The commissioning of the dose calculation algorithms of a treatment planning system is generally performed by entering the basic beam data into the system according to the methods and requirements described in the system user manual and by comparing the results of dose calculations with the entered data and with data that were measured specifically for this purpose.

The purpose of this paper is to present our experience with the commissioning and quality assurance (QA) of PrecisePLAN treatment planning system with multileaf collimator using step and shoot IMRT technique.

The Blue PhantomTM Radiation Field Analyser was accurately positioned in the treatment room and left there for at least 2 hours in order to reach temperature stability. The measured data were evaluated with great care in order to construct a data set with great consistency.

PrecisePLAN treatment planning system
A 3D treatment planning system (TPS) for intensity modulated radiotherapy using a multileaf collimator has been made available by Elekta. Two approaches of computerized treatment planning for step and shoot IMRT are generally applied: the first method is an extension of conventional treatment planning and is referred to as forward planning. Its definition of the segment shapes is performed manually similar to conventional planning. However, more than one segment is used from each beam direction. Afterwards, the weights of the segments are optimized using a computer optimization algorithm to achieve the desired dose distribution [6]. The second strategy, which we denote as inverse planning, usually starts with the optimization of fluence profiles from each beam direction by minimization of an objective function. Afterwards, sequencing transforms each optimized profile into a series of segments, which can be delivered with a multileaf collimator.

Dosimetry
The following basic dosimetric parameters were measured for the purpose of modeling the 6 MV photon beam for the treatment planning system used in this study.

Depth dose data
The depth dose data for open beams were measured for 3, 5, 6, 8, 10, 12, 15, 20, 25, 30, 40 (cm x cm) square field sizes, for depths from 0 to 22 cm. The depth dose data for 60° wedged fields for square field sizes of 3, 5, 6, 8, 10, 12, 15, 20, 25, and rectangular field 30 x 40 (cm x cm), for same depth.

Profiles
Three open beam depth profiles for square field sizes 10, 20, and 30 (cm x cm) at the depths of 10 and 20 cm were measured. The wedge profiles for maximum wedged field size of 30 x 40 cm were measured at 1.5 cm (dmax), 5 cm, 10 cm, and 20 cm. The off-axis-ratio (OAR) is the ratio of dose on a point at depth d, to the dose for the same field size and depth at the central axis. The OAR was measured at the beam profiles. The beam profiles were measured with high resolution (smaller step), minimum detector size (CC13 ionization chamber) and measured at different depths.

Output factors and scatter factors
The relative output factors were measured for square field sizes 3, 4, 5, 6, 8, 10, 12, 15, 18, 20, 25, 30, 35, and 40 (cm x cm) and normalized for 10 cm x 10 cm output. Two scatter factors Sc (Head Scatter Factor) and Sp (Phantom Scatter Factor) were needed. The Sc was measured with 3 cm diameter build-up cap with the ion chamber in an upright position. For those users of the data set for whom the treatment planning system required output factors at dmax, Sc,p was recalculated to the output defined at this depth using the ratios of percentage depth dose (PDD) values. Sp is obtained by dividing Sc,p by Sc for a given field size.

Bulk leaf transmission and back up jaw transmission
The bulk leaf transmission and back up jaw transmission were measured and entered into the treatment planning system.

Fig 1: Percentage depth dose of respective open field sizes using CC13 pin-point ion chamber

The percentage depth dose for wedged field sizes was measured and the variation found to be 4.2% between the 3 cm x 3 cm and 30 cm x 40 cm wedged field sizes at depth of 10 cm as shown in figure 2.

Fig 2: Percentage depth dose of respective wedged field sizes using CC13 pin-point ion chamber

The OAR were computed from the beam profiles for 10 cm x 10 cm, 20 cm x 20 cm, and 30 cm x 30 cm field sizes at 10 and 20 cm depths in the x (crossline) and y (inline) directions and were found within ±0.2 mm. The OAR values entered in the treatment planning system were the average of the x and y profiles. The x and y profiles were as shown in figure 3a and figure 3b. The wedged profiles were measured at 1.5 cm, 5 cm, 10 cm, and 20 cm depths as shown in Figure 3c.

