Search

The magnitude of inter- and intrafractional patient motion has been assessed for a broad set of immobilization devices. Data was analyzed for the three ordinal directions--left-right (x), sup-inf (y), and ant-post (z)--and the combined spatial displacement. We have defined "rigid" and "non-rigid" immobilization devices depending on whether they could be rigidly and reproducibly connected to the treatment couch or not. The mean spatial displacement for intrafractional motion for rigid devices is 1.3 mm compared to 1.9 mm for nonrigid devices. The modified Gill-Thomas-Cosman frame performed best at controlling intrafractional patient motion, with a 95% probability of observing a three-dimensional (3D) vector length of motion (v95) of less than 1.8 mm, but could not be evaluated for interfractional motion. All other rigid and nonrigid immobilization devices had a v95 of more than 3 mm for intrafractional patient motion. Interfractional patient motion was only evaluated for the rigid devices. The mean total interfractional displacement was at least 3.0 mm for these devices while v95 was at least 6.0 mm.

Purpose: The global burden of cancer is considerable, particularly in low and middle-income countries. Massachusetts General Hospital (MGH) and Botswana-Harvard AIDS Institute have partnered with the oncology community and government of Botswana to form BOTSOGO (BOTSwana Oncology Global Outreach) to address the rising burden of cancer in Botswana. Currently, radiation therapy (RT) is only available at a single linear accelerator (LINAC) in Gaborone Private Hospital (GPH). BOTSOGO worked to limit the absence of RT during a LINAC upgrade and ensure a safe transition to modern radiotherapy techniques. Methods: The existing Elekta Precise LINAC was decommissioned in November 2015 and replaced with a new Elekta VERSA-HD with IMRT/VMAT/CBCT capability. Upgraded treatment planning and record-and-verify systems were also installed. Physicists from GPH and MGH collaborated during an intensive on-site visit in Botswana during the commissioning process. Measurements were performed using newly purchased Sun Nuclear equipment. Photon beams were matched with an existing model to minimize the time needed for beam modeling and machine down time. Additional remote peer review was also employed. Independent dosimetry was performed by irradiating OSLDs, which were subsequently analyzed at MGH. Results: Photon beam quality agreed with reference data within 0.2%. Electron beam data agreed with example clinical data within 3%. Absolute dose calibration was performed using both IAEA and AAPM protocols. Absolute dose measurements with OSLDs agreed within 5%. Quentry cloud-based software was installed to facilitate remote review of treatment plans. Patient treatments resumed in February 2016. The time without RT was reduced, therefore likely resulting in reduced patient morbidity/mortality. Conclusion: A global physics collaboration was utilized to commission a modern LINAC in a resource-constrained setting. This can be a useful model in other areas with limited resources. Further use of technology and on-site exchanges will facilitate the introduction of more advanced techniques in Botswana. We acknowledge funding support from the AAPM International Educational Activities Committee and the NCI Federal Share Proton Beam Program Income Grant.

The magnitude of inter- and intrafractional patient motion has been assessed for a broad set of immobilization devices. Data was analyzed for the three ordinal directions--left-right (x), sup-inf (y), and ant-post (z)--and the combined spatial displacement. We have defined "rigid" and "non-rigid" immobilization devices depending on whether they could be rigidly and reproducibly connected to the treatment couch or not. The mean spatial displacement for intrafractional motion for rigid devices is 1.3 mm compared to 1.9 mm for nonrigid devices. The modified Gill-Thomas-Cosman frame performed best at controlling intrafractional patient motion, with a 95% probability of observing a three-dimensional (3D) vector length of motion (v95) of less than 1.8 mm, but could not be evaluated for interfractional motion. All other rigid and nonrigid immobilization devices had a v95 of more than 3 mm for intrafractional patient motion. Interfractional patient motion was only evaluated for the rigid devices. The mean total interfractional displacement was at least 3.0 mm for these devices while v95 was at least 6.0 mm.

