Calibration of MTT assay in proton beams using radiochromic films.

PURPOSE: This study provides methodology of calibrating as well as controlling the output for an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay irradiated in a low energy proton beam using EBT3-model GAFCHROMICTM film, without correcting for quenching effect. METHODS: A calibrated Markus ionization chamber was used to measure the depth dose and beam output for 26.5 MeV protons produced by a CS30 cyclotron. A time-controlled aluminum cylinder was added in front of the horizontal beam-exit serving as a radiation shutter. Following the TRS-398 reference dosimetry protocol for proton beams, the output was calibrated in water at a reference depth of 3 mm. EBT3 film was calibrated for doses up to 8 Gy at the same depth. To verify the dose distribution for each 96-well MTT assay plate, EBT3 film was placed at the reference depth during irradiation and cell doses were scaled by measured percent depth dose (PDD) data. RESULTS: The radiochromic film dosimetry system in this study provides dose measurements with an uncertainty better than 3.3% for doses higher than 1 Gy. From a single exposure and utilizing the Gaussian shape of the beam, multiple dose points can be obtained within different wells of the same plate ranging from 6.9 Gy (sigma ∼4%) in the central well, and 2 Gy (sigma ∼8%) for wells positioned closer to the periphery. CONCLUSIONS: We described a methodology for radiochromic film-based dose monitoring system, using low-energy protons, which can be used for the MTT assay in any proton beam, except within Bragg peak region.

Helical Tomotherapy versus Conventional Technique for Post Mastectomy Left Sided Breast Cancer; Dosimetric Study.

PURPOSE: To evaluate the feasibility of using Helical Tomotherapy (HT) for post mastectomy left-sided breast cancer. METHODS AND MATERIALS: Treatment plans were generated for 5 post mastectomy left sided breast cancer patients treated at king Faisal Specialist Hospital & Research Center, Riyadh, Saudi Arabia, comparing three-dimensional technique with the HT planning. The prescribed dose was 50 Gy in 25 fractions; the planning target volume (chest wall and nodal volumes) was contoured according to RTOG atlas and the heart, lungs, spinal cord and contralateral breast were contoured as Organs at Risks (OARs). RESULTS: Both plans achieved the planning criteria, however the HT plan increased the minimal dose to the PTV (78% vs. 45.4%, p=0.043), improving the dose conformity (0.76 vs. 0.38, p=0.034) and dose homogeneity (1.05 vs. 1.2, p=0.08), while a three-dimensional technique has better V20 of lung (15.8% vs. 22.6%, p=0.04), Mean Lung Dose (7.6Gy vs. 13.8Gy, p=0.043), contralateral breast dose (0.1 vs. 12.7, p=0.043) and comparable Mean Heart Dose (4.4Gy vs. 5.3Gy, p=0.136). CONCLUSION: Both plans achieved comparable target coverage and OARs sparing, however HT plans provided better dose conformity and homogeneity than did the three-dimensional plans at the cost of less sparing of the heart, left lung and contralateral breast for treatment of left-sided post mastectomy breast cancer.