Optimization of extracranial stereotactic radiation therapy of small lung lesions using accurate dose calculation algorithms

BACKGROUND: The aim of this study was to compare and to validate different dose calculation algorithms for the use in radiation therapy of small lung lesions and to optimize the treatment planning using accurate dose calculation algorithms. METHODS: A 9-field conformal treatment plan was generated on an inhomogeneous phantom with lung mimics and a soft tissue equivalent insert, mimicking a lung tumor. The dose distribution was calculated with the Pencil Beam and Collapsed Cone algorithms implemented in Masterplan (Nucletron) and the Monte Carlo system XVMC and validated using Gafchromic EBT films. Differences in dose distribution were evaluated. The plans were then optimized by adding segments to the outer shell of the target in order to increase the dose near the interface to the lung. RESULTS: The Pencil Beam algorithm overestimated the dose by up to 15% compared to the measurements. Collapsed Cone and Monte Carlo predicted the dose more accurately with a maximum difference of -8% and -3% respectively compared to the film. Plan optimization by adding small segments to the peripheral parts of the target, creating a 2-step fluence modulation, allowed to increase target coverage and homogeneity as compared to the uncorrected 9 field plan. CONCLUSION: The use of forward 2-step fluence modulation in radiotherapy of small lung lesions allows the improvement of tumor coverage and dose homogeneity as compared to non-modulated treatment plans and may thus help to increase the local tumor control probability. While the Collapsed Cone algorithm is closer to measurements than the Pencil Beam algorithm, both algorithms are limited at tissue/lung interfaces, leaving Monte-Carlo the most accurate algorithm for dose prediction

Dinamiche in famiglie con figli affetti da diverse forme di degenerazioni tapeto retiniche ereditarie

Poster presentato al 22-esimo Congresso Nazionale di Oftalmologia, Padova, 23 giugno 200

Impact of Residual Setup Error on Parotid Gland Dose in Intensity-Modulated Radiation Therapy with or without Planning Organ-at-Risk Margin

Purpose: To estimate the dosimetric impact of residual setup errors on parotid sparing in head-and-neck (H&N) intensity-modutated treatments and to evaluate the effect of employing an PRV (planning organ-at-risk volume) margin for the parotid gland. Patients and Methods: Ten patients treated for H&N cancer were considered. A nine-beam intensity-modutated radiotherapy (IMRT) was planned for each patient. A second optimization was performed prescribing dose constraint to the PRV of the parotid gland. Systematic setup errors of 2 mm, 3 mm, and 5 mm were simulated. The dose-volume histograms of the shifted and reference plans were compared with regard to mean parotid gland dose (MPD), normal-tissue complication probability (NTCP), and coverage of the clinical target volume (V(95%) and equivalent uniform dose [EUD]); the sensitivity of parotid sparing on setup error was evaluated with a probability-based approach. Results: MPD increased by 3.4%/mm and 3.0%/mm for displacements in the craniocaudal and lateral direction and by 0.7%/mm for displacements in the anterior-posterior direction. The probability to irradiate the parotid with a mean dose > 30 Gy was > 50%, for setup errors in cranial and lateral direction and < 10% in the anterior-posterior direction. The addition of a PRV margin improved parotid sparing, with a relative reduction in NTCP of 14%. The PRV margin compensates for setup errors of 3 mm and 5 mm (MPD <= 30 Gy in 87% and 60% of cases), without affecting clinical target volume coverage (V(95%) and EUD variations < 1% and < 1 Gy). Conclusion: The parotid gland is more sensitive to craniocaudal and lateral displacements. A setup error of 2 mm guarantees an MPD : 30 Gy in most cases, without adding a PRV margin. If greater displacements are expected/accepted, an adequate PRV margin could be used to meet the clinical parotid gland constraint of 30 Gy, without affecting target volume coverage

Late fecal incontinence after high-dose radiotherapy for prostate cancer : better prediction using longitudinal definitions

