A comprehensive comparison of IMRT and VMAT plan quality for prostate cancer treatment

Abstract

Purpose: We performed a comprehensive comparative study of the plan quality between volumetric-modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) for the treatment of prostate cancer.

Methods and materials: Eleven patients with prostate cancer treated at our institution were randomly selected for this study. For each patient, a VMAT plan and a series of IMRT plans using an increasing number of beams (8, 12, 16, 20, and 24 beams) were examined. All plans were generated using our in-house-developed automatic inverse planning (AIP) algorithm. An existing eight-beam clinical IMRT plan, which was used to treat the patient, was used as the reference plan. For each patient, all AIP-generated plans were optimized to achieve the same level of planning target volume (PTV) coverage as the reference plan. Plan quality was evaluated by measuring mean dose to and dose-volume statistics of the organs at risk, especially the rectum, from each type of plan.

Results: For the same PTV coverage, the AIP-generated VMAT plans had significantly better plan quality in terms of rectum sparing than the eight-beam clinical and AIP-generated IMRT plans (p < 0.0001). However, the differences between the IMRT and VMAT plans in all the dosimetric indices decreased as the number of beams used in IMRT increased. IMRT plan quality was similar or superior to that of VMAT when the number of beams in IMRT was increased to a certain number, which ranged from 12 to 24 for the set of patients studied. The superior VMAT plan quality resulted in approximately 30% more monitor units than the eight-beam IMRT plans, but the delivery time was still less than 3 min.

Conclusions: Considering the superior plan quality as well as the delivery efficiency of VMAT compared with that of IMRT, VMAT may be the preferred modality for treating prostate cancer.

Fig.1

Flowchart of the OFPA algorithm for prostate IMRT and VMAT, where i is the index of the optimization cycle, which has an upper limit of 6. D_threshold and f_weight are arbitrarily chosen to be 76.4 Gy and 5, respectively, based on experience.

Fig.2

Dose-volume histograms of three plans resulted from one AIP execution with different trade-offs between PTV coverage and rectum sparing.

Fig.3

Average PTV conformality index and heterogeneity index values for the 11 patients for each type of plan.

Fig.4

Number of patients who received better rectum sparing from each type of IMRT plan than from their VMAT plan.

Fig.5

Dose statistics in the rectum for the 11 patients from the 8-beam clinical IMRT plan (a), 8-(b), 12- (c), 16- (d), 20- (e) and 24-beam (f) AIP-generated IMRT plans, and the AIP-generated VMAT plan (g).

Fig.6

Dose statistics in the bladder for the 11 patients from the 8-beam clinical IMRT plan (a), 8- (b), 12- (c), 16- (d), 20- (e) and 24-beam (f) AIP-generated IMRT plans, and the AIP-generated VMAT plan (g).

Fig.7

Dose-volume histograms of the PTV, rectum, and bladder from IMRT plans with different numbers of beams and the AIP-generated VMAT plan for a typical patient. The numbers in parentheses in the legend give the total number of control points for each plan.

Fig.8

Dose distributions represented by isodose lines from the 8-beam clinical and AIP-generated IMRT plans, the 24-beam AIP-generated IMRT plan, and the AIP-generated VMAT plan.

Fig.9

Average total MUs of the 11 patients.

Fig.10

DVH curves from two VMAT plans with different MUs.

Fig.11

Dose-volume histograms of the PTV, rectum, and bladder from the single- and dual-arc VMAT plans for a typical patient.