E Brandner

Dosimetric evaluations of the interplay effect in respiratory‐gated intensity‐modulated radiation therapy

The interplay between a mobile target and a dynamic multileaf collimator can compromise the accuracy of intensity-modulated radiation therapy (IMRT). Our goal in this study is to investigate the dosimetric effects caused by the respiratory motion during IMRT. A moving phantom was built to simulate the typical breathing motion. Different sizes of the gating windows were selected for gated deliveries. The residual motions during the beam-on period ranged from 0.5 to 3 cm. An IMRT plan with five treatment fields from different gantry angles were delivered to the moving phantom for three irradiation conditions: Stationary condition, moving with the use of gating system, and moving without the use of gating system. When the residual motion was 3 cm, the results showed significant differences in dose distributions between the stationary condition and the moving phantom without gating beam control. The overdosed or underdosed areas enclosed about 33% of the treatment area. In contrast, the dose distribution on the moving phantom with gating window set to 0.5 cm showed no significant differences from the stationary phantom. With the appropriate setting of the gating window, the deviation of dose from the respiratory motion can be minimized. It appeals that limiting the residual motion to less than 0.5 cm is critical for the treatments of mobile structures.

FOUR-DIMENSIONAL COMPUTED TOMOGRAPHY-BASED INTERFRACTIONAL REPRODUCIBILITY STUDY OF LUNG TUMOR INTRAFRACTIONAL MOTION

PURPOSE To evaluate the interfractional reproducibility of respiration-induced lung tumors motion, defined by their centroids and the intrafractional target motion range. METHODS AND MATERIALS Twentythree pairs of four-dimensional/computed tomography scans were acquired for 22 patients. Gross tumor volumes were contoured, Clinical target volumes (CTVs) were generated. Geometric data for CTVs and lung volumes were extracted. The motion tracks of CTV centroids, and CTV edges along the cranio-caudal, anterior-posterior, and lateral directions were evaluated. The Pearson correlation coefficient for motion tracks along the cranio-caudal direction was determined for the entire respiratory cycle and for five phases about the end of expiration. RESULTS The largest motion extent was along the cranio-caudal direction. The intrafractional motion extent for five CTVs was <0.5 cm, the largest motion range was 3.59 cm. Three CTVs with respiration-induced displacement >0.5 cm did not exhibit the similarity of motion, and for 16 CTVs with motion >0.5 cm the correlation coefficient was >0.8. The lung volumes in corresponding phases for cases that demonstrated CTVs motion similarity were reproducible. No correlation between tumor size and mobility was found. CONCLUSION Target motion reproducibility seems to be present in 87% of cases in our dataset. Three cases with dissimilar motion indicate that it is advisable to verify target motion during treatment. The adaptive adjustment to compensate the possible interfractional shifts in a target position should be incorporated as a routine policy for lung cancer radiotherapy.

Intensity-modulated radiation therapy (IMRT) reduces the dose to the contralateral breast when compared to conventional tangential fields for primary breast irradiation: initial report.

PURPOSEThis study was designed to compare the dose received by the contralateral breast during primary breast irradiation using intensity-modulated radiotherapy with the dose received via conventional tangential field techniques. METHODS/MATERIALSBetween March 2003 and March 2004, 44 patients with breast carcinoma were treated using 6-, 10-, or mixed 6/18-MV photons (36 with tangential intensity-modulated radiotherapy technique and eight with three-dimensional technique using tangential fields with wedges) for primary breast irradiation after breast-conserving surgery. Paired thermoluminescent dosimeters were placed on each patients contralateral breast, 4 cm from the center of the medial border of the tangential field. The thermoluminescent dosimeters were left on the patient during a single fraction and then measured 24 hours later. RESULTSThe mean dose delivered with photons to the primary breast for all patients was 4998 cGy [SD = 52], and the mean single fraction dose was 200 cGy [SD = 9]. The mean percent of the prescribed dose to the contralateral breast was 7.74% (SD = 2.35) for patients treated with intensity-modulated radiotherapy, compared with 9.74% [SD = 2.04] for the patients treated with conventional tangential field techniques. This represented a 20% reduction in the mean dose to the contralateral breast with the use of intensity-modulated radiotherapy when compared with the dose received via the three-dimensional technique, a result that was statistically significant. CONCLUSIONPrimary breast irradiation with tangential intensity-modulated radiotherapy technique significantly reduces the dose to the contralateral breast when compared with conventional tangential techniques.