Fig 3a: Crossline dose profile

Fig 3b: Inline dose profile

Fig 3c: The wedged beam profiles measured at 1.5 cm, 10 cm, and 20 cm depths

The relative output factors (OF) for different field sizes was as shown in figure 4 and the scatter factor Sc(r) for open field sizes was as shown in figure 5.

Fig 4: The relative output factors for various field sizes

Figure 5: Scatter factors for open field sizes

Quality-assurance of dose calculation by measurements
Point dose calculations
The absolute point dose of the treatment plan was measured by using Head and Neck Cube supplied by Scanditronix Wellhofer. The dimension of this cube was 18 cm x 18 cm x 18 cm and had the facility to insert the ion chamber. The prostate plan was executed on this phantom and the point dose at the isocenter was calculated (TPS value). The same plan was executed on machine with same phantom by inserting 0.65 cc active volume Farmer type ion chamber (FC65-G). The dose at isocenter was measured. The calculated and the measured doses were compared and the variation was noted in table 1.

Table 1: Comparison of isocenter doses between the calculated the measured doses for 5 IMRT plans

The deviations between the calculated dose, D_calc, and the measured dose, D_meas at a specific depth was given by (D_calc – D_meas) x 100% / D_meas. In those cases where the points were outside the penumbra or under a block, the results of the comparison were expressed relatively to the dose measured at the same depth, but on the central axis of the open beam, D_meas,cax according to (D_calc – D_meas)x100% / D_meas,cax [7,8].

Isodose distribution comparison
Film measurement was performed for a prostate IMRT plan. Same prostate plan with seven non-coplanar beams were used. The Torso Phantom (Scanditronix Wellhofer’s) was used to compare the calculated and the measured dose distributions. The film (Kodak EDR) was stacked in between the two slabs. Isodose distributions were measured using Kodak EDR film. The exposed film was scanned with Vidar Scanner and the distributions were seen through Omnipro IMRT Software. The comparisons between the calculated and the measured dose distributions were made within Omnipro IMRT Software system [9]. The results for the prostate IMRT plan were as shown in figures 6A- 6D.

Fig 6A: Planned image generated from TPS for a prostate case

Fig 6B: Actual exposed image with Torso Phantom for a prostate case

Fig 6C: Profiles comparison for prostate case

Fig 6D: Comparison of isodoses for prostate case

From the figures shown in Figure 6B-6C, it is seen that the agreement between the calculated and the measured dose is within 2% in the high-dose region. For clinical patient specific QA we specify 3% dose difference and 3 mm distance acceptance scaling criteria [10].

The bulk leaf transmission and back up jaw transmission were 0.02% and 0.105%.

Treatment-planning system configuration parameters must be measured precisely. All the dosimetric tests have proved very useful and detected only minor deviations. The TPS calculated dose and the measured absolute dose should not deviate more than 3% to ensure safe treatment. It is necessary to maintain strict criteria to compare measured and calculated values. The patient pretreatment QA requires significant machine time for measurements and must be completed before treatment starts. In order to reduce patient pretreatment QA time, an independent monitor unit calculation program can be evaluated. The introduction of new analyzing tools, such as DTA (distance to agreement) or g (gamma factor), correlation coefficient can be useful to better quantify the comparison between measured versus calculated dose distributions [11,12]. The implementation of IMRT must not be underestimated. Every institution should adopt a QA protocol. The QA protocol presented here has proved adequate, and with it we have had no patient complications attributed to IMRT delivery.

ACKNOWLEDGEMENT
The authors would like to thank Vikash Pathak of Elekta Instrument (India) for his valuable suggestions and technical support to perform quantitative comparative analysis.

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