Four-dimensional dose calculations for dynamic tumour tracking with a gimbal-mounted linear accelerator.

PURPOSE: In this study we present a novel method for re-calculating a treatment plan on different respiratory phases by accurately modeling the panning and tilting beam motion during DTT (the "rotation method"). This method is used to re-calculate the dose distribution of a plan on multiple breathing phases to accurately assess the dosimetry. METHODS: sIMRT plans were optimized on a breath hold computed tomography (CT) image taken at exhale (BHexhale ) for 10 previous liver stereotactic ablative radiotherapy patients. Our method was used to re-calculate the plan on the inhale (0%) and exhale (50%) phases of the four-dimensional CT (4DCT) image set. The dose distributions were deformed to the BHexhale CT and summed together with proper weighting calculated from the patient's breathing trace. Subsequently, the plan was re-calculated on all ten phases using our method and the dose distributions were deformed to the BHexhale CT and accumulated together. The maximum dose for certain organs at risk (OARs) was compared between calculating on two phases and all ten phases. RESULTS: In total, 26 OARs were examined from 10 patients. When the dose was calculated on the inhale and exhale phases six OARs exceeded their dose limit, and when all 10 phases were used five OARs exceeded their limit. CONCLUSION: Dynamic tumor tracking plans optimized for a single respiratory phase leave an OAR vulnerable to exceeding its dose constraint during other respiratory phases. The rotation method accurately models the beam's geometry. Using deformable image registration to accumulate dose from all 10 breathing phases provides the most accurate results, however it is a time consuming procedure. Accumulating the dose from two extreme breathing phases (exhale and inhale) and weighting them properly provides accurate results while requiring less time. This approach should be used to confirm the safety of a DTT treatment plan prior to delivery.

Spinal cord tolerance to high-dose fractionated 3D conformal proton-photon irradiation as evaluated by equivalent uniform dose and dose volume histogram analysis.

PURPOSE: To evaluate cervical spinal cord tolerance using equivalent uniform dose (EUD) and dose volume histogram (DVH) analysis after proton-photon radiotherapy. METHODS AND MATERIAL: The 3D dose distributions were analyzed in 85 patients with cervical vertebral tumors. Mean follow-up was 41.3 months. The mean prescribed dose was 76.3 Cobalt Gray Equivalent (CGE = proton dose x RBE 1.1). Dose constraints to the center and the surface of the cervical cord were 55-58 CGE and 67-70 CGE, respectively. Dose parameters, DVH and EUD, were calculated for each patient. The spinal cord toxicity was graded using the European Organization for Research and Treatment of Cancer (EORTC) and Radiation Therapy Oncology Group (RTOG) late effects scoring system. RESULTS: Thirteen patients experienced Grade 1-2 toxicity. Four patients had Grade 3 toxicity. For the dose range used in this study, none of the dosimetric parameters was found to be associated with the observed distribution of cord toxicities. The only factor significantly associated with cord toxicity was the number of surgeries before irradiation. CONCLUSION: The data and our analysis suggest that the integrity of the normal musculoskeletal supportive tissues and vascular supply may be important confounding factors of toxicity at these dose levels. The results also indicate that the cervical spinal cord dose constraints used in treating these patients are appropriate for conformal proton-photon radiotherapy.

Computed tomography determination of prostate volume and maximum dimensions: a study of interobserver variability.

OBJECTIVE: (1) To evaluate the reproducibility of prostate volume, maximum dimensions and geometrical center coordinates determination using computed tomography (CT) and (2) to identify patterns of interobserver variability. MATERIALS AND METHODS: Forty patients, suitable for our brachytherapy program, were selected for the study. All patients underwent CT scanning and the prostate volumes were determined by three radiation oncologists. Measurements of geometrical center coordinates, maximum organ dimensions in the anterior-posterior (AP), lateral (Lat) and longitudinal (Long) axes as well as prostate volumes were recorded. This yielded 840 measurements of seven variables for analysis. The means and corresponding standard deviations (SD) of each variable were calculated for each patient. The SDs were then averaged and presented as indices of dispersion. Average variations from the mean were also calculated for each observer along with the SDs. RESULTS: Analysis of the geometrical center coordinates revealed acceptable variability amongst observers. For the AP, Lat and Long coordinates the SDs were 0.78, 0.89 and 1.72 mm, respectively. The corresponding values for the maximum organ dimensions were 2.54, 2.72 and 4.43 mm, respectively. While the volumes outlined by observer B were less than or equal to the mean in 95% of cases and those of observer C were greater than or equal to the mean in 93% of cases, the volumes of observer A were equally distributed above and below the mean (48% in both cases). CONCLUSION: The determination of the geometrical center coordinates was reproducible amongst observers. The largest variations were seen with the Long axis. The volume determination is more variable. However, a characteristic trend was seen amongst observers when their volumes were compared to the mean volumes of the group.