Intensity-modulated proton radiotherapy spares musculoskeletal structures in regional nodal irradiation for breast cancer: a dosimetric comparison.

BACKGROUND AND PURPOSE: Regional nodal irradiation (RNI) for breast cancer delivers radiation in proximity to the shoulder and torso, and radiation exposure may contribute to long-term upper extremity and postural morbidity. To date, no studies have assessed the differential dosimetric impact of proton versus photon radiation on shoulder and torso anatomy. This study examined clinically relevant musculoskeletal (MSK) structures and assessed the dose delivered with each modality. PATIENTS/MATERIAL AND METHODS: Ten MSK structures were contoured on IMPT (intensity-modulated proton therapy) and VMAT (volumetric modulated arc therapy) plans for 30 patients receiving RNI. Relevant dose metrics were compared for each of the structures. Intensity-modulated proton therapy dose was calculated using the relative biological effective value of 1.1. Hypo-fractionated plans were scaled to the equivalent dose in 2 Gy fractions (EQD2) using an alpha/beta ratio of four. Wilcoxon signed rank sum tests compared doses. Select three-dimensional and optimised VMAT plans were also informally compared. RESULTS AND INTERPRETATION: Each of the 10 structures received a statistically significantly lower dose with the use of IMPT compared with VMAT. Differences were greatest for posterior structures, including the trapezius, latissimus dorsi and glenohumeral joint. Mean absolute differences were as great as 23 Gy (supraspinatus D5cc) and up to 30-fold dose reductions were observed (deltoid D50cc). An average 3.7-fold relative dose reduction existed across all structures. Measures of low/intermediate dose (V15Gy and D50cc) showed the largest differences. Intensity-modulated proton therapy results in statistically lower radiation exposure to relevant shoulder and torso anatomy compared to photon radiation for patients requiring RNI. Prospective study is needed to correlate functional outcomes with radiation dose.

Case study of a bolus helmet used to maintain optic chiasm and nerve sparing while improving target coverage using IMPT.

The purpose of this study was to examine if the use of a bolus helmet when treating the head with intensity modulated proton therapy (IMPT) will maintain organs at risk (OAR) sparing while improving the clinical target volume (CTV) coverage. A bolus helmet is a device that aims to improve on the traditionally used range shifter in proton therapy by improving dose distribution characteristics. Ten patients were retrospectively selected who had 2 separate treatment planning scans performed, a scan with the bolus helmet and a second scan without. Plans were created using both scans. Dose to organs at risk (OAR) including the left optic nerve, right optic nerve, optic chiasm, and normal brain minus CTV (brain-CTV), as well as CTV coverage were compared between the 2 plans. The use of the bolus helmet displayed lower mean OAR doses as well as higher CTV coverage, suggesting that use of the bolus helmet provides benefit when treating the head with IMPT.