Simplifying EPID dosimetry for IMRT treatment verification.

PURPOSE: Electronic portal imaging devices (EPIDs) are increasingly used for IMRT dose verification, both pretreatment and in vivo. In this study, an earlier developed backprojection model has been modified to avoid the need for patient-specific transmission measurements and, consequently, leads to a faster procedure. METHODS: Currently, the transmission, an essential ingredient of the backprojection model, is estimated from the ratio of EPID measurements with and without a phantom/patient in the beam. Thus, an additional irradiation to obtain "open images" under the same conditions as the actual phantom/patient irradiation is required. However, by calculating the transmission of the phantom/patient in the direction of the beam instead of using open images, this extra measurement can be avoided. This was achieved by using a model that includes the effect of beam hardening and off-axis dependence of the EPID response on photon beam spectral changes. The parameters in the model were empirically obtained by performing EPID measurements using polystyrene slab phantoms of different thickness in 6, 10, and 18 MV photon beams. A theoretical analysis to verify the sensitivity of the model with patient thickness changes was performed. The new model was finally applied for the analysis of EPID dose verification measurements of step-and-shoot IMRT treatments of head and neck, lung, breast, cervix, prostate, and rectum patients. All measurements were carried out using Elekta SL20i linear accelerators equipped with a hydrogenated amorphous silicon EPID, and the IMRT plans were made using PINNACLE software (Philips Medical Systems). RESULTS: The results showed generally good agreement with the dose determined using the old model applying the measured transmission. The average differences between EPID-based in vivo dose at the isocenter determined using either the new model for transmission and its measured value were 2.6 +/- 3.1%, 0.2 +/- 3.1%, and 2.2 +/- 3.9% for 47 patients treated with 6, 10, and 18 MV IMRT beams, respectively. For the same group of patients, the differences in mean gamma analysis (3% maximum dose, 3 mm) were 0.16 +/- 0.26%, 0.21 +/- 0.24%, and 0.02 +/- 0.12%, respectively. For a subgroup of 11 patients, pretreatment verification was also performed, showing similar dose differences at the isocenter: -1.9 +/- 0.9%, -1.4 +/- 1.2%, and -0.4 +/- 2.4%, with somewhat lower mean gamma difference values: 0.01 +/- 0.09%, 0.01 +/- 0.07%, and -0.09 +/- 0.10%, respectively. Clinical implementation of the new model would save 450 h/yr spent in measurement of open images. CONCLUSIONS: It can be concluded that calculating instead of measuring the transmission leads to differences in the isocenter dose generally smaller than 2% (2.6% for 6 MV photon beams for in vivo dose) and yielded only slightly higher gamma-evaluation parameter values in planes through the isocenter. Hence, the new model is suitable for clinical implementation and measurement of open images can be omitted.

3D Dosimetric verification of volumetric-modulated arc therapy by portal dosimetry.

BACKGROUND AND PURPOSE: To demonstrate the feasibility of back-projection portal dosimetry for accurate 3D dosimetric verification of volumetric-modulated arc therapy (VMAT), pre-treatment as well as in vivo. MATERIALS AND METHODS: Several modifications to our existing approach were implemented to make the method applicable to VMAT: (i) gantry angle-resolved data acquisition, (ii) calculation of the patient transmission, (iii) compensation for detector 'flex' and (iv) 3D dose reconstruction and evaluation. RESULTS: Planned and EPID-(Electronic Portal Image Detector)-reconstructed dose distributions show good agreement for pre-treatment verification of two prostate, a stereotactic lung and a head-and-neck VMAT plan and for in vivo verification of VMAT treatments of prostate and lung cancer. Averaged over pre-treatment verifications, planned and measured isocentre dose ratios were -1.2% (range [-4.7%,1.8%]). 3D gamma analysis (3% maximum dose, 3mm) revealed mean gamma gamma(mean)=0.37 [0.34,0.39], maximum 1% gamma gamma(1%)=0.72 [0.66,0.81] and percentage of points with gamma1 P(gamma)(1)=99% [97%,100%]. For in vivo verification, the average isocentre dose ratio was -1.2% [-0.8%,-1.7%], gamma(mean)=0.52 [0.40,0.64], gamma(1%)=0.92 [0.76,1.08] and P(gamma)(1)=96% [93%,100%]. CONCLUSIONS: Our portal dosimetry method was successfully adapted for verification of VMAT treatments, pre-treatment as well as in vivo.

Limitations of the planning organ at risk volume (PRV) concept.

