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Purpose: Measurements of internal organ motion have demonstrated that daily organ deformation exists throughout the course of radiation treatment. However, a method of constructing the resultant dose delivered to the organ volume remains a difficult challenge. In this study, a model to quantify internal organ motion and a method to construct a cumulative dose in a deforming organ are introduced. Methods and Materials: A biomechanical model of an elastic body is used to quantify patient organ motion in the process of radiation therapy. Intertreatment displacements of volume elements in an organ of interest is calculated by applying an finite element method with boundary conditions, obtained from multiple daily computed tomography (CT) measurements. Therefore, by incorporating also the measurements of daily setup error, daily dose delivered to a deforming organ can be accumulated by tracking the position of volume elements in the organ. Furthermore, distribution of patient-specific organ motion is also predicted during the early phase of treatment delivery using the daily measurements, and the cumulative dose distribution in the organ can then be estimated. This dose distribution will be updated whenever a new measurement becomes available, and used to reoptimize the ongoing treatment. Results: An integrated process to accumulate dosage in a daily deforming organ was implemented. In this process, intertreatment organ motion and setup error were systematically quantified, and incorporated in the calculation of the cumulative dose. An example of the rectal wall motion in a prostate treatment was applied to test the model. The displacements of volume elements in the rectal wall, as well as the resultant doses, were calculated. Conclusion: This study is intended to provide a systematic framework to incorporate daily patient-specific organ motion and setup error in the reconstruction of the cumulative dose distribution in an organ of interest. The realistic dose distribution in an organ of interest gives the true dose–volume relationship, and may play an important role in the evaluation of the dose response of human organs. Dose reconstruction during the course of treatment delivery can also be used as an important feedback for the online optimization of individual treatment plans.

To compare CT and MRI target volumes for nasopharyngeal carcinoma (NPC) and evaluate the role of intensity-modulated radiotherapy (IMRT) in treating composite CT+MRI targets.CT and T(1)/T(2)-weighted MRI scans were obtained for 8 consecutive NPC patients. Using CT, MRI, and fused CT/MRI, various target volumes (gross target volume, clinical target volume, and planning target volume [PTV]) and critical structures were outlined. For each patient, three treatment plans were developed: (1) a three-dimensional conformal RT (3D-CRT) plan using CT-based targets; (2) a 3D-CRT plan using composite CT+MRI targets; and (3) a IMRT plan using CT+MRI targets. The prescription dose was 57.6 Gy and 70.2 Gy to the initial and boost PTV, respectively. Treatment plans were compared using the PTV dose to 95% volume (D(95)), critical structure dose to 5% organ volume (D(5)), and mean dose.Compared with CT, the MRI-based targets were 74% larger, more irregularly shaped, and did not always include the CT targets. For CT-based targets, 3D-CRT plans, in general, achieved adequate target coverage and sparing of critical structures. However, when these plans were evaluated using CT+MRI targets, the average PTV D(95) was approximately 60 Gy (14% underdosing), and critical structure doses were significantly worse. The use of IMRT for CT+MRI targets resulted in marked improvement in the PTV coverage and critical structure sparing: average PTV D(95) improved to 69.3 Gy, brainstem D(5) to43 Gy (19% reduction), spinal cord D(5) to37 Gy (19% reduction), and the mean dose to the parotids and cochlea reduced to below tolerance (23.7 Gy and 35.6 Gy, respectively).CT/MRI fusion improved the determination of target volumes in NPC. In contrast to 3D-CRT, IMRT planning resulted in significantly improved coverage of composite CT+MRI targets and sparing of critical structures.

Results: Median biological effective dose (BED) at an a/b ratio of 10 as the prescribed dose was 84.5GyE10 (range, 60-96.8GyE10). Eleven (40%) patients developed acute Grade 1, 12 (45%) patients Grade 2, and 4(15%) patients Grade 3 acute dermatitis. In uni-variate analysis, factors associated with onset of Grade 2 or higher acute dermatitis included BED (GyE10) as prescription dose (p Z 0.030), skin dose parameter (V45-70GyE10) (p Z 0.044, 0.039, 0.031, 0.015, 0.010, and 0.022, respectively), and gradient dose index (GDI65GyE and 70GyE) (p Z 0.031, and 0.048, respectively). When plotting the p value of the logistic regression analysis between the variables, V65 GyE10 of skin surface area show the strongest statistical significance (p & 0.01). Conclusions: High incidence of severe (Grade 2 or higher) radiation dermatitis (60% of entire cohort) was observed during clinical use of PBT in HNC patients. The entrance portal area to skin irradiated volume with 45-70GyE10 was possible predictor for acute skin toxicity and higher dose volume of superficial regions over 65GyE10 should be considered in the treatment planning as sensitive organ volume in PBT. Author Disclosure: H. Inokuchi: None. T. Nakamura: None. T. Tomoda: None. A. Takada: None. K. Takayama: None. C. Makita: None. M. Shiomi: None. T. Kato: None. N. Fuwa: None.