Does breast size affect the scatter dose to the ipsilateral lung, heart, or contralateral breast in primary breast irradiation using intensity-modulated radiation therapy (IMRT)?

Purpose:To evaluate the relationship between the primary breast volume and dose received by the ipsilateral lung, heart (for left-breast cancers), and contralateral breast during primary breast irradiation using intensity-modulated radiation therapy (IMRT). Methods and Materials:Sixty-five patients with breast carcinoma were treated using 6-MV photons with IMRT technique using the Eclipse Planning System following breast conserving surgery. All patients had a treatment planning CT scan. The primary breast, ipsilateral lung, and heart were contoured on the axial CT slices. The primary breast volume was calculated using the Eclipse Planning System. The mean ipsilateral lung and heart doses were obtained from the dose-volume histogram. The contralateral breast dose was measured using paired thermoluminescent dosimeters (TLDs) placed on the patients contralateral breast, 4 cm from the center of the medial border of the primary breast irradiation field. Results:The mean dose delivered with photons to the primary breast for all patients was 49.97 Gy. The mean volume of the primary irradiated breast was 1167.9 cc. As a percentage, the mean ipsilateral lung, heart, and contralateral breast doses were 11.2%, 6.1%, and 7.2%, respectively. The primary breast volume positively correlated with the contralateral breast dose (P < 0.0005). There was no significant correlation between the breast volume and the ipsilateral lung or heart dose (P = 0.463 and 0.943, respectively). Conclusion:This study suggests that the primary breast size significantly affects the scatter dose to the contralateral breast but not the ipsilateral lung or heart dose when using IMRT for breast irradiation.

Four-dimensional computed tomography-based respiratory-gated whole-abdominal intensity-modulated radiation therapy for ovarian cancer: a feasibility study.

This study assesses the feasibility and implementation of respiratory-gated whole-abdominal intensity-modulated radiation therapy (RG-WAIMRT). Three patients were treated with RG-WAIMRT. The planning target volume (PTV1) included the entire peritoneal cavity and a pelvic boost field was created (PTV2). The dose prescribed was 30 Gy to PTV1 and 14.4 Gy to PTV2. For comparison, a conventional three-dimensional (3D) plan was generated for each patient. In the WAIMRT plan, an average of 90% of PTV1 received 30 Gy compared to 70% for the conventional 3D plan. The percent volume receiving 30 Gy (V30) for liver averaged 54% (WAIMRT) vs 43% (3D). The percent volume receiving 20 Gy (V20) for kidneys averaged 19% vs 0%, and the mean V20 for bone marrow was 74% vs 83%, respectively. Major acute toxicities were anemia (grade 2: 1/3), leukopenia (grade 3: 2/3 patients), and thrombocytopenia (grade 2: 1/3 patients, grade 3: 1/3 patients). One patient could not complete the whole-abdomen field after 19.5 Gy because of persistent nausea. No major subacute toxicity has been reported. WAIMRT demonstrated superior target coverage and reduced dose to bone marrow, with a slightly increased dose to liver and kidneys. WAIMRT is a novel and feasible technique for ovarian cancer treatment.

