 nonverrucous glottic carcinoma is to use a single fraction of 2.25 Gy and a total dose ranging 56.25-63 Gy according to tumor size, with a continuous course.