Linac-based Volumetric Arc Therapy Research Articles

Effect of changes in monitor unit rate and energy on dose rate of total marrow irradiation based on Linac volumetric arc therapy

BackgroundThis study set out to evaluate the effect of dose rate on normal tissues (the lung, in particular) and the variation in the treatment efficiency as determined by the monitor unit (MU) and energy applied in Linac-based volumetric arc therapy (VMAT) total marrow irradiation (TMI).MethodsLinac-based VMAT plans were generated for the TMI for six patients. The planning target volume (PTV) was divided into six sub-volumes, each of which had their own isocenter. To examine the effect of the dose rate and energy, a range of MU rates (40, 60, 80, 100, 300, and 600 MU/min) were selected for 6, 10, and 15 MV. All the plans were verified by portal dosimetry.ResultsThe dosimetric parameters for the target and normal tissue were consistent in terms of the energy and MU rate. The beam-on time was changed from 59.6 to 6 min for 40 and 600 MU/min. When 40 MU/min was set for the lung, the dose rate delivered to the lung was less than 6 cGy/min (that is, 90%), while the beam-on time was approximately 10 min. The percentage volume of the lung receiving 20 cGy/min was 1.47, 3.94, and 6.22% at 6, 10, and 15 MV, respectively. However, for 600 MU/min, the total lung volume received over 6 cGy/min regardless of the energy, and over 20 cGy/min for 10 and 15 MV (i.e., 54.4% for 6 MV).ConclusionsIn TMI treatment, reducing the dose rate administered to the lung can decrease the incidence of pulmonary toxicity. To reduce the probability of normal tissue complications, the selection of the lowest MU rate is recommended for fields including the lung. To minimize the total treatment time, the maximum MU rate can be applied to other fields.

Is linac-based volumetric modulated arc therapy better than helical tomotherapy in the radiotherapy of nasopharyngeal carcinoma?

Volumetric arc therapy (VMAT) and helical tomotherapy (HT) are currently the two state-of-the-art rotational intensity-modulated radiotherapy (IMRT). When compared with conventional IMRT, linac-based VMAT can shorten the treatment time while HT has been claimed to produce a better dosimetric outcome. The aim of our study was to evaluate whether the more “efficient” VMAT can produce a comparable or better dosimetric outcome when compared with HT for the treatment of nasopharyngeal carcinoma (NPC) patients. Sixteen primary NPC patients treated by HT were recruited. In addition to the original HT plans, two-arc VMAT plans using the same patient data and dose requirements were generated for each patient. The dosimetric evaluation of the plans included the comparison of conformity index (CI), homogeneity index (HI), dose to 2 and 98 % volume (D 2 and D 98) of the planning target volume (PTV) of the NP, and left and right lymphatic regions. For the organs at risk (OARs), maximum dose or D 2 and mean dose were used for serial and parallel organs, respectively. Student’s paired t test or Wilcoxon signed-rank test was used to check for significant difference. The dose to the NP target was similar between the two techniques. For the nodal targets, the VMAT plans produced higher mean D 2 and lower HI values. For the OARs, the spinal cord and spinal cord doses were higher in the VMAT plans than in the HT plans, whereas the opposite happened in the optic chiasm and pituitary gland. The doses to the brain stem, lens, and optic nerve did not show a significant difference. Both systems delivered satisfactory dose distributions and kept the dose to OARs below their tolerance. However, if the parotid gland toxicity is a concern in the treatment, HT would be more favorable because it is relatively more effective in sparing the parotid gland. Otherwise, the linac-based double-arc VMAT would be recommended because of its shorter treatment time.