CHAPTER 4 PRE TREATMENT PATIENT SPECIFIC QUALITY ASSURANCE OF RAPIDARC PLANS

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1 47 CHAPTER 4 PRE TREATMENT PATIENT SPECIFIC QUALITY ASSURANCE OF RAPIDARC PLANS 4.1 INTRODUCTION Advanced treatment techniques use optimized radiation beam intensities to conform dose distribution to the irregular shaped planning target volume (PTV). This is especially useful when the PTV has a concave shape or if organs at risk (OAR) lie very close to the PTV. When IMRT is delivered by means of a multileaf collimator (MLC), its success strongly depends on the behaviour of the leaves, namely their precise positioning and, in the case of dynamic treatments, the accuracy of the leaf speed control. The dose distribution is very sensitive to a number of factors, among which are the leaf end design, the transmission through the leaves and between two adjacent leaves, and the gap between two opposed leaves. Unlike conformal homogeneous beams, intensity-modulated beams may have areas of high dose gradient anywhere in the field and not at the boundaries so that a correct modelling of the beam penumbra by the treatment planning system (TPS) is especially important. Thus, intensity modulated radiation delivery requires strict quality assurance (QA) hardware tests for MLC and specific patient treatment plan validation procedures. The latter generally involve mapping the plan fields onto a phantom that has been computed tomography scanned, creating a so called hybrid plan, and comparing the results with measurements made on that phantom. It is assumed that the validity of the results for the phantom can be extrapolated to the patient.

2 48 Ionization chambers are the preferred dosimeters for measuring absorbed dose in intensity modulated radiation therapy deliveries. Although other methods such as one-dimensional or two-dimensional arrays of detectors, film, and electronic portal imaging devices, among others, have proved increasingly useful, they are more suited to the measurement of relative dose distributions. Dose measurements with ionization chambers (ICs) reflect the average dose value over their volumes. Since the RapidArc delivery involves complex treatment delivery procedures like variable dose rate and variable gantry speed, the patient specific quality assurance for individual patient should be done. In this paper we present our methodology for RapidArc pre-treatment patient specific QA procedure for hundred different cases using semiflex ionization chamber. 4.2 MATERIALS AND METHOD Pre Treatment Rapid Arc Quality Assurance Hundred different RapidArc plans conforming to the clinical standards were selected for the pre treatment patient specific quality assurances study using semiflex ionization chambers (0.125 cm 3 ). A treatment plan is considered clinically acceptable if the dose within the PTV is uniform to within predefined limits, typically ± 5%, and the doses to OARs are lower than those which would cause unacceptable normal tissue complications. Verification plan was created for each treatment plan with the chamberphantom combinations CT scanned (Figure 4.1). All plans were aimed to deliver 200 cgy at the isocentre. All the verification plans has were done using the Eclipse planning system version 8.6 using the AAA algorithm. All measurements have been done using Varian 2100 C/D linear accelerator. The semiflex ionization chamber was inserted inside the Octavius CT phantom (Octavius Phantom without compensating cavity) in such a way that the central axis of radiation beam will be coincided with the centre of the

3 49 sensitive volume of the chamber. Experimental setup is shown in Figure 4.2. The source to detector effective point of measurement is kept at 100cm and the source to surface distance (SSD) at 84cm. The ionization chamber was connected to the array interface. From the array interface data has been transferred to the PTW tansoft software using the RS 232 cable. Figure 4.1 Verification plan window in eclipse treatment planning system for the pre treatment patient quality assurance using semiflex ionization chamber and octavius CT phantom The semiflex chambers are air vented waterproof thimble chambers used for measuring high-energy photon and electron radiation. The chamber used in this study has a sensitive volume of cm 3, with a flat energy response within a wide energy range. The nominal useful energy range is from 30 kv to 50 MV photons and 6 MeV to 50 MeV electrons. The wall material is graphite with a protective acrylic cover. The guard rings are designed up to the measuring volume. The cylindrically shaped chamber has

