Radiotherapy treatment of early-stage prostate cancer with IMRT and protons: a treatment planning comparison

Abstract

Purpose: To compare intensity-modulated photon radiotherapy (IMRT) with three-dimensional conformal proton therapy (3D-CPT) for early-stage prostate cancer, and explore the potential utility of intensity-modulated proton therapy (IMPT).

Methods and materials: Ten patients were planned with both 3D-CPT (two parallel-opposed lateral fields) and IMRT (seven equally spaced coplanar fields). Prescribed dose was 79.2 Gy (or cobalt Gray-equivalent, [CGE] for protons) to the prostate gland. Dose-volume histograms, dose conformity, and equivalent uniform dose (EUD) were compared. Additionally, plans were optimized for 3D-CPT with nonstandard beam configuration, and for IMPT assuming delivery with beam scanning.

Results: At least 98% of the planning target volume received the prescription dose. IMRT plans yielded better dose conformity to the target, whereas proton plans achieved higher dose homogeneity and better sparing of rectum and bladder in the range below 30 Gy/CGE. Bladder volumes receiving more than 70 Gy/CGE (V70) were reduced, on average, by 34% with IMRT vs. 3D-CPT, whereas rectal V70 were equivalent. EUD from 3D-CPT and IMRT plans were indistinguishable within uncertainties for both bladder and rectum. With the use of small-angle lateral-oblique fields in 3D-CPT and IMPT, the rectal V70 was reduced by up to 35% compared with the standard lateral configuration, whereas the bladder V70 increased by less than 10%.

Conclusions: In the range higher than 60 Gy/CGE, IMRT achieved significantly better sparing of the bladder, whereas rectal sparing was similar with 3D-CPT and IMRT. Dose to healthy tissues in the range lower than 50% of the target prescription was substantially lower with proton therapy.

Conflict of interest statement

Conflict of Interest Notification: The authors declare NO conflict of interest in relation to this manuscript.

Figures

Figure 1

A proton range compensator map: (a) designed based on the radiological depth of the target’s distal edge; and (b) smeared to counter the range uncertainties. The letters indicate anterior (A), posterior (P), inferior (I) and superior (S) directions.

Figure 2

Patient 1: dose distribution in the transversal isocenter section from (a) IMRT, (b) 3D-CPT and (c) IMPT plans. Dashed white lines show the contours of the prostate, PTV1, rectum, bladder and femoral heads.

Figure 3

Patient 1: dose-volume histograms from IMRT, 3D-CPT (parallel-opposed lateral beam configuration, labeled “lat”) and IMPT plans

Figure 4

Patient 2: dose distributions from (a) IMRT, (b) 3D-CPT plan using parallel-opposed lateral beam configuration; the difference between doses delivered by these two plans is shown in (c). Dose distribution from the 3D-CPT plan using lateral-anterior-oblique beam configuration, with the beams rotated by 20° towards the anterior, is shown in (d). The outlines of the prostate, PTV1, rectum and femoral heads are designated as dashed white lines in (a, b, d), and solid green lines in (c).

Figure 5

Patient 2: dose-volume histograms from IMRT and 3D-CPT plans with lateral parallel-opposed (“lat”), and anterior-oblique configurations with the beams rotated by 20° (“ao20”) and 50° (“ao50”) towards the anterior. DVH are shown for the gross tumor volume, bladder, right femoral head, whole rectum, anterior and posterior rectal walls.

Figure 6

Patient 2: dose-volume histograms for the whole irradiated body volume.

Figure 7

Dose-volume histograms for the rectum (a-c) and bladder (d-f). Individual DVH from ten 3D-CPT and IMRT plans are shown in (a, d) and (c, f), respectively. Plots (b) and (e) show curves obtained by averaging, over the irradiated volume, of the DVH from 10 plans, as well as one-standard-deviation variability bounds (dashed lines).

Figure 8

Patient 1: IMPT dose distributions delivered by (a) the right lateral and (b) left lateral beams. Dashed purple lines designate the outlines of the prostate, PTV1, rectum, bladder and femoral heads.