Objective: In the quality assurance (QA) of stereotactic radiosurgery (SRS) and radiotherapy (SRT) plans involving small targets or multiple metastasis, MapCHECK2 and SRS MapCHECK (Sun Nuclear Corporation, Melbourne, FL) are filmless solutions. Previously at our center, MapCHECK2 was used for patient specific SRS/SRT QA. MapCHECK2 has a larger array size (260x320 mm), larger 0.8 mm detector size, and coarser orthogonal (10 mm) and diagonal (7.07 mm) detector spacing. SRS MapCHECK has a smaller array size (77 x 77 mm), smaller 0.47 mm detector size, and finer 2.47 mm orthogonal detector spacing. In addition, SRS MapCHECK accounts and corrects for angular dependence - allowing accurate dose measurements from any angle, including vertex fields. In this work, we have evaluated SRS MapCHECK for patient-specific SRS/SRT QA.

Methods: The Sun Nuclear SRS MapCHECK in StereoPHAN was scanned using a 0.625mm slice width with a GE Discovery 590RT CT scanner.  For QA plan calculations, the StereoPHAN and SRS MapCHECK densities were overridden with a value of 1.20 g/cm3, while the CT data set supplied from Sun Nuclear Corporation for MapCHECK2 in MapPHAN was used.
Six volumetric modulated arc therapy (VMAT) plans using single isocenter planning technique were selected from our database of measurements using MapCHECK2 and SRS MapCHECK. Three plans have a single target, one plan has two targets, one plan has three targets, and one plan has four targets. In the case of the 4-target plan; the distance between each target and isocenter was 6.80 cm, 5.72 cm, 6.35 cm, and 5.97 cm, respectively. The longest distance between targets was 12.5 cm. The distance between the most posterior tumor and isocenter in AP direction was 5.7 cm. For multiple target plans, horizontal measurements were done at isocenter and each target center, vertical measurement was only done at isocenter. External groove lines on MapPHAN were used for alignment of MapCHECK2, while cone beam computed tomography (CBCT) was used for SRS MapCHECK alignment. Note that the QA plans using SRS MapCHECK were delivered with the actual planned couch angles, while MapCHECK2 only allows measurements done at couch zero. Gamma analysis was calculated using 5% dose threshold, 3%/1mm criteria.  

Results: The average gamma passing rates for SRS MapCHECK and MapCHECK2 were 99.2±1.2% and 98.0±2.0% respectively. The equivalent target volume diameters were in the range of 1.1 to 2.1 cm and no dependence on target size was observed. Our tolerance limit for the gamma passing rate is ≥ 97% using a 3%/1mm and a 5% dose threshold. For MapCHECK2, two plans did not meet the tolerance limit, whereas all the SRS MapCHECK measurements met the limit. The four-target plan reaches the distance limit of measuring multiple targets using SRS MapCHECK.  In addition, the vertical distance between the posterior tumor and the isocenter results in a table drop of more than 4 cm and causes potential clearance issues. A dry run is needed before doing the QA measurement using SRS MapCHECK.

Conclusion: Both SRS MapCHECK and MapCHECK2 can be used for SRS/SRT patient specific QA. SRS MapCHECK is more suited to small targets due to its smaller detector size, smaller array size, and finer orthogonal detector spacing. MapCHECK2 is better suited to larger targets due to its larger array size, larger detector size, and coarser detector spacing. The recommended target size criteria are: PTV <4 cm for SRS MapCHECK and PTV ≥4cm for MapCHECK2. If the distance between a posterior target and isocenter in AP direction is more than 4 cm, MapCHECK2 may be needed to avoid collision issues.