Assessment of radiobiological metrics applied to patient-specific QA process of VMAT prostate treatments.

VMAT is a powerful technique to deliver hypofractionated prostate treatments. The lack of correlations between usual 2D pretreatment QA results and the clinical impact of possible mistakes has allowed the development of 3D verification systems. Dose determination on patient anatomy has provided clinical predictive capability to patient-specific QA process. Dose-volume metrics, as evaluation criteria, should be replaced or complemented by radiobiological indices. These metrics can be incorporated into individualized QA extracting the information for response parameters (gEUD, TCP, NTCP) from DVHs. The aim of this study is to assess the role of two 3D verification systems dealing with radiobiological metrics applied to a prostate VMAT QA program. Radiobiological calculations were performed for AAPM TG-166 test cases. Maximum differences were 9.3% for gEUD, -1.3% for TCP, and 5.3% for NTCP calculations. Gamma tests and DVH-based comparisons were carried out for both systems in order to assess their performance in 3D dose determination for prostate treatments (high-, intermediate-, and low-risk, as well as prostate bed patients). Mean gamma passing rates for all structures were bet-ter than 92.0% and 99.1% for both 2%/2 mm and 3%/3 mm criteria. Maximum discrepancies were (2.4% ± 0.8%) and (6.2% ± 1.3%) for targets and normal tis-sues, respectively. Values for gEUD, TCP, and NTCP were extracted from TPS and compared to the results obtained with the two systems. Three models were used for TCP calculations (Poisson, sigmoidal, and Niemierko) and two models for NTCP determinations (LKB and Niemierko). The maximum mean difference for gEUD calculations was (4.7% ± 1.3%); for TCP, the maximum discrepancy was (-2.4% ± 1.1%); and NTCP comparisons led to a maximum deviation of (1.5% ± 0.5%). The potential usefulness of biological metrics in patient-specific QA has been explored. Both systems have been successfully assessed as potential tools for evaluating the clinical outcome of a radiotherapy treatment in the scope of pretreatment QA.

Dosimetric validation and clinical implementation of two 3D dose verification systems for quality assurance in volumetric-modulated arc therapy techniques.

A pretreatment quality assurance program for volumetric techniques should include redundant calculations and measurement-based verifications. The patient-specific quality assurance process must be based in clinically relevant metrics. The aim of this study was to show the commission, clinical implementation, and comparison of two systems that allow performing a 3D redundant dose calculation. In addition, one of them is capable of reconstructing the dose on patient anatomy from measurements taken with a 2D ion chamber array. Both systems were compared in terms of reference calibration data (absolute dose, output factors, percentage depth-dose curves, and profiles). Results were in good agreement for absolute dose values (discrepancies were below 0.5%) and output factors (mean differences were below 1%). Maximum mean discrepancies were located between 10 and 20 cm of depth for PDDs (-2.7%) and in the penumbra region for profiles (mean DTA of 1.5 mm). Validation of the systems was performed by comparing point-dose measurements with values obtained by the two systems for static, dynamic fields from AAPM TG-119 report, and 12 real VMAT plans for different anatomical sites (differences better than 1.2%). Comparisons between measurements taken with a 2D ion chamber array and results obtained by both systems for real VMAT plans were also performed (mean global gamma passing rates better than 87.0% and 97.9% for the 2%/2 mm and 3%/3 mm criteria). Clinical implementation of the systems was evaluated by comparing dose-volume parameters for all TG-119 tests and real VMAT plans with TPS values (mean differences were below 1%). In addition, comparisons between dose distributions calculated by TPS and those extracted by the two systems for real VMAT plans were also performed (mean global gamma passing rates better than 86.0% and 93.0% for the 2%/2 mm and 3%/ 3 mm criteria). The clinical use of both systems was successfully evaluated.

Impact of neoadjuvant hormonal therapy on dose-volume histograms in patients with localized prostate cancer under radical radiation therapy.

