Clinical evaluation of monitor unit software and the application of ...

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DEVIATION [%] 6% 4% 2% 0% -2% -4% -6% -8% -10% Pelvic fields (157) fields were checked for step-and-shoot IMRT delivery and dynamic MLC IMRT delivery. Mean deviations (incl. standard deviations) between MUV and the local TPS were 1.0 ± 7.3% for dynamic IMRT delivery and 1.3 ± 3.2% for step-and-shoot IMRT delivery, respectively. Step-and-shoot IMRT results in the thorax were again biased by tissue inhomogeneity; no dynamic IMRT was performed in the thoracic region. For dynamic IMRT cases in the pelvis good agreement was obtained between MUV and the local TPS (mean: 1.6 ± 1.5%). In order to verify IMRT fields against ionisation chamber measurements and to compare the achievable accuracy for IM fields with the one for open beams, a few treatment plans were recalculated in a homogeneous phantom with both MUV and the TPS. In other words, recalculated IMRT verification plans were considered in this part. Table 4 summarizes the results for open static beams and IMRT cases, both on an individual field basis and for composite treatment plans. These results confirm the high accuracy of the independent dose/MU calculation software MUV in homogeneous conditions, i.e. when comparing ionisation chamber measurements with dose calculations in identical conditions D. Georg et al. / Radiotherapy and Oncology 85 (2007) 306–315 311 Thoracic fields (65) Head & neck fields (83) Fig. 2. Illustration of the improvements for radiological depth corrections at the field level. Median and quartiles for pelvic fields (total 157), thoracic fields (total 65) and head-and-neck fields (total 83) when using geometric or radiological depth for independent dose calculations (filled circles...with radiological path length correction, filled diamonds...all data). Table 3 Summary of deviations between MUV and the local treatment planning system, as a function of conformal treatment technique and treatment site (RL... radiological depth) Pelvic fields Thorax fields Head-and-neck fields All data RL data All data RL data All data RL data Open beams 0.5 ± 1.3% (273) 0.6 ± 1.3% (107) 2.6 ± 7.6% (95) 1.6 ± 2.2% (51) 0.5 ± 2.1% (75) 1.0 ± 1.1% (45) Physical wedges 0.9 ± 2.3% (46) 1.3 ± 2.6% (31) 1.1 ± 1.0% (17) 1.2 ± 0.9% (8) 0.6 ± 1.5% (48) 1.1 ± 1.5% (24) Dynamic wedges 0.2 ± 2.6% (38) 1.3 ± 1.5% (19) 3.2 ± 7.8% (36) 0.6 ± 2.5% (6) 0.0 ± 2.0% (22) 0.3 ± 1.4% (14) The numbers in brackets indicate the total number of fields per category in a water phantom or a solid verification phantom without homogeneities. There are, however, additional uncertainties in both TPS calculations and calculations performed with MUV, due to the extent and accuracy in modelling rounded leaf ends, tongue-and-groove effects, leaf transmission or the distribution of the direct source (X-ray target). When compared to open beams, larger mean deviations and standard deviations for individual IMRT fields (see Table 4a) are also influenced by the larger overall experimental uncertainty for ionisation chamber measurements in IMRT treatments [24]. Large relative deviations are also influenced by the beam contribution to the overall treatment plan and for composite treatment plans excellent agreement was found between ionisation chamber measurements and calculations performed with MUV, see Table 4b. Clinical action levels Fig. 3a and b show treatment site dependent frequency distributions of deviations between MUV and TPS calculations. The dashed lines indicate a deviation level of ±3% or ±5%. For all treatment sites there were systematic deviations which are biased by dose calculation uncertainties
312 Clinical evaluation of fluence based MU software Table 4 Comparison of dose calculations and ionisation chamber (IC) measurements for individual treatment fields performed under identical conditions in a water phantom or a solid water equivalent verification phantom Treatment technique (a) Individual fields Number of fields (MUV IC)/IC (mean ± 1 SD) (%) (TPS IC)/IC (mean ± 1 SD) (%) Open irregular fields 47 0.7 ± 0.6 0.3 ± 1.0 Dynamic IMRT 56 0.7 ± 4.1 1.0 ± 5.8 Step-and-shoot IMRT 48 0.6 ± 2.5 1.1 ± 3.3 Number of plans (MUV IC)/IC (mean ± 1 SD) (%) (TPS IC)/IC (mean ± 1 SD) (%) (b) Composite treatment plan Open irregular fields 12 0.4 ± 0.6 0.4 ± 1.1 Dynamic IMRT 9 0.1 ± 1.1 1.5 ± 3.0 Step-and-shoot IMRT 7 0.3 ± 1.0 1.7 ± 0.9 and the fact that dose calculations in a homogeneous flat water equivalent medium were compared with dose calculations that were based on CT information. Best agreement between TPS and