3D in vivo dosimetry using megavoltage cone-beam CT and EPID dosimetry.

Department of Radiation Oncology (MAASTRO), GROW Research Institute, University Medical Centre Maastricht, Maastricht, The Netherlands.
International journal of radiation oncology, biology, physics (Impact Factor: 4.26). 05/2009; 73(5):1580-7. DOI: 10.1016/j.ijrobp.2008.11.051
Source: PubMed


To develop a method that reconstructs, independently of previous (planning) information, the dose delivered to patients by combining in-room imaging with transit dose measurements during treatment.
A megavoltage cone-beam CT scan of the patient anatomy was acquired with the patient in treatment position. During treatment, delivered fields were measured behind the patient with an electronic portal imaging device. The dose information in these images was back-projected through the cone-beam CT scan and used for Monte Carlo simulation of the dose distribution inside the cone-beam CT scan. Validation was performed using various phantoms for conformal and IMRT plans. Clinical applicability is shown for a head-and-neck cancer patient treated with IMRT.
For single IMRT beams and a seven-field IMRT step-and-shoot plan, the dose distribution was reconstructed within 3%/3mm compared with the measured or planned dose. A three-dimensional conformal plan, verified using eight point-dose measurements, resulted in a difference of 1.3 +/- 3.3% (1 SD) compared with the reconstructed dose. For the patient case, planned and reconstructed dose distribution was within 3%/3mm for about 95% of the points within the 20% isodose line. Reconstructed mean dose values, obtained from dose-volume histograms, were within 3% of prescribed values for target volumes and normal tissues.
We present a new method that verifies the dose delivered to a patient by combining in-room imaging with the transit dose measured during treatment. This verification procedure opens possibilities for offline adaptive radiotherapy and dose-guided radiotherapy strategies taking into account the dose distribution delivered during treatment sessions.

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Available from: Sebastiaan Nijsten
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    • "Radiotherapy pre-treatment plan verification is an important aspect in Quality Assurance (QA) of intensity modulated radiotherapy (IMRT) [1]. In literature, various dose verification techniques suitable for patient specific QA have been described [2] [3] [4] [5]. Commonly , patient specific QA is performed by a physicist who accepts or rejects a treatment plan based on QA results. "
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    ABSTRACT: Background and purpose: Treatment plan verification of intensity modulated radiotherapy (IMRT) is generally performed with the gamma index (GI) evaluation method, which is difficult to extrapolate to clinical implications. Incorporating Dose Volume Histogram (DVH) information can compensate for this. The aim of this study was to evaluate DVH-based treatment plan verification in addition to the GI evaluation method for head and neck IMRT. Materials and methods: Dose verifications of 700 subsequent head and neck cancer IMRT treatment plans were categorised according to gamma and DVH-based action levels. Fractionation dependent absolute dose limits were chosen. The results of the gamma- and DVH-based evaluations were compared to the decision of the medical physicist and/or radiation oncologist for plan acceptance. Results: Nearly all treatment plans (99.7%) were accepted for treatment according to the GI evaluation combined with DVH-based verification. Two treatment plans were re-planned according to DVH-based verification, which would have been accepted using the evaluation alone. DVH-based verification increased insight into dose delivery to patient specific structures increasing confidence that the treatment plans were clinically acceptable. Moreover, DVH-based action levels clearly distinguished the role of the medical physicist and radiation oncologist within the Quality Assurance (QA) procedure. Conclusions: DVH-based treatment plan verification complements the GI evaluation method improving head and neck IMRT-QA.
    Full-text · Article · Aug 2014 · Radiotherapy and Oncology
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    • "In the work of McDermott et al. [25] measured EPID images were back-projected to multiple planes in a (kilovoltage) CBCT, assuming a water-equivalent electron density. Van Elmpt et al. [26,27] acquired megavoltage CBCT scans with calibrated electron densities to derive the entrance energy fluence for each treatment beam by back-projection of measured PDIs. These fluences were then used for reconstruction of the dose distribution in the CBCT using a Monte Carlo dose algorithm. "
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    ABSTRACT: To investigate for prostate cancer patients the comparison of 'in-vivo' measured portal dose images (PDIs) with predictions based on a kilovoltage cone-beam CT scan (CBCT), acquired during the same treatment fraction, as an alternative for pre-treatment verification. For evaluation purposes, predictions were also performed using the patients' planning CTs (pCT). To get reliable CBCT electron densities for PDI predictions, Hounsfield units from the pCT were mapped onto the CBCT, while accounting for non-rigidity in patient anatomy in an approximate way. PDI prediction accuracy was first validated for an anatomical phantom, using IMRT treatment plans of ten prostate cancer patients. Clinical performance was studied using data acquired for 50 prostate cancer patients. For each patient, 4--5 CBCTs were available, resulting in a total of 1413 evaluated images. Measured and predicted PDIs were compared using gamma-analyses with 3% global dose difference and 3 mm distance to agreement as reference criteria. Moreover, the pass rate for automated PDI comparison was assessed. To quantify improvements in IMRT fluence verification accuracy results from multiple fractions were combined by generating a gamma-image with values halfway the minimum and median gamma values, pixel by pixel. For patients, CBCT-based PDI predictions showed a high agreement with measurements, with an average percentage of rejected pixels of 1.41% only. In spite of possible intra-fraction motion and anatomy changes, this was only slightly larger than for phantom measurements (0.86%). For pCT-based predictions, the agreement deteriorated (average percentage of rejected pixels 2.98%), due to an enhanced impact of anatomy variations. For predictions based on CBCT, combination of the first 2 fractions yielded gamma results in close agreement with pre-treatment analyses (average percentage of rejected pixels 0.63% versus 0.35%, percentage of rejected beams 0.6% versus 0%). For the pCT-based approach, only combination of the first 5 fractions resulted in acceptable agreement with pre-treatment results. In-room acquired CBCT scans can be used for high accuracy IMRT fluence verification based on in-vivo measured EPID images. Combination of gamma results for the first 2 fractions can largely compensate for small accuracy reductions, with respect to pre-treatment verification, related to intra-fraction motion and anatomy changes.
    Full-text · Article · Sep 2013 · Radiation Oncology
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    • "For the fluence reconstruction, it is not necessary to relate measured PDI pixel values directly to dose in the patient. For reconstruction of the delivered 3D patient dose we use our clinical planning system, while others use an independent dose calculation algorithm [15] [16] [28]. The advantage of this latter approach might be that errors in the dose calculation can also be detected at once. "

    Full-text · Article · Nov 2009 · International Journal of Radiation OncologyBiologyPhysics
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