INTRODUCTION: Prostate volume involves a defined toxicity predictor in the radiation therapy of localized prostate cancer. Neoadjuvant hormone therapy (nHT) can reduce prostate volume and, therefore, the planned volume. The objective of this study was to establish if the value of nHT reduces the planned volume and if this reduction correlates with a reduction of the dose received in the target organs. MATERIAL AND METHODS: 28 patients diagnosed of localized prostate cancer and referred to our departments for radiation therapy with radical intention, in the period ranging between April 2002 and October 2003, were included prospectively. The patients received nHT (triptorelin + flutamide) for 2 months and adjuvant HT until completing 2 years in the high-risk cases. A transrectal ultrasound study was performed in all patients, simulation CT and planning before the start of HT and after 2 months of treatment. The radiation therapy was carried out with 6 or 18 MV LINAC photons, with a dose fractioning scheme of 5 x 180-200 cGy, a total dosage of 66-72 Gy to prostate, 56 Gy to seminal vesicles and, in the high-risk cases, 46 Gy to pelvic lymph nodes. RESULTS: The distribution according to risk group was: low risk 3.6%, intermediate risk 28.6% and high risk 67.9%. By transrectal ultrasound, prostate volume on diagnosis was 50.65 cc pre HT and 38.97 cc post HT (p < 0.001), which means a volume reduction of 24%. The comparative analysis of the dose-volume histograms of the first versus the second CT shows a reduction in the planned volume GTV1 (prostate) (81.33 cc vs 63.96 cc, p < 0.05), PTV1 (prostate and margin) (197.51 cc vs 168.38 cc, p < 0.001) and PTV2 (prostate, vesicles and margin) (340.5 cc vs 307.26 cc, p < 0.05), a reduction of the maximum dose in the seminal vesicles (70.2 versus 68.75 Gy, p < 0.05), a reduction of the mean dose in the seminal vesicles (65.07 Gy versus 63.07 Gy, p < 0.05), PTV2 (67.72 Gy versus 66.9 Gy, p < 0.01) and PTV3 (prostate, vesicles, pelvic lymph nodes and margin) (58.86 Gy versus 57.21 Gy, p < 0.01), a reduction of the D90 in the seminal vesicles (61.83 Gy versus 60.06 Gy, p < 0.05) and PTV2 (61.04 Gy versus 59.45 Gy, p < 0.05) and a reduction of V60 of the rectum (32.45% versus 28.22%, p < 0.05) and V60 of the bladder (41.78% versus 31.67%, p < 0.005). CONCLUSIONS: Neoadjuvant hormone therapy reduces significantly prostate volume and as a result the planned volume and consequently the rectal and bladder V60 can be significantly reduced.

Impact of neoadjuvant hormonal therapy on dose-volume histograms in patients with localized prostate cancer under radical radiation therapy

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Introduction. Prostate volume involves a definedtoxicity predictor in the radiation therapy of localizedprostate cancer. Neoadjuvant hormone therapy(nHT) can reduce prostate volume and, therefore,the planned volume. The objective of thisstudy was to establish if the value of nHT reducesthe planned volume and if this reduction correlateswith a reduction of the dose received in thetarget organs.Material and methods. 28 patients diagnosed of localizedprostate cancer and referred to our departmentsfor radiation therapy with radical intention,in the period ranging between April 2002 andOctober 2003, were included prospectively. The patientsreceived nHT (triptorelin + flutamide) for 2months and adjuvant HT until completing 2 yearsin the high-risk cases. A transrectal ultrasoundstudy was performed in all patients, simulation CTand planning before the start of HT and after 2months of treatment. The radiation therapy wascarried out with 6 or 18 MV LINAC photons, with adose fractioning scheme of 5 x 180-200 cGy, a totaldosage of 66-72 Gy to prostate, 56 Gy to seminalvesicles and, in the high-risk cases, 46 Gy to pelviclymph nodes.Results. The distribution according to risk groupwas: low risk 3.6%, intermediate risk 28.6% and highrisk 67.9%. By transrectal ultrasound, prostate volumeon diagnosis was 50.65 cc pre HT and 38.97 ccpost HT (p < 0.001), which means a volume reductionof 24%. The comparative analysis of the dosevolumehistograms of the first versus the second CTshows a reduction in the planned volume GTV1(prostate) (81.33 cc vs 63.96 cc, p < 0.05), PTV1(prostate and margin) (197.51 cc vs 168.38 cc, p <0.001) and PTV2 (prostate, vesicles and margin)(340.5 cc vs 307.26 cc, p < 0.05), a reduction of themaximum dose in the seminal vesicles (70.2 versus68.75 Gy, p < 0.05), a reduction of the mean dose inthe seminal vesicles (65.07 Gy versus 63.07 Gy, p <0.05), PTV2 (67.72 Gy versus 66.9 Gy, p < 0.01) andPTV3 (prostate, vesicles, pelvic lymph nodes andmargin) (58.86 Gy versus 57.21 Gy, p < 0.01), a reductionof the D90 in the seminal vesicles (61.83 Gyversus 60.06 Gy, p < 0.05) and PTV2 (61.04 Gy versus59.45 Gy, p < 0.05) and a reduction of V60 of the rectum(32.45% versus 28.22%, p < 0.05) and V60 of thebladder (41.78% versus 31.67%, p < 0.005).Conclusions. Neoadjuvant hormone therapy reducessignificantly prostate volume and as a resultthe planned volume and consequently the rectaland bladder V60 can be significantly reduced