Comparison of dosimetric characteristics between flattening filter‐free and flattening filter mode volumetric‐modulated arc therapy plans in rectal cancer

We aimed to compare the dosimetric characteristics of 6‐MV flattening filter mode (FF) and flattening filter‐free mode (FFF) volumetric‐modulated arc therapy (VMAT) plans in preoperative radiotherapy for rectal cancer using an Edge linear accelerator.


INTRODUCTION
Rectal cancer is one of the most common malignant tumors of the digestive system. The location of the rectum within the bony pelvis and its proximity to vital structures present significant therapeutic challenges. 1,2 Preoperative radiotherapy can increase the radical resection rate and significantly reduce the local recurrence rate.
The combination of preoperative radiotherapy and chemotherapy, named neoadjuvant therapy, has become the standard treatment for stage II/III rectal cancer. 3,4 Currently, two main preoperative radiotherapy schemes are available for the treatment of rectal cancer 5 : (i) short-course neoadjuvant radiotherapy, with a prescribed dose of 25 and improved treatment efficiency. 12 Studies have also shown that a non-uniform beam (FFF) can reduce the dose to normal tissue and the incidence of secondary cancer. 13 However, when the field was less than 10 × 10 cm 2 , the incident dose was higher than that of the uniform beam, although the difference was not significant for larger fields. 14 Currently, FFF is mostly used in stereotactic radiotherapy.
In the present study, 15 patients with rectal cancer were selected to compare the dosimetry differences of VMAT between 6-MV flattening filter mode (FF) and heterogeneous integration mode (FFF) of the Varian Edge linear accelerator in preoperative radiotherapy for rectal cancer.

Patient selection and contouring
The present study included 15 patients with a median age of 62 years.
All patients were immobilized with a thermoplastic mask in the supine position. Computed tomography (CT) images with 5.0-mm slice thicknesses were acquired using a 16-slice CT scanner (GE Healthcare, Waukesha, WI, USA). The scanning range was from the upper edge of the lumbar 1 vertebral body to the inferior ischial tubercle The clinical target volumes (CTVs) were delineated by an experienced radiation oncologist according to the Radiation Therapy Oncology Group guidelines. 15 The CTV included the gross tumor volume, gross tumor lymph node, mesorectum, presacral area, obturator drainage area, and some bilateral iliac and internal and external lymphoid drainage areas. The planning target volume (PTV) was defined as a 0.5-cm expansion of the unified CTV. The organs at risk (OARs) included the bladder, intestine, colon, and bilateral femoral heads.

Treatment planning
Two-arc FF-VMAT and FFF-VMAT plans were generated for each patient using the Philips Pinnacle (version 9.10) treatment plan-

Dose delivery
The total MUs and BOT of the FF-VMAT and FFF-VMAT plans were analyzed.

Statistical analysis
All statistical analyses were carried out using IBM SPSS Statistics for Windows, version 22.0 (IBM Corp., Armonk, NY, USA). The plan evaluation parameters for each structure and deviation from the dose constraints were calculated for each plan. Student's t-tests were used for data that obey normal distribution. Statistical significance was set at P < 0.05.

PTV dose distributions and evaluations
Both plans reached the clinical constraints, with similar dose coverages of the PTV (Figure 1). Table 1 lists the dosimetry parameters of the PTV.
The FF-VMAT plan achieved better conformity, with higher CIs, than the FFF-VMAT plan (Table 1).
In addition, the FF-VMAT plan achieved better homogeneity, with a lower homogeneity index, than the FFF-VMAT plan. The V 105% signifi-

F I G U R E 2
Box plot shows the volume of planning target volume covered by 105% isodose lines (cm 3 ) of flattening filter mode volumetric-modulated arc therapy (FF-VMAT) and flattening filter-free mode volumetric-modulated arc therapy (FFF-VMAT) plans for 15 patients cantly increased in the FFF-VMAT plan than in the FF-VMAT plan (Figure 1). The V 105% of the FFF-VMAT plan was significantly higher than that of the FF-VMAT plan (Figure 2).

Dose sparing of the OARs
There was no significant difference in the V 30 and V 40 of the bladder between the FF-VMAT and FFF-VMAT plans (Figure 3). The V 30 and V 40 of the small intestine increased by 5.5% and 2.1% (

Beam delivery efficiency
The total MUs and BOT were collected and analyzed ( Table 3)

DISCUSSION
Both FF-VMAT and FFF-VMAT techniques provide improved dose conformity, homogeneity, and sparing of high-dose irradiation. VMAT technology reduces the therapeutic time and MUs, and is associated with an enhanced tumor gain ratio. [12][13][14] Conventional VMAT uses the FF, which is associated with some disadvantages, including prolonged delivery time, reduced treatment dose rate, decreased photon intensity, and enhanced treatment dose scattering. 16,17 Although the flattening filter in the linear accelerator can make the dose distribution uniform at a specific depth of the phantom, the X-ray quality hardens and scattering increases after the X-rays pass through the homogenizer. 18,19 With the development of intensity modulation technology, a uniform dose distribution can be achieved using an MLC. When the flattening filter is removed, the X-ray quality becomes softer and scattering is reduced. 20 VMAT technology combined with the FFF has been widely used in stereotactic radiotherapy because of its good conformal degree and high efficiency. 21,22 To investigate whether the FFF-VMAT plan is applicable for rectal cancer, FF-VMAT and FFF-VMAT plans were generated for each patient using the Philips Pinnacle treatment planning system for the Edge linear accelerator equipped with high-definition MLCs.
Both the plans reached the clinical constraints, with similar dose coverages of the PTV. The FF-VMAT plan achieved better conformity and homogeneity, with higher CIs, than the FFF-VMAT plan. The V 105% of the FFF-VMAT plan was significantly higher than that of the FF-VMAT plan. The non-flattened beam has a greater number of lowenergy photons than the flattened beam because of minimal beam hardening owing to the absence of the flattening filter. Thus, the nonflattened beam requires slightly more intensity than the flattened beam for the same dose. 17 The rectum target was located deep within the bony pelvis. Thus, to meet the target dose in the deep volume, the cumulative dose was increased in the ray path. Furthermore, the accelerator also needed to output more machine hops to meet the dose in the deep target area, leading to a significantly higher V 105% and MU in the FFF-VMAT plan than in the FF-VMAT plan. The dose sparing of the OARs did not differ significantly between the FF-VMAT and FFF-VMAT plans. The FF-VMAT plan delivered approximately half of the MU of the FFF-VMAT plan. However, the BOT of the FFF-VMAT plan decreased slightly owing to its higher dose rate compared with the FF-VMAT plan.
In conclusion, the present study generated FF-VMAT and FFF-VMAT plans for 15 patients with rectal cancer using the Philips Pinnacle treatment planning system for the Edge linear accelerator equipped with high-definition MLCs. Both the plans met the clinical constraints.
The FF-VMAT plan achieved better conformity and homogeneity than the FFF-VMAT plan. The FFF-VMAT plan showed no significant advantage in terms of dose sparing of the OARs compared with the FF-VMAT plan. The FFF did not significantly shorten the treatment time.