Motion management strategies and technical issues associated with stereotactic body radiotherapy of thoracic and upper abdominal tumors: A review from NRG oncology

&NA; The efficacy of stereotactic body radiotherapy (SBRT) has been well demonstrated. However, it presents unique challenges for accurate planning and delivery especially in the lungs and upper abdomen where respiratory motion can be significantly confounding accurate targeting and avoidance of normal tissues. In this paper, we review the current literature on SBRT for lung and upper abdominal tumors with particular emphasis on addressing respiratory motion and its affects. We provide recommendations on strategies to manage motion for different, patient‐specific situations. Some of the recommendations will potentially be adopted to guide clinical trial protocols.

Breast skin doses from brachytherapy using MammoSite® HDR, intensity modulated radiation therapy, and tangential fields techniques

Skin doses from brachytherapy using MammoSite® HDR, Intensity Modulated Radiation Therapy (IMRT), and conventional tangential fields techniques were compared. For each treatment technique, skin doses were measured using paired thermoluminescent dosimeters placed on the patients skin: (i) directly above the balloon catheter during MammoSite® HDR brachytherapy treatments and (ii) 4 cm inside the treatment borders during the IMRT and conventional breast treatments. The mean dose measured was about 58% of the prescription dose for the patients treated using the MammoSite® technique. On the other hand, for patients treated with IMRT and tangential fields, the mean dose was found to be about 69% and 71% of the corresponding prescription dose. This study suggests that in breast cancer radiation treatments the MammoSite® HDR technique reduces skin doses compared to IMRT and tangential field techniques. PACS numbers: 87.53.Jw, 87.53.Xd, 87.66.Sq

Clinical Application of Ultrasound Imaging in Radiation Therapy

Radiation therapy plays an important role in cancer treatment. It is well known that good local control is achieved when planned dose of radiation is delivered to the target. Recent advances in technology, in particular in image guided radiation therapy (IGRT), has significantly improved the accuracy of target localization for daily radiation treatment. Daily localization of the target is critical to the delivery of the prescription dose to the target. Thus, to achieve accurate targeting and reduce the irradiation of normal tissues and to potentially escalate dose to target volumes, IGRT needs to be implemented for daily use in the clinic. Various IGRT techniques are currently available. One of the techniques integrates an On Board low kilovoltage imaging capability into the linear accelerator that produces diagnostic quality images. However, this imaging technique requires that extra radiation dose be delivered to the patient. Another IGRT imaging system that has been integrated recently with the linear accelerator is 3D ultrasound imaging. This technique is non invasive, requires no extra radiation to a patient and provides capabilities for daily target localization and verification prior to the delivery of radiation treatment. Currently, 3D ultrasound imaging is used for target localization and verification of prostate, gynecological and breast cancers. Since the late nineties, 2D ultrasound imaging has been used for prostate localization only, but only recently has 3D ultrasound localization been available. This has also led to the use of 3D ultrasound for localizing other treatment sites as well; although, each site requires unique methods. For the prostate cancer treatment, the prostate can move daily compared to the reference planning CT (radiation therapy planning and dose calculation for all disease sites is done with CT image based)due to bladder and rectal filling. Ultrasound imaging can be used to image and localize the prostate target daily. The prostate and bladder can all typically be well visualized and compared to the reference ultrasound image or CT image. Shifts are then identified and made to reposition the patient to the point that the treatment volumes identified each day are aligned with where they were on the planning CT images relative to the linear accelerator’s isocenter. This isocenter is the point in space about which the linear accelerator rotates and all planning and radiation beam delivery for a daily treatment is performed relative to it. For gynecological cancer treatment, the ultrasound image does not show the entire target because target definition is complex and composed of multiple structures. The purpose of

SU‐GG‐J‐84: Evaluation of Comparing Daily Ultrasound Images with a Reference Ultrasound Image for Prostate Localization