PURPOSE: Previously, we determined a planning target volume (PTV) margin recipe for geometrical errors in radiotherapy equal to M(T) = 2 Sigma + 0.7 sigma, with Sigma and sigma standard deviations describing systematic and random errors, respectively. In this paper, we investigated margins for organs at risk (OAR), yielding the so-called planning organ at risk volume (PRV). METHODS AND MATERIALS: For critical organs with a maximum dose (D(max)) constraint, we calculated margins such that D(max) in the PRV is equal to the motion averaged D(max) in the (moving) clinical target volume (CTV). We studied margins for the spinal cord in 10 head-and-neck cases and 10 lung cases, each with two different clinical plans. For critical organs with a dose-volume constraint, we also investigated whether a margin recipe was feasible. RESULTS: For the 20 spinal cords considered, the average margin recipe found was: M(R) = 1.6 Sigma + 0.2 sigma with variations for systematic and random errors of 1.2 Sigma to 1.8 Sigma and -0.2 sigma to 0.6 sigma, respectively. The variations were due to differences in shape and position of the dose distributions with respect to the cords. The recipe also depended significantly on the volume definition of D(max). For critical organs with a dose-volume constraint, the PRV concept appears even less useful because a margin around, e.g., the rectum changes the volume in such a manner that dose-volume constraints stop making sense. CONCLUSION: The concept of PRV for planning of radiotherapy is of limited use. Therefore, alternative ways should be developed to include geometric uncertainties of OARs in radiotherapy planning.

Geometrical uncertainties, radiotherapy planning margins, and the ICRU-62 report.

In this paper, we elaborate on the proposals in the ICRU-62 report concerning planning target volume (PTV) margins for geometrical uncertainties during radiotherapy, such as variations in patient set-up and internal organ motion. According to the ICRU, these margins should be such that the planned dose in the PTV is representative of the real dose in the 'moving' clinical target volume (CTV). We demonstrate that the dosimetrical consequences of systematic and random geometrical uncertainties are fundamentally different, which should be reflected in margin calculations. The recommendation in the ICRU-62 report, to quadratically add standard deviations for systematic (Sigma(tot)) and random (sigma(tot)) errors to determine an overall standard deviation for margin calculations, is therefore generally not valid. Instead, a previously published recipe for PTV margin calculation, M = 2Sigma(tot) + 0.7sigma(tot), does indeed account for the different impact of systematic and random errors on the dose in the CTV. If, for both random and systematic uncertainties, the internal and external errors are uncorrelated and quantified by the standard deviations sigma(int), sigma(ext), Sigma(int), Sigma(ext), then Sigma(tot) = square root (Sigma(int)(2) + Sigma(ext)(2)) and sigma(tot) = square root (sigma(int)(2) + sigma(ext)(2)). If the PTV margin thus acquired is deliberately reduced to spare normal tissues, the planned PTV dose is not representative of the CTV anymore. Therefore, we recommend to also report the minimum dose in the volume originally defined by the recipe (designated RTV, i.e. representative target volume).

Investigation of the added value of high-energy electrons in intensity-modulated radiotherapy: four clinical cases.

PURPOSE: Intensity-modulated radiotherapy (IMRT) with photon beams is currently pursued in many clinics. Theoretically, inclusion of intensity- and energy-modulated high-energy electron beams (15-50 MeV) offers additional possibilities to improve radiotherapy treatments of deep-seated tumors. In this study the added value of high-energy electron beams in IMRT treatments was investigated. METHODS AND MATERIALS: In a comparative treatment planning study, conventional treatment plans and various types of IMRT plans were constructed for four clinical cases (cancer of the bladder, pancreas, chordoma of the sacrum, and breast). The conventional plans were used for the actual treatment of the patients. The IMRT plans were optimized using the Orbit optimization code (Löf et al., 2000) with a radiobiologic objective function. The IMRT plans were either photon or combined electron and photon beam plans, with or without dose homogeneity constraints assuming standard or increased radiosensitivities of organs at risk. RESULTS: Large improvements in expected treatment outcome are found using IMRT plans compared to conventional plans, but differences in tumor control probability (TCP) and normal tissue complication probabilities (NTCP) values between IMRT plans with and without electrons are small. However, the use of electrons improves the dose-volume histograms for organs at risk, especially at lower dose levels (e.g., 0-40 Gy). CONCLUSIONS: This preliminary study indicates that addition of higher energy electrons to IMRT can only marginally improve treatment outcome for the selected cases. The dose-volume histograms of organs at risk show improvements for IMRT with higher energy electrons, which may reduce tumor induction but does not substantially reduce NTCP.