Purpose: In-vivo dose and beam range verification in proton therapy could play significant roles in proton treatment validation and improvements. Invivo beam range verification, in particular, could enable new treatment techniques one of which, for example, could be the use of anterior fields for prostate treatment instead of opposed lateral fields as in current practice. We have developed and commissioned an integrated system with hardware, software and workflow protocols, to provide a complete solution, simultaneously for both in-vivo dosimetry and range verification for proton therapy. Methods: The system uses a matrix of diodes, up to 12 in total, but separable into three groups for flexibility in application. A special amplifier was developed to capture extremely small signals from very low proton beam current. The software was developed within iMagX, a general platform for image processing in radiation therapy applications. The range determination exploits the inherent relationship between the internal range modulation clock of the proton therapy system and the radiological depth at the point of measurement. The commissioning of the system, for in-vivo dosimetry and for range verification was separately conducted using anthropomorphic phantom. EBT films and TLDs were used for dose comparisons and range scan of the beam distal fall-off was used as ground truth for range verification. Results: For in-vivo dose measurement, the results were in agreement with TLD and EBT films and were within 3% from treatment planning calculations. For range verification, a precision of 0.5mm is achieved in homogeneous phantoms, and a precision of 2mm for anthropomorphic pelvic phantom, except at points with significant range mixing. Conclusion: We completed the commissioning of our system for in-vivo dosimetry and range verification in proton therapy. The results suggest that the system is ready for clinical trials on patient.

Purpose: We assessed the magnitude of inter‐ and intra‐fractional patient motion for a broad set of immobilization devices. Method and Materials: In our practice for protonradiotherapy, we verify patient setup prior to each treatment field by means of digital imaging. Deviations from the “expected” patient position (with 6 degrees of freedom on our patient positioner) are stored in a database and were retrospectively analyzed for patient setup accuracy. Data was analyzed for each ordinal direction and for the combined spatial displacement. Immobilization devices were separated into “rigid” and “non‐rigid” devices depending on their connection to the patient positioner. Results: Systematic offsets in intra‐fractional motion are small for all immobilization devices, and less than 0.2mm for translations and less than 0.1 degrees for any rotational axis. Although the mean translation per ordinal direction is often smaller than 1mm, the mean spatial displacement for intra‐fractional motion for rigid devices is 1.3mm compared to 1.9mm for non‐rigid devices. The modified Gill‐Thomas‐Cosman frame controlled intra‐fractional patient motion best, with a 95% probability (v95) of observing less than 1.8mm of motion. For the widely used IC‐mask, the v95 value is below 3mm. All other immobilization devices have a v95 greater than 3.0mm for intra‐fractional patient motion, while the mean spatial inter‐fractional displacement (rigid devices only) was at least 3mm with a v95 of at least 6mm. Conclusion: Intra‐fractional patient motion is generally smaller than inter‐fractional patient motion, which indicates that immobilization devices are better at maintaining a certain patient position than at reproducing this position between fractions. Immobilization devices that rigidly connect to the patient positioner ensure better immobility than those that do not. However, none of the immobilization devices achieved sub‐millimeter immobility. Patient position verification remains necessary to achieve high setup accuracy. Research supported in part by NCI grant 5_PO1‐CA21239‐24

The predictions of an analytic technique for calculating fluence and dose distributions beneath thin inhomogeneities are presented for a number of structures, including a rectangular cavity or bar, a cylinder, a disk, and an angled or diffuse edge. Experiments with both electrons and protons for several geometries are presented and compared with predictions based on this technique. We offer some clinical guidelines for avoiding large perturbations due to scattering effects.

Methods for obtaining flattened charged-particle dose distributions over large areas are described. The system being used at Harvard for proton radiotherapy is discussed in detail. It is an extension of usual multiple-scattering techniques to include blocking out some of the central peak, followed by rescattering to fill in the profile, resulting in flat distributions up to 30 cm in diameter. The unusually long source-to-skin distance (4.6-m SSD) plus the small lateral spread (2 mm rms) from multiple scattering in the patient results in little divergence of the beam throughout the treatment volume.