4 50 an inner diameter of 5.5 mm. The cm 3 chamber is ideal for 3D dosimetry, since the measuring volume is approximately spherical resulting in a flat angular response over an angle of ± 160 and a uniform spatial resolution along all three axes. The specification of the ionization chambers, phantoms, and software used for the study is specified in Table 4.1. Before the measurements the chambers were cross calibrated with a secondary standard at our clinic for the photon beam. All the results were cross compared with fluence measurements using 2D seven29 array combined with Octavius LINAC phantom. The gamma analysis was done using the PTW verisoft software (version 3.2). Table 4.1 Specifications of the ionization chambers, phantoms and software used in the study Ionization chamber PTW Semiflex Chamber ( ) PTW 2D-Array 729 (T10024) Type Semiflex Chamber vented cubic ion chamber Array Electrometer/software PTW - TanSoft V 1.2 PTW- Matrix Scan (S080050) Active volume cm cm 3 (single ion chamber) Polarizing voltage 400V 400V Wall material PMMA + Graphite Graphite Phantom used PTW Octavius CT phantom PTW Octavius LINAC phantom

5 51 Figure 4.2 Set-up for the pre-treatment quality assurance tests for rapid arc treatment delivery. Semiflex ionization chamber inserted inside the Octavius CT phantom. SSD was kept at 84 cm Gamma Analysis Gamma analysis was done to compare two 2D dose maps. Generally the dose distribution comparison is sub divided into regions of high and low dose gradients. In low gradient regions, the doses are compared directly with an acceptance tolerance placed on the difference between the two dose maps. In high dose gradient regions, a small spatial error, in either of the dose maps or small miss alignment results in large dose difference between the dose maps. Dose difference in the high dose gradient regions may therefore be relatively unimportant and the concept of DTA distribution is used to determine the acceptability of agreement. The DTA is the distance between a dose point in a dose map and the nearest point in the other dose

6 52 map that exhibits the same dose. The dose difference and DTA evaluations complement each other when used as determinants of agreement accuracy between the dose maps. The simultaneous use of DTA and a percent dose difference (DD) is proposed by Daniel A Low et al (1998). These parameters can help evaluate the agreement of the two distributions in terms of misalignment and difference, respectively. So in this study the various gamma index constraints which are a combination of particular DTA value with specific dose difference tolerance value were used. 4.3 RESULTS Figures 4.3, 4.4, and 4.5 shows the percentage variation of absolute dose between planned and measured for head and neck, thoracic and abdomen cases respectively. The statistical analysis for absolute dose measured with semiflex ionization chamber is shown in Table 4.2. Positive absolute mean dose variation of 0.56 % was observed with thorax cases with a standard deviation (SD) of ± 1.13 between the plans with a range of -1.78% to 2.70%. Negative percentage dose errors were found with head and neck and abdomen cases, with a mean variation of % (SD ± 1.50), (range % to 2.85 %) and % (SD ± 1.48), (range % to 2.65 %) for head and neck and abdomen cases respectively. Positive variations in measured absolute doses were observed with thoracic cases. Most of the cases selected in this category for planning were oesophagus. Also negative variations in mean dose were observed with head and neck and abdomen cases, where the MLC movement is stringent and more number of critical organs was involved. The results were compared with the fluence measurement using the 2D array seven29.

7 53 Figure 4.3 Percentage variation for planned and measured isocentre absolute dose using semiflex ionization chamber for head and neck cases Figure 4.4 Percentage variation for planned and measured isocentre absolute dose using semiflex ionization chamber for thoracic cases

8 54 Figure 4.5 Percentage variation for planned and measured isocentre absolute dose using semiflex ionization chamber for abdomen cases Table 4.2 Statistical analysis for absolute dose measured with semiflex ionization chamber Statistical analysis for semiflex ionization chamber Head and neck Thorax Abdomen maximum negative deviation (%) maximum positive deviation (%) mean deviation (%) standard deviation sample variance count negative positive negative to positive ratio confidence level Kurtosis (for all 100 patients) Skewness (for all 100 patients) 0.22

