Stereotactic radiosurgery with MLC‐defined arcs: Verification of dosimetry, spatial accuracy, and end‐to‐end tests

Abstract

Purpose To measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator‐(MLC) equipped linear accelerator without cones or other special attachments or modifications. Methods Dosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ‐index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC‐shaped fields ranging from 1.6 × 5 to 30 × 30 mm2 were planned and delivered to an in‐house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple‐arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm2 fields applied at six different couch angles. For end‐to‐end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine‐arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy. Results In the single‐arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ‐index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm2 and rose to 4.4% as field size increased to 20 × 20 mm2. Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine‐arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end‐to‐end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained <1 at all pixels. To date, more than 50 functional SRS treatments using MLC‐shaped static field arcs have been delivered. Conclusion Stereotactic radiosurgery (SRS) can be planned and delivered on a standard linac without cones or other modifications with better than...

Optical surface guidance for submillimeter monitoring of patient position during frameless stereotactic radiotherapy

“…21 Variability in longitudinal RTDs amongst patients also suggested a cause other than couch walkout. Couch walkout was investigated as a potential source of the increased longitudinal offset, but the magnitude of longitudinal walkout was <0.4 mm when evaluated via dial gauge.…”

“…Couch walkout was investigated as a potential source of the increased longitudinal offset, but the magnitude of longitudinal walkout was <0.4 mm when evaluated via dial gauge. 21 Variability in longitudinal RTDs amongst patients also suggested a cause other than couch walkout. The distance between the ROI and isocenter was investigated as a potential cause of interpatient variation.…”

Technological quality requirements for stereotactic radiotherapy

“…The geometric accuracy in these reports ranges from 0.28 ± 0.36 mm to 1.5 ± 0.7 mm depending on system, test scenario, and measurement device. Adding recent literature to these guidelines, one can safely assume on one hand that well-calibrated dedicated stereotactic radiotherapy equipment can reach the required level of mechanical precision of 1 mm in system-specific static end-to-end tests for SRS [189][190][191][192][193][194][195][196][197]. Additionally, a geometric accuracy of Ä1 mm is clearly necessary for SRS in order to keep the overall PTV margin Ä2 mm when also considering patient motion during treatment before side effects increase significantly for the high SRS doses [9].…”

“…Concerning the dosimetric accuracy of the treatment systems used for stereotactic radiotherapy, high-quality data arise mostly from clinical trial audits. In homogeneous phantoms, the dosimetric end-to-end or delivery-quality-assurance accuracy was found to be well below the required 3% limit [190,197,[205][206][207]. However, when looking at moving targets and especially those targets surrounded by heterogeneous tissue, it becomes apparent that the 3% dosimetric accuracy cannot always be reached without type-C dose calculation algorithms (see "Dose calculation" section) [205].…”

Methodology for radiochromic film analysis using FilmQA Pro and ImageJ

“…These attributes have proven useful for a variety of clinical tasks such as dosimetry for total skin electron therapy [9,10] or total body irradiation [11]. RCF is also used in patient specific quality assurance (QA) measurements for delivery techniques such as volumetric modulated arc therapy (VMAT) [12] or stereotactic radiosurgery (SRS) [13,14] where high spatial resolution is necessary and has proven useful as a research tool in phantom and radiobiological studies to verify dosimetry in unconventional geometries that do not easily accommodate conventional dosimeters [15][16][17][18][19][20][21].…”