Single-Isocenter Volumetric Modulated Arc Therapy (VMAT) Radiosurgery for Multiple Brain Metastases: Potential Loss of Target(s) Coverage Due to Isocenter Misalignment

Abstract

Purpose A single-isocenter volumetric modulated arc therapy (VMAT) treatment to multiple brain metastatic patients is an efficient stereotactic radiosurgery (SRS) option. However, the current clinical practice of single-isocenter SRS does not account for patient setup uncertainty, which degrades treatment delivery accuracy. This study quantifies the loss of target coverage and potential collateral dose to normal tissue due to clinically observable isocenter misalignment. Methods and materials Nine patients with 61 total tumors (2-16 tumors/patient) who underwent Gamma Knife® SRS were replanned in Eclipse™ using 10 megavoltages (MV) flattening-filter-free (FFF) bream (2400 MU/min), using a single-isocenter VMAT plan, similar to HyperArc™ VMAT plan. Isocenter was placed in the geometric center of the tumors. The prescription was 20 Gy to each tumor. Average gross tumor volume (GTV) and planning target volume (PTV) were 1.1 cc (0.02-11.5 cc) and 1.9 cc (0.11-18.8 cc), respectively, derived from MRI images. The average isocenter to tumor distance was 5.5 cm (1.6-10.1 cm). Six-degrees of freedom (6DoF) random and systematic residual set up errors within [±2 mm, ±2 o ] were generated using an in-house script in Eclipse based on our pre-treatment daily cone-beam CT imaging shifts and recomputed for the simulated VMAT plan. Relative loss of target coverage as a function of tumor size and distance to isocenter were evaluated as well as collateral dose to organs-at risk (OAR). Results The average beam-on time was less than six minutes. However, loss of target coverage for clinically observable setup errors were, on average, 7.9% (up to 73.1%) for the GTV (p < 0.001) and 21.5% for the PTV (up to 93.7%; p < 0.001). The correlation was found for both random and systematic residual setup errors with tumor sizes; there was a greater loss of target coverage for small tumors. Due to isocenter misalignment, OAR doses fluctuated and potentially receive higher doses than the original plan. Conclusion A single-isocenter VMAT SRS treatment (similar to HyperArc™ VMAT) to multiple brain metastases was fast with < 6 min of beam-on time. However, due to small residual set up errors, single-isocenter VMAT, in its current use, is not an accurate SRS treatment modality for multiple brain metastases. Loss of target coverage was statistically significant, especially for smaller lesions, and may not be clinically acceptable if left uncorrected. Further investigation of correction strategies is underway.

SBRT treatment of abdominal and pelvic oligometastatic lymph nodes using ring‐mounted Halcyon Linac

“…On the other hand, our TrueBeam Linac is capable of applying both rotational and translational couch corrections and may better align multiple lesions on a single daily CBCT scan and minimize the patient set up errors in the treatment of LNs SBRT. A few recent studies have shown loss of target coverage due to rotational set up errors in a single‐isocenter/multilesions setting 46–48 . However, the exact magnitude of dosimetric discrepancy in using six‐degrees‐of‐freedom (6DOF) couch corrections for treating multiple LNs SBRT treatment on Halcyon is not yet known.…”

Predicting the effect of indirect cell kill in the treatment of multiple brain metastases via single‐isocenter/multitarget volumetric modulated arc therapy stereotactic radiosurgery

“…Treating m‐bm with a single‐isocenter plan comes with many challenges, including dosimetric disparities, due to patient positioning errors 14–20 . This presents concerns in terms of local tumor control and unexpected dosing to the normal brain and other adjacent critical structures in the brain as described above.…”

“…Treating m‐bm with a single‐isocenter plan comes with many challenges, including dosimetric disparities, due to patient positioning errors. 14 , 15 , 16 , 17 , 18 , 19 , 20 This presents concerns in terms of local tumor control and unexpected dosing to the normal brain and other adjacent critical structures in the brain as described above. The QUANTEC guidelines for normal brain tissue cite a study relating V12Gy to radiation‐induced necrosis, where the risk of NTCP increases from 23% for V12Gy between 0 and 5 cc and 54% for V12Gy at 10 to 15 cc.…”

“… 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 The most recent simulation study demonstrated that there was a large loss of target(s) coverage (30% average, but up to 70% for small lesions) due to both rotational and translational setup errors while using SIMT‐VMAT SRS for m‐bm. 20 This is a challenge in lining up all tumors correctly using a single daily cone‐beam CT scans, especially since skull‐based rigid alignment is required. In addition, soft tissue visibility is low inside the brain on low‐quality cone‐beam CT images.…”

“…Although the previous studies have demonstrated the dosimetric effects of translational and rotational errors, they did not provide the effect of ICK in the treatment of m‐bm using SIMT‐VMAT plans. 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 Therefore, the relationship between ICK and patient setup errors was used to define an uncertainty cut‐off value. It is clinically useful to know the allowable setup uncertainties for multilesion SRS treatments.…”