Monte Carlo-based software for 3D personalized dose calculations in image-guided radiotherapy.

BACKGROUND AND PURPOSE: Image-guided radiotherapy (IGRT) involves frequent in-room imaging sessions contributing to additional patient irradiation. The present work provided patient-specific dosimetric data related to different imaging protocols and anatomical sites. MATERIAL AND METHODS: We developed a Monte Carlo based software able to calculate 3D personalized dose distributions for five imaging devices delivering kV-CBCT (Elekta and Varian linacs), MV-CT (Tomotherapy machines) and 2D-kV stereoscopic images from BrainLab and Accuray. Our study reported the dose distributions calculated for pelvis, head and neck and breast cases based on dose volume histograms for several organs at risk. RESULTS: 2D-kV imaging provided the minimum dose with less than 1 mGy per image pair. For a single kV-CBCT and MV-CT, median dose to organs were respectively around 30 mGy and 15 mGy for the pelvis, around 7 mGy and 10 mGy for the head and neck and around 5 mGy and 15 mGy for the breast. While MV-CT dose varied sparsely with tissues, dose from kV imaging was around 1.7 times higher in bones than in soft tissue. Daily kV-CBCT along 40 sessions of prostate radiotherapy delivered up to 3.5 Gy to the femoral heads. The dose level for head and neck and breast appeared to be lower than 0.4 Gy for every organ in case of a daily imaging session. CONCLUSIONS: This study showed the dosimetric impact of IGRT procedures. Acquisition parameters should therefore be chosen wisely depending on the clinical purposes and tailored to morphology. Indeed, imaging dose could be reduced up to a factor 10 with optimized protocols.

Fractionated stereotactic radiotherapy of benign skull-base tumors: a dosimetric comparison of volumetric modulated arc therapy with Rapidarc® versus non-coplanar dynamic arcs.

BACKGROUND: Benign tumors of the skull base are a challenge when delivering radiotherapy. An appropriate choice of radiation technique may significantly improve the patient's outcomes. Our study aimed to compare the dosimetric results of fractionated stereotactic radiotherapy between non-coplanar dynamic arcs and coplanar volumetric modulated arctherapy (Rapidarc®). METHODS: Thirteen patients treated with Novalis TX® were analysed: six vestibular schwannomas, four pituitary adenomas and three meningioma. Two treatment plans were created for each case: dynamic arcs (4-5 non coplanar arcs) and Rapidarc® (2 coplanar arcs). All tumors were >3 cm and accessible to both techniques. Patients had a stereotactic facemask (Brainlab) and were daily repositioned by Exactrac®. GTV and CTV were contoured according to tumor type. A 1-mm margin was added to the CTV to obtain PTV. Radiation doses were 52.2-54 Gy, using 1.8 Gy per fraction. Treatment time was faster with Rapidarc®. RESULTS: The mean PTV V95 % was 98.8 for Rapidarc® and 95.9 % for DA (p = 0.09). Homogeneity index was better with Rapidarc®: 0.06 vs. 0.09 (p = 0.01). Higher conformity index values were obtained with Rapidarc®: 75.2 vs. 67.9 % (p = 0.04). The volume of healthy brain that received a high dose (V90 %) was 0.7 % using Rapidarc® vs. 1.4 % with dynamic arcs (p = 0.05). Rapidarc® and dynamic arcs gave, respectively, a mean D40 % of 10.5 vs. 18.1 Gy (p = 0.005) for the hippocampus and a Dmean of 25.4 vs. 35.3 Gy (p = 0.008) for the ipsilateral cochlea. Low-dose delivery with Rapidarc® and dynamic arcs were, respectively, 184 vs. 166 cm(3) for V20 Gy (p = 0.14) and 1265 vs. 1056 cm(3) for V5 Gy (p = 0.67). CONCLUSIONS: Fractionated stereotactic radiotherapy using Rapidarc® for large benign tumors of the skull base provided target volume coverage that was at least equal to that of dynamics arcs, with better conformity and homogeneity and faster treatment time. Rapidarc® also offered better sparing of the ipsilateral cochlea and hippocampus. Low-dose delivery were similar between both techniques.

In vivo dose verification from back projection of a transit dose measurement on the central axis of photon beams.

PURPOSE: In vivo dose verification is used to prevent major deviations between the prescribed dose and the dose really delivered to the patient. This work presents a quick and simple alternative method for verification of dose delivered to the patient using photon beams. During the treatment session, a transit dose is measured with the EPID and the dose in the patient is estimated from back projection of the portal dose. METHODS AND MATERIALS: The formalism for dose calculation is described. It is independent of the detector and has been validated for different beam energies using an ionization chamber (IC). Central axis doses estimated by this formalism were compared with measured dose. Subsequently, the IC was replaced by the EPID appropriately calibrated. The feasibility of the method and its applicability in clinical use has been evaluated on 3 8 patients treated with conformal therapy for various localizations. RESULTS: Ratios between stated and measured doses are reported. They are within the accepted tolerance of classical in vivo dosimetry (SD of 3.5%). CONCLUSIONS: The proposed method for in vivo dose verification is very simple to implement and to use in clinics. Measurements can be repeated during several sessions giving the opportunity to built new strategies for the validation by statistical evaluation of the data. The trending of in vivo dose along the treatment becomes also possible. The number of checkable beams is also increased by this method.

Performance optimization of the Varian aS500 EPID system.

Today, electronic portal imaging devices (EPIDs) are widely used as a replacement to portal films for patient position verification, but the image quality is not always optimal. The general aim of this study was to optimize the acquisition parameters of an amorphous silicon EPID commercially available for clinical use in radiation therapy with the view to avoid saturation of the system. Special attention was paid to selection of the parameter corresponding to the number of rows acquired between accelerator pulses (NRP) for various beam energies and dose rates. The image acquisition system (IAS2) has been studied, and portal image acquisition was found to be strongly dependent on the accelerator pulse frequency. This frequency is set for each "energy - dose rate" combination of the linear accelerator. For all combinations, the image acquisition parameters were systematically changed to determine their influence on the performances of the Varian aS500 EPID system. New parameters such as the maximum number of rows (MNR) and the number of pulses per frame (NPF) were introduced to explain portal image acquisition theory. Theoretical and experimental values of MNR and NPF were compared, and they were in good agreement. Other results showed that NRP had a major influence on detector saturation and dose per image. A rule of thumb was established to determine the optimum NRP value to be used. This practical application was illustrated by a clinical example in which the saturation of the aSi EPID was avoided by NRP optimization. Moreover, an additional study showed that image quality was relatively insensitive to this parameter.