PURPOSE: To model late fecal incontinence after high-dose prostate cancer radiotherapy (RT) in patients accrued in the AIROPROS (prostate working group of the Italian Association of Radiation Oncology) 0102 trial using different endpoint definitions. METHODS AND MATERIALS: The self-reported questionnaires (before RT, 1 month after RT, and every 6 months for 643 years after RT) of 586 patients were available. The peak incontinence (P_INC) and two longitudinal definitions (chronic incontinence [C_INC], defined as the persistence of Grade 1 or greater incontinence after any Grade 2-3 event; and mean incontinence score [M_INC], defined as the average score during the 3-year period after RT) were considered. The correlation between the clinical/dosimetric parameters (including rectal dose-volume histograms) and P_INC (Grade 2 or greater), C_INC, and M_INC of 651 were investigated using multivariate logistic analyses. Receiver operating characteristic curves and the area under the curve were used to assess the predictive value of the different multivariate models. RESULTS: Of the 586 patients, 36 with a Grade 1 or greater incontinence score before RT were not included in the present analysis. Of the 550 included patients, 197 (35.8%) had at least one control with a Grade 1 or greater incontinence score (M_INC &gt;0). Of these 197 patients, 37 (6.7%), 22 (4.0%), and 17 (3.1%) were scored as having P_INC, M_INC 651, and C_INC, respectively. On multivariate analysis, Grade 2 or greater acute incontinence was the only predictor of P_INC (odds ratio [OR], 5.9; p = .0009). Grade 3 acute incontinence was predictive of C_INC (OR, 9.4; p = .02), and percentage of the rectal volume receiving &gt;40 Gy of 6580% was predictive of a M_INC of 651 (OR, 3.8; p = .008) and of C_INC (OR, 3.6; p = .03). Previous bowel disease, previous abdominal/pelvic surgery, and the use of antihypertensive (protective factor) correlated highly with both C_INC and M_INC 651. The predictive values of the models for C_INC (area under the curve, 0.83) and M_INC 651 (area under the curve, 0.73) were greater than the ones for P_INC (area under the curve, 0.62) and more reliable (p = .0001-.0003 against p = .02). Nomograms for the two longitudinal definitions were derived. CONCLUSIONS: The longitudinal definitions of fecal incontinence (C_INC and M_INC 651) were helpful in accounting for both the persistence and the severity of the incontinence. A significant fraction of peak events was consequential to acute incontinence, and a longer duration of symptoms mainly depended on the rectal dose bath (percentage of rectal volume receiving &gt;40 Gy), and pretreatment clinical factors

Inclusion of clinical risk factors into NTCP modelling of late rectal toxicity after high dose radiotherapy for prostate cancer

Background and purpose: To fit an NTCP model including clinical risk factors to late rectal toxicities after radiotherapy for prostate cancer. Methods and materials: Data of 669 patients were considered. The probability of late toxicity within 36 months (bleeding and incontinence) was fitted with the original and a modified Logit-EUD model, including clinical factors by fitting a subset specific TD 50s: the ratio of TD 50s with and without including the clinical variable was the dose-modifying factor (D mod). Results: Abdominal surgery (surg) was a risk factor for G2-G3 bleeding, reflecting in a TD 50 = 82.7 Gy and 88.4 Gy for patients with and without surg (D mod = 0.94; 0.90 for G3 bleeding); acute toxicity was also an important risk factor for G2-G3 bleeding (D mod = 0.93). Concerning incontinence, surg and previous diseases of the colon were the clinical co-factors. D mod(surg) and D mod(colon) were 0.50 and 0.42, respectively for chronic incontinence and 0.73 and 0.64, respectively for mean incontinence score \ue2\u89\ua51. Best-fit n values were 0.03-0.05 and 1 for bleeding and incontinence, respectively. The inclusion of clinical factors always improved the predictive value of the models. Conclusions: The inclusion of predisposing clinical factors improves NTCP estimation; the assessment of other clinical and genetic factors will be useful to reduce parameter uncertainties

Clinical and dosimetric predictors of late rectal toxicity after conformal radiation for localized prostate cancer : results of a large multicenter observational study

Purpose: Assessing the predictors of late rectal toxicity after high-dose conformal radiotherapy for prostate cancer. Methods: One thousand one hundred thirty-two patients entered a prospective observational multicentric study; late rectal toxicity was evaluated by a self-reported questionnaire. Results concerning bleeding and faecal incontinence of 718/1132 patients with a complete follow-up at 36 months were analysed. The correlation between a number of clinical-dosimetric parameters and moderate/severe toxicity was investigated by univariate and multivariate logistic analyses. Results: Fifty-two (7.2%) and 57/718 (7.9%) patients were scored as moderate/severe bleeders and faecal incontinents, respectively; 19/57 incontinent patients showed persistent incontinence at 36 months. Bleeding was mainly correlated with V75 Gy while severe bleeding was mainly correlated with the previous abdominal/pelvic surgery; a different rectal dose-volume relationship in the two groups of patients (with/without surgery) was found. Moderate/severe acute toxicity was weakly correlated to late bleeding. The best predictor of faecal incontinence was acute toxicity (OR = 4 and 7 for chronic and actuarial incontinence, respectively). Conclusion: The application of rectal dose-volume constraints limited the incidence of rectal bleeding. The risk of bleeding may be further reduced by limiting V75 Gy &lt; 5% and, in the case of patients previously submitted to abdominal/pelvic surgery, V70 Gy &lt; 15-20%. Faecal incontinence seems to be mainly a consequential effect after acute toxicity