Purpose: To evaluate if acquiring an ultrasoundimage at the time of CT simulation for comparison with daily ultrasoundimages improves daily localization of prostates. Method and Materials: Resonant Medicals® ultrasound localization system was installed and implemented in our clinic. The technique relies on acquiring a 3D ultrasoundimage at the time of CT simulation for daily comparison whereas other ultrasound localization techniques compare daily ultrasoundimages to the original CTimage.Treatment planning is done on the CT. DRRs are also constructed from the CT, and fiducials implanted in the prostate are outlined on the DRRs. Each day a 3D ultrasoundimage was acquired and compared to the ultrasound that was acquired at the time of CT simulation. Daily, if the ultrasoundimage was approved by the physician, the couch was shifted to align the current prostate location with its location at the time of simulation. After the ultrasound, ports were taken as often as prescribed by the physician. The fiducial locations as seen in the ports were compared to their locations on the DRRs. Any necessary shifts were made to align the fiducials. Following the treatments, an analysis was made of the ultrasound localization as compared to the fiducial localization. 22 patients had 7 or more days in which both ultrasound and ports of fiducials were acquired and are included in this analysis. Results: The measured average difference between the ultrasound localization and the localization based on ports of fiducials is 7.2 mm. This is comparable to what is reported in literature for other ultrasound localization techniques. Conclusion: Using a 3D ultrasoundimage acquired at the time of CT simulation does not improve ultrasound localization accuracy as compared to techniques that compare daily ultrasoundimages to the simulation CT for localization.

A 3D correction method for predicting the readings of a PinPoint chamber on the CyberKnife® M6™ machine

The use of small fields in radiation therapy techniques has increased substantially in particular in stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). However, as field size reduces further still, the response of the detector changes more rapidly with field size, and the effects of measurement uncertainties become increasingly significant due to the lack of lateral charged particle equilibrium, spectral changes as a function of field size, detector choice, and subsequent perturbations of the charged particle fluence. This work presents a novel 3D dose volume-to-point correction method to predict the readings of a 0.015 cc PinPoint chamber (PTW 31014) for both small static-fields and composite-field dosimetry formed by fixed cones on the CyberKnife® M6™ machine. A 3D correction matrix is introduced to link the 3D dose distribution to the response of the PinPoint chamber in water. The parameters of the correction matrix are determined by modeling its 3D dose response in circular fields created using the 12 fixed cones (5 mm-60 mm) on a CyberKnife® M6™ machine. A penalized least-square optimization problem is defined by fitting the calculated detector reading to the experimental measurement data to generate the optimal correction matrix; the simulated annealing algorithm is used to solve the inverse optimization problem. All the experimental measurements are acquired for every 2 mm chamber shift in the horizontal planes for each field size. The 3D dose distributions for the measurements are calculated using the Monte Carlo calculation with the MultiPlan® treatment planning system (Accuray Inc., Sunnyvale, CA, USA). The performance evaluation of the 3D conversion matrix is carried out by comparing the predictions of the output factors (OFs), off-axis ratios (OARs) and percentage depth dose (PDD) data to the experimental measurement data. The discrepancy of the measurement and the prediction data for composite fields is also performed for clinical SRS plans. The optimization algorithm used for generating the optimal correction factors is stable, and the resulting correction factors were smooth in the spatial domain. The measurement and prediction of OFs agree closely with percentage differences of less than 1.9% for all the 12 cones. The discrepancies between the prediction and the measurement PDD readings at 50 mm and 80 mm depth are 1.7% and 1.9%, respectively. The percentage differences of OARs between measurement and prediction data are less than 2% in the low dose gradient region, and 2%/1 mm discrepancies are observed within the high dose gradient regions. The differences between the measurement and prediction data for all the CyberKnife based SRS plans are less than 1%. These results demonstrate the existence and efficiency of the novel 3D correction method for small field dosimetry. The 3D correction matrix links the 3D dose distribution and the reading of the PinPoint chamber. The comparison between the predicted reading and the measurement data for static small fields (OFs, OARs and PDDs) yield discrepancies within 2% for low dose gradient regions and 2%/1 mm for high dose gradient regions; the discrepancies between the predicted and the measurement data are less than 1% for all the SRS plans. The 3D correction method provides an access to evaluate the clinical measurement data and can be applied to non-standard composite fields intensity modulated radiation therapy point dose verification.