Investigating the impact of treatment delivery uncertainties on treatment effectiveness for lung SABR.

To quantify the impact of treatment delivery uncertainties on lung stereotactic ablative body radiotherapy (SABR) plans for step-and-shoot intensity-modulated radiotherapy (ssIMRT) and volumetric modulated arc therapy (VMAT). Baseline ssIMRT and VMAT treatment plans were generated for a cohort of 18 lung SABR patients. Modified plans were generated for each baseline plan by systematically varying gantry and collimator angles between - 5 and + 5 degrees, as well as multi-leaf collimator (MLC) leaf position errors of magnitude between 1 and 5 mm in both directions (i.e. leaf banks shifted either in the same (Type 1) or opposite (Type 2) directions). Planning target volume (PTV), spinal cord and healthy lung dose-volume histogram (DVH) metrics were compared between the modified and baseline plans. Collimator and gantry angle uncertainties did not significantly impact any of the PTV DVH metrics considered. MLC shifts of 5 mm resulted in average V95% changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) and average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) for ssIMRT and VMAT plans. Comparatively, MLC shifts of - 2 mm resulted in average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) and average [Formula: see text] changes of [Formula: see text] (Type 1) and [Formula: see text] (Type 2) for ssIMRT and VMAT plans. ssIMRT gantry angle uncertainties impacted spinal cord DVH metrics the most, with increases in [Formula: see text] of [Formula: see text] occurring for a 1 degree shift. Type 2 MLC modifications impacted all OAR DVH metrics substantially with differences in spinal cord [Formula: see text] (ssIMRT) and healthy lung [Formula: see text] (VMAT) exceeding [Formula: see text] for 5 mm shifts. Uncertainties in MLC leaf positions affected target and OAR DVH metrics more than collimator or gantry angle uncertainties for lung SABR plans. Less patient-to-patient variation occurred from delivery uncertainties in VMAT than ssIMRT.

Comparison of setup accuracy of three different image assessment methods for tangential breast radiotherapy.

INTRODUCTION: To compare the differences in setup errors measured with electronic portal image (EPI) and cone-beam computed tomography (CBCT) in patients undergoing tangential breast radiotherapy (RT). Relationship between setup errors, body mass index (BMI) and breast size was assessed. METHODS: Twenty-five patients undergoing postoperative RT to the breast were consented for this study. Weekly CBCT scans were acquired and retrospectively registered to the planning CT in three dimensions, first using bony anatomy for bony registration (CBCT-B) and again using breast tissue outline for soft tissue registration (CBCT-S). Digitally reconstructed radiographs (DRR) generated from CBCT to simulate EPI were compared to the planning DRR using bony anatomy in the V (parallel to the cranio-caudal axis) and U (perpendicular to V) planes. The systematic (Σ) and random (σ) errors were calculated and correlated with BMI and breast size. RESULTS: The systematic and random errors for EPI (ΣV = 3.7 mm, ΣU = 2.8 mm and σV = 2.9 mm, σU = 2.5) and CBCT-B (ΣV = 3.5 mm, ΣU = 3.4 mm and σV = 2.8 mm, σU = 2.8) were of similar magnitude in the V and U planes. Similarly, the differences in setup errors for CBCT-B and CBCT-S in three dimensions were less than 1 mm. Only CBCT-S setup error correlated with BMI and breast size. CONCLUSIONS: CBCT and EPI show insignificant variation in their ability to detect setup error. These findings suggest no significant differences that would make one modality considered superior over the other and EPI should remain the standard of care for most patients. However, there is a correlation with breast size, BMI and setup error as detected by CBCT-S, justifying the use of CBCT-S for larger patients.

Hypofractionated versus conventionally fractionated radiotherapy for ductal carcinoma in situ (DCIS) of the breast.

INTRODUCTION: Hypofractionated radiotherapy (RT) in the setting of early invasive breast cancer has been shown to have similar local control rates and cosmetic outcomes as conventionally fractionated RT. This study compares ipsilateral recurrence rates between hypofractionated and conventional RT, with and without a boost. The effect of hypofractionated RT and chest wall separation (CWS) on cosmetic outcome was also assessed. METHODS: All patients with ductal carcinoma in situ (DCIS) treated between 1998 and 2012 across two sites of a single cancer institution were retrospectively studied. Patients were analysed according to those receiving conventional RT (≤2 Gy per fraction) and those receiving hypofractionated RT (>2 Gy per fraction), as well as the presence or absence of a tumour bed boost. Data were collected through electronic medical records and local cancer registry. Cosmetic outcome was scored by physicians on a four-point scale during clinical follow-up appointments. RESULTS: One hundred and ninety-seven patients were treated for DCIS during the study period. One hundred and forty-one were treated with conventional RT, and 56 with hypofractionated RT. After a median follow up of 4.4 years, there were 12 ipsilateral recurrences, of which seven were invasive disease and five DCIS. Ten recurrences occurred in patients who received conventional RT (7.1% recurrence rate) and two in those who received hypofractionated RT (3.6% recurrence rate) (P = 0.48). Cosmetic outcomes were not significantly different between conventional and hypofractionated RT (P = 0.06). CONCLUSIONS: Hypofractionation represents a suitable alternative for treating DCIS in the absence of randomised data.

Assessment of MRI image quality for various setup positions used in breast radiotherapy planning.

This study investigates breast magnetic resonance imaging (MRI) image quality for 3 different breast radiotherapy positions (prone, supine flat and supine inclined) and associated choice of breast coils. Supine breast MRI has comparable image quality to prone breast MRI for the purposes of radiotherapy delineation for T2-weighted sequences.