Evaluation of a composite Gel‐Alanine phantom on an end‐to‐end test to treat multiple brain metastases by a single isocenter VMAT technique

Purpose This work aims to evaluate the application of a cylindrical phantom made of dosimetric gel containing alanine pellets distributed inside the gel volume during an end‐to‐end test of a single isocenter VMAT for simultaneous treatment of multiple brain metastases. The evaluation is based on the comparison of the results obtained with the composite phantom with the treatment planning system ( TPS ) dose distribution validated by using the clinical conventional quality control with point and planar dose measurements. Methods A cylindrical MAGIC ‐f gel phantom containing alanine dosimeters (composite phantom) was used to design the VMAT plan in the treatment planning system ( TPS ). The alanine dosimeters were pellets with radius of 2.5 mm and height of 3 mm, and played the role of brain metastasis inside the gel cylinder, which simulated the cerebral tissue. Five of the alanine dosimeters were selected to simulate five lesions; five planning target volumes ( PTV s) were created including the dosimeters and irradiated with different doses. Conventional quality assurance ( QA ) was performed on the TPS plan and on the composite phantom; a phantom containing only gel (Gel 1 phantom) was also irradiated. One day after irradiation, magnetic resonance images were acquired for both phantoms on a 3T scanner. An electron spin resonance spectrometer was used to evaluate alanine doses. Calibration curves were constructed for the alanine and the gel dosimeters. All the gel only measurement was repeated (Gel 2 phantom) in order to confirm the previous gel measurement. Results The VMAT treatment plan was approved by the conventional QA . The doses measured by alanine dosimeters on the composite gel phantom agreed to the TPS on average within 3.3%. The alanine dose for each lesion was used to calibrate the gel dosimeter measurements of the concerned PTV . Both gel dose volume histograms ( DVH ) achieved for each PTV were in agreement with the expected TPS DVH , except for a small discrepancy observed for the Gel 2 curve of PTV 1 and the Gel 1 curve of PTV 5. In a 3D gamma analyses performed for each PTV volume independently, comparing both the gels' measurements to the TPS and using 3%/3 mm, 5%/2 mm, and 7%/2 mm, more than 90% of the points were approved for all the PTV s, except for the PTV 5 comparison in the Gel 1 measurement and for the PTV 2 comparison in the Gel 2 measurement. A 3D gamma analysis was also applied for each PTV independently, to compare both gel measurements in order to evaluate the consistence of repeated gel measurements of the same plan, and more than 94.5% of the points were approved. Conclusions The composite Gel‐Alanine phantom can be used for the end‐to‐end test of a single isocenter VMAT for simultaneous treatment of multiple brain metastases. The use of the alanine as the lesion cores for the treatment planning provided the precise dose measurements inside each lesion and allowed the conversion of the gel R2 values based on an accurate dose measurement inside the target.