Purpose: To compare two 3D Gamma Index calculation schemes for VMAT plans. Methods: VMAT (Volumetric Modulated Arc Therapy) treatment technique was commissioned for our clinic on an Elekta Synergy machine. We selected to use ScandiDos Delta4TM system for our VMAT patient QAs. Delta4 is a pseudo 3D diode detector array which calculates Gamma index from 3D dose distributions. Delta4 offers two ways to calculate Gamma Index: global Gamma Index and local Gamma Index. The global gamma calculates the dose difference relative to the normalization dose, which can be iso‐center dose, prescribed or maximum dose. The local gamma calculates the dose difference relative to the current measurement point under examination. Thirty two test plans for the treatments of different sites including prostates, HN, lung, abdomen, pancreas node, anus, brain post fossa, medium sternum and liver were selected for this study. Pass rates from the two Gamma Index calculation schemes with 3% and 3mm criterion were compared. Results: The pass rate from global Gamma Index calculation is high than those from the global Gamma Index (p<0.001). Correlation coefficient of the two pass rates is 0.78. The differences of pass rates range from 0% to 9.5%. The most dramatic differences happen for high modulated (more complex) plans and where there are some low dose plateau regions. Conclusions: Global Gamma Index doesnˈt describe the details of high modulated plans with some low dose regions. Pass rate of local Gamma Index should be examined in addition to global Gamma Index. Pass rates of local Gamma Index for different structures are desired.

Purpose: Simultaneous integrated boost (SIB) fractionation schemes for the treatment of head and neck cancer have become widespread. This usually results in increased total dose to elective regions of the neck. We hypothesized that a sequential boost plan (SEQ) using a lower total dose to the elective neck at a standard fractional dose would result in improved dosimetry to normal tissues. Methods: Nine patients with locally advanced head and neck cancer with primaries in the oropharynx and larynx were selected for this study. SEQ radiotherapy plans were generated to compare with SIB plans. Normal tissue and target dose constraints were kept identical, except for dose to the elective neck target (56 Gy vs. 45 Gy). SIB dose fractionation schema to planning target volumes (PTV) included 70 Gy / 2 Gy fractions to PTV_High, 63 Gy / 1.8 Gy fractions to PTV_Intermediate, and 56 Gy / 1.6 Gy fractions to PTV_elective. SEQ dose fractionation schema included 50 Gy / 2 Gy fractions to PTV_High, 45 Gy / 1.8 Gy fractions to PTV_Intermediate, and 45 Gy / 1.8 Gy fractions to PTV_elective, followed by a boost of 20 Gy / 2 Gy fractions to PTV_High and 18 Gy / 1.8 Gy fractions to PTV_Intermediate. Results: SEQ plans were more likely to have higher percent coverage of the PTV_elective at the 110% and 100% (p < 0.01) prescription dose levels, but not at 95% or 93%. Mean doses to the contralateral parotid, larynx and inferior pharyngeal constrictor were lower with SEQ (p<0.05). Conclusions: The use of SEQ intensity modulated radiotherapy with a previous standard lower total dose (45 Gy in 25 fractions) for treatment of the elective neck in locally advanced head and neck cancer patients may improve the ability to spare normal tissues such as the parotid, larynx, and inferior pharyngeal constrictor.

Purpose: To compare three array calibration methods of MapCheck for an Elekta Beam Modulator LINAC. Methods: Both Elekta Synergy and Elekta Beam Modulator LINACs are used in our clinic. While an Elekta Synergy LINAC is a conventional Linac with the maximu field size 40 × 40 cm2, an Elekta Beam Modulator LINAC only has a maximum field size of 21 × 16 cm2. Because the MapCheck requires a field size of 26 × 26 cm2 for array calibration according to its user manual, it is a challenge to calibrate MapCheck array for Beam Modulator. We have tested three methods for this Purpose: 1) use the array calibration file from Elekta Synergy; 2) calibrate the MapCheck using Beam Modulator at 135cm SSD; 3) calibrate the MapCheck with both Beam Modulator and Elekta Synergy, then combine the calibration files. For the third method, the MapCheck was first calibrated with Beam Modulator at both collimator angles 0 and 90 degrees at 110cm SSD. Then the MapCheck was calibrated with Elekta Synergy using the standard method at 100cm SSD. The three array calibration files were combined such that it best describes the response of MapCheck detectors to Beam Modulator and all the detectors are covered. After calibrating the MapCheck detector array with the three methods, four prostate and four H&N IMRT treatment plans were delivered. The measurements were compared with the calculations. The Gamma Index (Gamma Index < 1) pass rates with a criterion of 3% and 3mm from all the three calibration methods were compared. Results: The pass rate of Gamma Index ranges from 90.4% to 100%. The null hypothesis that all three methods come with the same QA pass rates can not be rejected (p>0.1). Conclusions: For the testing cases, no significant difference was found among the three array calibration methods in term of Gamma Index pass rate.