independent dose calculations was observed for pelvic fields. When using radiological depth information the majority (98%) of all deviations was within ±5%, for pelvic and head-and-neck fields even within ±3% ( 92% of all fields). From the results of ionisation chamber measurements presented in Table 4a and from previously published experimental verification of MUV in homogeneous conditions, treatment technique dependent confidence intervals were defined as follows. For open beams the confidence interval in a homogeneous medium was even as small as ±2%. However, for the implementation of clinical action levels additional uncertainties need to be considered that account for different dose calculation conditions for the TPS and independent verification calculations, i.e. effects related to patient anatomy and inhomogeneities. These additional tolerances were estimated from the results presented in Figs. 1–3 and Table 3. When applying radiological depth corrections a clinical action level of ±3% (open) and ±5% (wedged and IMRT), respectively, seems to be suitable for pelvic and head-and-neck treatments. For thorax treatments, even with radiological depth correction, a site specific additional uncertainty of 3% seems to be necessary for all treatment techniques. If no radiological depth correction is performed site dependent additional uncertainties of 2% are recommended for head-and-neck treatments and 5% for thorax treatments. Discussion Dose calculations performed with a treatment planning system (TPS) can be verified with different approaches. Although quality assurance (QA) of TPS has been recognized as an important topic for many years, practical guidelines or recommendations for performance verification and periodic QA of TPS have become available only recently [10,15,16]. The detection of systematic errors in TPS calculations requires extensive or ‘‘stress’’ testing, which is difficult to perform in practice because no specific recommendation is available addressing the number of tests, test conditions, etc. On the other hand, if systematic uncertainties under specific conditions remain overlooked, the quality of a radiotherapy treatment is limited and no corrective actions can be set. An intelligent design to carry out such stress tests in an efficient way for a particular TPS requires a priori knowledge of the dose calculation algorithm. For those reasons stress tests are very often based on experimental techniques. In that aspect dedicated dose calculation software, which is designed to achieve as high dose calculation accuracy in homogeneous conditions as possible, can be very helpful in order to keep the workload at an acceptable level. From the results obtained within the present study and from previously published articles from the same authors [17–20], we conclude that MUV can be considered as such a tool. Besides systematic TPS tests and QA during the commissioning phase it is also recommended to use a second independent MU calculation to verify the MU data produced by the TPS as part of patient specific QA. For many years empirical factor based formalisms or in-vivo dosimetry have been in use for independent dose verification e.g. [3,5]. These procedures were governed by fairly simple treatment techniques with rather uncomplicated planning procedures. In the light of advanced treatment techniques based on MLC technology and complex planning procedures the general properties of independent dose calculation need to be reconsidered. Demands on accuracy could be shifted towards the standards of today’s planning dose calculations or even higher; single point dose verification in a semi-infinite slab approximation could be replaced by 2D (or 3D) calculations taking into account the patients’ anatomy. In other words, independent dose calculation algorithms can be designed to provide a high level of accuracy, even when patient anatomy is included. Depending on how well the independent dose calculation is integrated into the clinical workflow long calculation times may not constitute an actual problem. For example, QA-procedures for dose calculation can be designed to run automatically, i.e. without demanding any manual operations, by executing verification calculations on all treatment plans prior to the treatment start from a ‘‘QA robot’’, which is for example linked to the database of the record and verify (R&V) system. Treatment plans exceeding a certain deviation could be marked in the database and a notification is sent to clinical physicists. This is also an attractive concept to fulfil QA and pa-
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