Assessment of different IMRT boost delivery methods on target coverage and normal-tissue sparing

Abstract

Purpose: Because of biologic, medical, and sometimes logistic reasons, patients may be treated with 3D conformal therapy or intensity-modulated radiation therapy (IMRT) for the initial treatment volume (PTV(1)) followed by a sequential IMRT boost dose delivered to the boost volume (PTV(2)). In some patients, both PTV(1) and PTV(2) may be simultaneously treated by IMRT (simultaneous integrated boost technique). The purpose of this work was to assess the sequential and simultaneous integrated boost IMRT delivery techniques on target coverage and normal-tissue sparing.

Materials and methods: Fifteen patients with head-and-neck (H&N), lung, and prostate cancer were selected for this comparative study. Each site included 5 patients. In all patients, the target consisted of PTV(1) and PTV(2). The prescription doses to PTV(1) and PTV(2) were 46 Gy and 66 Gy (H&N cases), 45 Gy and 66.6 Gy (lung cases), 50 Gy and 78 Gy (prostate cases), respectively. The critical structures included the following: spinal cord, parotid glands, and brainstem (H&N structures); spinal cord, esophagus, lungs, and heart (lung structures); and bladder, rectum, femurs (prostate structures). For all cases, three IMRT plans were created: (1) 3D conformal therapy to PTV(1) followed by sequential IMRT boost to PTV(2) (sequential-IMRT(1)), (2) IMRT to PTV(1) followed by sequential IMRT boost to PTV(2) (sequential-IMRT(2)), and (3) Simultaneous integrated IMRT boost to both PTV(1) and PTV(2) (SIB-IMRT). The treatment plans were compared in terms of their dose-volume histograms, target volume covered by 100% of the prescription dose (D(100%)), and maximum and mean structure doses (D(max) and D(mean)).

Results: H&N cases: SIB-IMRT produced better sparing of both parotids than sequential-IMRT(1), although sequential-IMRT(2) also provided adequate parotid sparing. On average, the mean cord dose for sequential-IMRT(1) was 29 Gy. The mean cord dose was reduced to approximately 20 Gy with both sequential-IMRT(2) and SIB-IMRT. Prostate cases: The volume of rectum receiving 70 Gy or more (V(>70 Gy)) was reduced to 18.6 Gy with SIB-IMRT from 22.2 Gy with sequential-IMRT(2). SIB-IMRT reduced the mean doses to both bladder and rectum by approximately 10% and approximately 7%, respectively, as compared to sequential-IMRT(2). The mean left and right femur doses with SIB-IMRT were approximately 32% lower than obtained with sequential-IMRT(1). Lung cases: The mean heart dose was reduced by approximately 33% with SIB-IMRT as compared to sequential-IMRT(1). The mean esophagus dose was also reduced by approximately 10% using SIB-IMRT as compared to sequential-IMRT(1). The percentage of the lung volume receiving 20 Gy (V(20 Gy)) was reduced to 26% by SIB-IMRT from 30.6% with sequential-IMRT(1).

Conclusions: For equal PTV coverage, both sequential-IMRT techniques demonstrated moderately improved sparing of the critical structures. SIB-IMRT, however, markedly reduced doses to the critical structures for most of the cases considered in this study. The conformality of the SIB-IMRT plans was also much superior to that obtained with both sequential-IMRT techniques. The improved conformality gained with SIB-IMRT may suggest that the dose to nontarget tissues will be lower.