TBI treatment planning using the ADAC pinnacle treatment planning system

Organ sparing of linac-based targeted marrow irradiation over total body irradiation

PURPOSE

Targeted marrow irradiation (TMI) is an alternative conditioning regimen to total body irradiation (TBI) before bone marrow transplantation in hematologic malignancies. Intensity-modulation methods of external beam radiation therapy are intended to permit significant organ sparing while maintaining adequate target coverage, improving the therapeutic ratio. This study directly compares the dose distributions to targets and organs at risk from TMI and TBI, both modalities conducted by general-use medical linacs at our institution.

METHODS

TMI treatments were planned for 10 patients using multi-isocentric feathered volumetric arc therapy (VMAT) plans, delivered by 6 MV photon beams of Elekta Synergy linacs. The computed tomography (CT) datasets used to obtain these plans were also used to generate dose distributions of TBI treatments given in the AP/PA extended-field method. We compared dose distributions normalized to the same prescription for both plan types. The generalized equivalent uniform dose (gEUD) of Niemierko for organs and target volumes was used to quantify effective whole structure dose and dose savings.

RESULTS

For the clinical target volume (CTV), no significant differences were found in mean dose or gEUD, although the radical dose homogeneity index (minimum dose divided by maximum dose) was 31.7% lower (P = 0.002) and the standard deviation of dose was 28.0% greater (P = 0.027) in the TMI plans than in the TBI plans. For the TMI plans, gEUD to the lungs, brain, kidneys, and liver was significantly lower (P < 0.001) by 47.8%, 33.3%, 55.4%, and 51.0%, respectively.

CONCLUSION

TMI is capable of maintaining CTV coverage as compared to that achieved in TBI, while significantly sparing organs at risk. Improvement on sparing organs at risk permits a higher prescribed dose to the target or the maximum number of times marrow conditioning may be delivered to a patient while maintaining similar typical tissue complication rates.

Prescribed and Measured Dose Differences for an AP-PA TBI Protocol with Compensation Filter and Ergonomic Patient Support

INTRODUCTION

A treatment scheme is presented for clinical applications of total body radiation using 6 MV photons.

METHODS

The two-field technique, anteroposterior (AP) and posteroanterior (PA), was applied. In vivo dose verification with TLD-100 dosimeters was developed during the first treatment sessions. Dosimetry was performed in several anatomical regions such as head, shoulder, thorax, abdomen, hip, thigh, knee, and ankle in 13 patients as preconditioning for bone marrow transplantation. A prescribed dose of 100 cGy was considered at the level of the patient's coronal plane. Values with least differences between measured and prescribed dose were obtained for the shoulder.

RESULTS

Standard deviation percentages of 0.3% and 2.6% were recorded between these doses for AP and PA, respectively. There were larger differences for other contours. In general, a measured dose less than the prescribed dose was observed in all the contours.

CONCLUSION

Differences between prescribed and measured doses showed in this study can contribute to a required understanding about the accurate doses in AP-PA total body irradiation (TBI) treatments.

A depth-sensing technique on 3D-printed compensator for total body irradiation patient measurement and treatment planning

PURPOSE

The purpose of total body irradiation (TBI) techniques is to deliver a uniform radiation dose to the entire volume of a patient's body. Due to variations in the thickness of the patient, it is difficult to produce such a uniform dose distribution throughout the body. In many techniques, a compensator is used to adjust the dose delivered to various sections of the patient. The current study aims to develop and validate an innovative method of using depth-sensing cameras and 3D printing techniques for TBI treatment planning and compensator fabrication.

METHODS

A tablet with an integrated depth-sensing camera and motion tracking sensors was used to scan a RANDO™ phantom positioned in a TBI treatment booth to detect and store the 3D surface in a point cloud format. The accuracy of the detected surface was evaluated by comparing extracted body thickness measurements with corresponding measurements from computed tomography (CT) scan images. The thickness, source to surface distance, and off-axis distance of the phantom at different body section were measured for TBI treatment planning. A detailed compensator design was calculated to achieve a uniform dose distribution throughout the phantom. The compensator was fabricated using a 3D printer, silicone molding, and a mixture of wax and tungsten powder. In vivo dosimetry measurements were performed using optically stimulated luminescent detectors.

RESULTS

The scan of the phantom took approximately 30 s. The mean error for thickness measurements at each section of phantom relative to CT was 0.48 ± 0.27 cm. The average fabrication error for the 3D-printed compensator was 0.16 ± 0.15 mm. In vivo measurements for an end-to-end test showed that overall dose differences were within 5%.

CONCLUSIONS

A technique for planning and fabricating a compensator for TBI treatment using a depth camera equipped tablet and a 3D printer was demonstrated to be sufficiently accurate to be considered for further investigation.