9 55 Ionization chamber and 2D array relative dose measurement results were highly comparable. Relative dose measurements with 2D array agreed well with the TPS calculated dose for all the cases. The measured and calculated fluence maps were both normalized to the same point and analysis was done. Evaluation was done using PTW verisoft software by keeping the passing criteria as 3mm DTA, 3% DD, for 95% of the evaluated dose points. Figure 4.6 shows the gamma analysis results for the hundred different RapidArc plans. The maximum percentage value failed in gamma analysis was found to be 4.95, 4.75, and 4.88 for head and neck, thorax, and abdomen cases respectively. The results indicate that the measured values agree well with that of the calculated by the treatment planning system in the treated volume region. In all the cases analysed the percentage dose points failed the gamma criteria is less than 5% (Table 4.3). Also the higher variation observed may be because of the complex MLC movements with small effective opening which will be more challenging to deliver the radiation. Also the larger deviations were observed particularly in the low dose regions outside the treatment volume. So by properly defining the region of interest (planning target volume) for the gamma analysis, it is possible to increase the pass percentage. Figure 4.6 Gamma analysis for 3mm DTA, 3% DD for all the hundred patients

10 56 Table 4.3 Gamma analysis results for hundred cases using PTW verisoft software Gamma analysis (% dose points failed) Head and neck Thorax Abdomen minimum maximum mean (SD) DISCUSSION The differences between the planned and measured doses can be attributed to three different sources of error: dosimeter, delivery system, and dose calculation system. However, if the dosimeter is properly chosen, commissioned, and maintained, errors related to calculation, delivery, or a combination of the two can be reduced. The selection of ionization chambers for the dose verification process for RapidArc involves issues particular to the dosimetry of dynamically delivered small fields, varying dose rates, and the summation of multiple low MU segments. Detector characteristics, such as energy dependence, the size of the collecting volume, charge leakage, design, and materials are important considerations. In addition, volumetric modulated arc therapy dosimetry conditions are radically different from the open field conditions under which the chambers are calibrated, and may invalidate the use of such calibration factors in these radiation beams. Dose discrepancies between the TPS and measurements also arise due to inaccurate beam and component modeling, the dose calculation algorithm, and beam delivery.

11 57 During RapidArc delivery, the detector is often located either outside of the field or in penumbra regions, resulting in volume averaging which is especially important over gradient regions. To overcome this problem suitable volume chambers are suggested for RapidArc absolute dose measurements. Semiflex ionization chamber with cm 3 proved to be ideal for RapidArc absolute dosimetry. Also in gamma analysis the percentage of passing points depends on choice of normalization point. The requirements for dosimetric accuracy in gamma analysis are highest in regions of low dose gradient and that the requirements for geometric accuracy are highest in regions of high dose gradient. There are therefore two criteria for accuracy. Dose points situated in the penumbra region were found to be responsible for low passing criteria. Study published by Danielle Fraser et al (2009) suggests that the degree of underestimation was the greatest for the smallest volume chamber for intensity modulated type of radiation delivery. Ion chamber based detector arrays are known to have insignificant energy and dose-rate dependence for megavoltage photon beams, but required a large sensitive volume, with diameter of the order of 5mm for each chamber, to gain signal and will therefore exhibit a volume averaging effect in steep dose gradient regions. Therefore the 2D array was used for fluence measurement rather than the absolute point dose measurement in the present study. For point dose measurements, the semiflex ionization chamber was kept inside the Octavius CT phantom without the compensating cavity. A trend for measured dose values to be lower than the calculated ones was observed mainly with head and neck cases. This observation may be explained by the leaves shielding the ICs from the primary beam during a fraction of the irradiation. Others potential cause for under response may be the ionization chamber leakage. Leakage is more marked for small volume ionization chambers, because chamber sensitivity is proportional to volume.

12 58 In practice, absolute dose discrepancy between a treatment planning system and measurement can be clinically accounted by applying TPS correction factor that adjusts the analytical algorithm to better match systematically different intensity modulated measurements. Alternatively, treatment plan specific correction factors can be applied to ionization chamber to account for fluence perturbation effects in RapidArc delivery. In this study the average discrepancy between TPS calculated and measured remains within ICRU Report 24 recommendations for clinical accuracy of ± 5%. Also it is important to normalize the dose distribution in the high dose region and exclude the area receiving a dose less than a certain minimum dose for the RapidArc QA analysis. 4.5 CONCLUSION On the basis of the studies performed, it can be concluded that the semiflex ionization chamber having a volume of 0.125cm 3 can be used efficiently for performing the pre-treatment quality assurance of RapidArc plans for all the sites. The results provide an overall accuracy when compared to fluence measurement done using 2D array seven29.