Volumetric‐modulated arc therapy and intensity‐modulated radiation therapy treatment planning for prostate cancer with flattened beam and flattening filter free linear accelerators

Abstract This study on patients with localized prostate cancer was set up to investigate valuable differences using flattened beam (FB) and flattening filter free (FFF) mode in the application of intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT). For ten patients, four different plans were calculated with Oncentra planning system of Elekta, using Synergy machines: IMRT and VMAT, with and without flattening filter. Homogeneity and conformity indexes, dose to the organs at risk, and measurements of peripheral dose and dosimetric plan verification including record of the delivery times were analyzed and statistically evaluated. The indexes for homogeneity and conformity (CTV and PTV) are either advantageous or not significantly different for FFF compared to FB with one minor exception. Regarding the doses to the organs at risk and the measured peripheral dose, equivalent or lower doses were delivered for FFF than with FB. Furthermore, the delivery times were significantly shorter for FFF. VMAT compared to IMRT reveals benefits or at least equivalent values. VMAT‐FFF combines the most advantageous plan quality parameters with the shortest delivery times and reduced peripheral dose and is therefore recommended for the given equipment and cancer localization.

Although there are some other publications regarding the treatment of patients with prostate carcinoma using linacs with FFF mode, they refer to other types of rotational technique and manufacturers and different treatment planning systems (TPS). [11][12][13] However, the design of the linac head affects the PD 14 and the penumbra of the beam. The desktop software, the hardware of the linac, and the TPS influence the treatment time. Kry et al. 15 showed that there is also a dependency of the PD on the field size and the amount of modulation. Therefore investigations with the same or similar equipment, 16,17 but different targets cannot simply be transferred. This work demonstrates for the first time a comparison of FB and FFF plans for prostate cancer therapy with the given equipment of linacs and TPS.

2.A | Patients, regions of interest (ROI), and dose prescription
Ten consecutive patients with histologically proven, previously untreated localized prostate cancer were included in this planning study. All patients had given their informed consent to take part in the study. DICOM data sets including delineated regions of interest were taken from a former investigation. 18 The mean age of the patients was 71 yr. For the planning CT study, they were immobilized in supine position in a vacuum mattress (Blue-BAG TM BodyFIX â , Medical Intelligence, Schwabm€ unchen, Germany) according to Boehmer et al. 19 In each slice with an effective distance of 2.5 mm, the following volumes of interest were delineated following the description of Bos et al. 20 : As target volumes the gross target volume (GTV: prostate gland and seminal vesicles), the clinical target volume (CTV: 5-mm three-dimensional margin added to the GTV excluding the rectal volume), the planning target volume (PTV: 10-mm threedimensional margin added to the GTV without respect to the rectum), and as organs at risk (OAR) the rectal volume (according to Guckenberger et al. 21 ), urinary bladder and femoral heads. To achieve high plan quality, two additional volumes were constructed. First, the PTV with an added margin of 5 mm was subtracted from the rectum volume, resulting in the posterior part of the rectum; second, the CTV was subtracted from the PTV (PTV À CTV) to model the dose gradient in this region.
Criteria were formulated to build a complete set of accepted values which represents the dose prescription in the sense of ICRU report 83: 22 The average dose to the CTV should be in the range of 71.5 Gy to 73.7 Gy, allowing a deviation of 1.5% in the total dose.
For the PTV, 56.4 Gy were set as acceptance value for D 98% PTV . For the OAR, we required the volume of the rectum receiving up to 70 Gy to be smaller than 20%, the volume of the urinary bladder and the femoral heads receiving up to 50 Gy to be smaller than 50%.

2.B | Linear accelerator
The linac used in the TPS and for the measurements is an Elekta Synergy TM with Agility TM head (Elekta AB, Stockholm, Sweden), which is equipped with 80 interdigitating leaf pairs, having 5 mm leaf width projected to the isocenter. 6 MV photons were applied in FB and FFF mode. It has been shown earlier that the beam quality is very similar for both modes, 23 as it is common for Elekta linacs. 24 The maximum dose rate is 500 monitor units (MU) per minute in FB mode and 1700 MU per minute in FFF. The desktop software is Integrity 3.1 and the record and verify system Mosaiq 2.50.

2.C | Treatment planning system
The treatment planning was performed with Oncentra â External Beam v4.5 (Nucletron â , an Elekta AB). The system has been commissioned especially for VMAT application with BeamModulator TM head first 25 and later also for Agility TM head. 23 The software supports variable gantry speed, which was set to a maximum value

2.D | Planning
The dose optimization parameters and fractionation schedule were taken from the above mentioned study: 18  which are part of the optimization process as well. 30 As also stated for another TPS, 31

2.E | Plan evaluation and statistics
For the evaluation of the plan quality, the following parameters were   37 The percentage of pixels in range (c ≤ 1) of the gamma evaluation was registered.

| 309
For the evaluation of the PD, we followed the description of Dobler et al. 16  To assess the delivery times (DT), they were measured from pressing the start button to the last beam off.

| RESULTS
All plans met the acceptance criteria as formulated for dose prescription. Due to the fact that the focus is on the comparison of FB and FFF mode, the statistical results are grouped in Table 2 correspondingly. Differences between IMRT and VMAT are given in the text only to gain clarity. A DVH for a specific patient is shown in Fig. 2.

3.A | Plan quality
The indexes for homogeneity and conformity (HI, CI CTV , and CI PTV )  Partly this can be traced to a smaller part of the PTV receiving the minimum dose required by the objectives and results therefore in a smaller CI PTV . However, these differences are small too, in most cases less than 1% of the volume of the OAR, especially in comparison to the large standard deviation. Therefore, the toxicity will be similar.

3.B | Plan verification and efficiency
We explained in section 2.D that the minimum and maximum dose values for the CTV were set stronger than could be achieved.
Therefore, the HI as documented in Table 2  The average values and sample standard deviation are over all ten plans per group. Values which are superior with statistical significance for the comparison of FB and FFF are highlighted with bold letters.   13 However, they did not find clinically relevant differences compared to their mArc rotation technique (which is described different from VMAT), which also resulted in shorter delivery times and therefore was recommended.
Kragl et al. 9 and Georg et al. 10  Closer to the isocenter, the patient scatter becomes more and more dominating.
VMAT compared to IMRT results in a further reduction of the PD which seems contradictory to the increased number of MU. This is again in accordance with the results of Dobler et al. 16 at another localization and Bell et al. 13 with another rotation technique (mArc).
Reduced PD will result in a reduced secondary cancer risk.