The patient-specific IMRT treatment quality assurance procedure often consists of the measurement of 2-D intensity map and absolute dose. The data is compared with the computed results from TPS using a standard pass-or-fail criterion. However, the same dose discrepancy may not have the same clinical significance depending on its location. For instance, the consequence of a hot-spot in PTV is different than in spinal cord. In this project, we reconstruct the 3D dose distribution in patient planning CT from the intensity maps obtained from the 2D IMRT QA measurement. A QA statistics method is proposed to include the location of dose discrepancy points. This approach promises a new IMRT QA standard that considers the clinical significance of dose discrepancy measured in IMRT QA.

Purpose: a novel 4-bank micro-multileaf collimator (mMLC) has been commissioned for stereotactic radiosurgery (SRS). Leafs of each bank of mMLC can be individually controlled, which allows to form more complex filed than conventional 2-bank MLC or circular cone. For cases with multiple brain masses, pre-treatment QA and treatment are always time consuming. In this study we investigate if multiple tumor sites can share one isocenter and if they can be treated simultaneously without sacrificing the quality of dosimetry.

Method and Materials: The microMLC (Alayna Enterprise Corp) consists of 96 tungsten leaves aligned in four banks. Three treated patients, each with two adjacent brain masses, were selected retrospectively in this study. Three plans were made on each patient. Plan 1 used conventional method that was to create two beam sets each with its own isocenter. Treatment was to be delivered in a sequential way. In Plan 2, tumors shared single isocenter but were treated by its respective beam set. In Plan 3, single isocenter was chosen and two masses were to be irradiated simultaneously by one set of beam. For comparisons, the treatment and setup time were measured or estimated. The isodose curves and dose volume histograms (DVH) were analyzed.

Results: Plan 2 and Plan 3 save significant time in QA and treatment (~30mins). Plan 1 and Plan 2 have equally satisfying dosimetric outcome, while the DVHs of Plan 3 degrade significantly. Plan 1 and Plan 2 have individual control of the dose of each target, while Plan 3 doesn’t have.

Conclusion: We have studied three scenarios of treating multiple isocenter SRS using 4-bank microMLC. The best method found is to share a common isocenter, but treat the targets individually. This method can reduce the QA and treatment time significantly, and achieve the similar dose coverage as the conventional technique.

Purpose: High dose rate brachytherapy (HDR) is a very effective cancer treatment method. Localization of source positions is essential for dosimetric accuracy. Nucletron’s PLATO TPS provides multiple means to reconstruct and localize source positions. Three commonly used methods are: (1) catheter describing using film, (2) catheter tracking using film, and (3) seed identifying on CT slices. Although all of these methods have been used in the clinic, the dose variance among them is unknown. The purpose of this project is to study quantitively the dose discrepancy of these different source reconstruction methods.

Results: Dose points from CT based plan was used as a reference. When comparing plans by the source reconstruction method, we found significant dose difference on certain dose point between film based planning and CT based planning. The biggest dose difference was 10.3% on fiducial A.

Conclusion: Through a series of planning on a phantom, we found the doses can differ significantly between the film and CT based- method. At certain locations, the film based- plan may underestimate the dose.

Purpose:Radical radiation therapy that combines external beam therapy (EBRT) and brachytherapy (BRT) is effective in managing local-regional confined cervical cancer. Depending on the cancer stage and disease volume, two therapies are used in a concurrent or sequential way. Although CT is widely used for EBRT treatment planning, traditional 2D film is still commonly used today in many institutions for BRT treatment planning. The incompatible image information between the BRT and EBRT leads to great difficulty in computation of the cumulative radiation dose from both treatments. To date, doses to target and critical structures are approximated by adding the point doses of BRT to the EBRT plan. This results in significant uncertainty of the dose evaluation of treatment plans. In this project, we propose to register the orthogonal films of BRT and CT of EBRT so as to accumulate the doses in a more accurate way.

Method and Materials:Five patients with cervical cancer were selected retrospectively for this study. Two in house made planning softwares were used in planning. BRT used two orthogonal films to reconstruct source positions. The doses on points “A” were prescribed at 4000 cGy. 3D conformal EBRT plan was generated based on CT to deliver 4500 cGy to pelvis. Two digitally-reconstructed radiographs (DRRs) were created with the same central ray and gantry angle as BRT films. A landmark-based image registration tool was developed to register films and DRRs of CT. The transformations, including translation, rotation and scaling, were calculated when registering two pairs of images (AP to AP, and LAT to LAT). The transformation matrix was applied on the dose grid of BRT and it was then merged with the dose grid of EBRT.

Results:A composite plan with accumulated doses of BRT and EBRT was created for each patient. The accuracy of the dose accumulation was tested in two means. First, the doses of relevant points, such as “A” points, were verified and they are correct. Second, the dose prescription of EBRT was intentionally set to zero before plan merging. The composite plan shows the isodose curves on CT are identical to those of original BRT plan. The curves are located in right anatomic regions of CT. The DVHs of target and critical structures were analyzed, and sampled points on DVHs agree with the estimation.

Conclusion:We have developed a method to accumulate the dose distributions from film-based brachytherapy and CT-based external beam radiotherapy. This yields a more realistic estimate of the cumulative dose received by the patient from both treatments. It also provides a more quantitative guidance for the design of subsequent radiotherapy plans.