Defining the Role of Intensity Modulation in Electron Conformal Therapy for the Treatment of Head and Neck Cancer

Abstract

The use of bolus electron conformal therapy (BECT) in the treatment of cancers of the head and neck is often limited by an inability to reduce dosimetric hot spots resulting from surface or tissue heterogeneity. In this study, we examined the potential benefits of using a recently patented, low-cost form of Intensity Modulation for electron therapy (IM-BECT) to reduce treatment hotspots in patients undergoing electron beam therapy for cancer of the Head and Neck (HN). The treatment plans from twelve patients with HN cancer previously treated with BECT were identified (treatment energies ranged from 6-18 MeV and field sizes of 36-625 cm2). Each case included the treatment targets and at least one primary OAR that were defined by the original treating radiation oncologist. Additionally, a target + 2 cm rind structure was created for analysis of the dose deposition in areas immediately surrounding the target volume as a measure of conformality. Each patient plan was transferred into a novel IM-BECT planning software and each case was recomputed as per the original prescription, gantry, couch, collimator angles, and original clinically used bolus. Next, each case was replanned with the inclusion of intensity modulation, as well as a new custom conformal bolus that was designed for optimized range compensation. The patient plans were then normalized such that 100% equals the prescription dose value and then transferred to a plan analysis software for comparison of the target coverage/dose and OAR dose. Comparison of the BECT and IM-BECT treatment plans demonstrated that IM-BECT was able to significantly reduce dosimetric hotspots for this cohort of patients undergoing radiation therapy for HN cancer, bringing the average maximum dose down from 130.6% to only 120.6% (p = 0.044, paired t-test). Moreover, the impact of IM-BECT appeared to be most substantial in the patients with the highest baseline maximum dose. For patients who had a hotspot of 125% or greater, the hotspot was on average reduced by 19%. Further dosimetric analysis demonstrated a small resultant increase in the low dose deposition to the surrounding normal tissues. For BECT, the average primary OAR mean dose and Target+2cm rind mean dose were 27.5% and 60.0%, respectively. For IM-BECT, the average primary OAR mean dose and Target+2cm rind mean dose increased slightly to 30.9% and 64.6%, respectively [Primary OAR mean (P = 0.0008), and Target+2cm rind mean (P = 0.0001), paired t-test]. IM-BECT is an effective method of reducing dosimetric hotspots in patients undergoing radiation therapy for cancer of the HN. This improvement came at the expense of a small increase in dose to the underlying tissues. This retrospective planning study represents the first example of IM-BECT being applied to real patient cases and suggests further development of IM-BECT is warranted.