Total body irradiation-an attachment free sweeping beam technique

Introduction

A sweeping beam technique for total body irradiation in standard treatment rooms and for standard linear accelerators (linacs) is introduced, which does not require any accessory attached to the linac. Lung shielding is facilitated to reduce the risk of pulmonary toxicity. Additionally, the applicability of a commercial radiotherapy planning system (RTPS) is examined.

Material and Methods

The patient is positioned on a low couch on the floor, the longitudinal axis of the body in the rotational plane of the linac. Eight arc fields and five additional fixed beams are applied to the patient in supine and prone position respectively. The dose distributions were measured in a solid water phantom and in an Alderson phantom. Diode detectors were calibrated for in-vivo dosimetry. The RTPS Oncentra was employed for calculations of the dose distribution.

Results

For the cranial 120 cm the longitudinal dose profile in a slab phantom measured with ionization chamber varies between 94 and 107 % of the prescription dose. These values were confirmed by film measurements and RTPS calculations. The transmittance of the lung shields has been determined as a function of the thickness of the absorber material. Measurements in an Alderson phantom and in-vivo dosimetry of the first patients match the calculated dose.

Discussion and conclusion

A treatment technique with clinically good dose distributions has been introduced, which can be applied with each standard linac and in standard treatment rooms. Dose calculations were performed with a commercial RTPS and should enable individual dose optimization.

Going the distance: validation of Acuros and AAA at an extended SSD of 400 cm

Accurate dose calculation and treatment delivery is essential for total body irradiation (TBI). In an effort to verify the accuracy of TBI dose calculation at our institution, we evaluated both the Varian Eclipse AAA and Acuros algorithms to predict dose distributions at an extended source‐to‐surface distance (SSD) of 400 cm. Measurements were compared to calculated values for a 6 MV beam in physical and virtual phantoms at 400 cm SSD using open beams for both 5×5 and 40×40 cm2 field sizes. Inline and crossline profiles were acquired at equivalent depths of 5 cm, 10 cm, and 20 cm. Depth‐dose curves were acquired using EBT2 film and an ion chamber for both field sizes. Finally, a RANDO phantom was used to simulate an actual TBI treatment. At this extended SSD, care must be taken using the planning system as there is good relative agreement between measured and calculated profiles for both algorithms, but there are deviations in terms of the absolute dose. Acuros has better agreement than AAA in the penumbra region.

PACS number(s): 87.55.kd

Radiation caries in nasopharyngeal carcinoma patients after intensity-modulated radiation therapy: A cross-sectional study

Background/purpose

The exact dose of intensity-modulated radiation therapy (IMRT) associated with tooth damage is mostly unknown. We aim to evaluate the severity of dental lesions after IMRT and the correlation with the radiation dose to the dentition in patients with nasopharyngeal carcinoma (NPC).

Materials and methods

This was a cross-sectional study of 42 patients with NPC who completed IMRT in 2011. Each premolar tooth was divided into 13 sites. Teeth were evaluated using a validated index and subsequently categorized at each divided site. The relationship between dose distribution and the caries severity score was analyzed using logistic models. The odds of developing caries damage were evaluated using odds ratios.

Results

A total of 4342 sites from 334 premolar teeth were evaluated. For sites exposed to 30–60 Gy, the odds of developing caries damage were 12–200 times greater compared with sites unexposed to IMRT. A new radiation caries lesion was likely to occur when the dose was >35.8 Gy after 17 days' radiation therapy (P < 0.05).

Conclusion

The findings suggest that new tooth damage was likely to occur at doses > 35.8 Gy, and care should be taken throughout the treatment planning process to limit tooth doses to < 50 Gy in NPC patients.

Benefits of online in vivo dosimetry for single-fraction total body irradiation

Use of a patient test dose before single-fraction total body irradiation (TBI) allows review of in vivo dosimetry and modification of the main treatment setup. However, use of computed tomography (CT) planning and online in vivo dosimetry may reduce the need for this additional step. Patients were treated using a supine CT-planned extended source-to-surface distance (SSD) technique with lead compensators and bolus. In vivo dosimetry was performed using thermoluminescent dosimeters (TLDs) and diodes at 10 representative anatomical locations, for both a 0.1-Gy test dose and the treatment dose. In total, 28 patients were treated between April 2007 and July 2013, with changes made in 10 cases (36%) following test dose results. Overall, 98.1% of measured in vivo treatment doses were within 10% of the prescribed dose, compared with 97.0% of test dose readings. Changes made following the test dose could have been applied during the single-fraction treatment itself, assuming that the dose was delivered in subportions and online in vivo dosimetry was available for all clinically important anatomical sites. This alleviates the need for a test dose, saving considerable time and resources.

In vivo dosimetry for total body irradiation: five-year results and technique comparison

The aim of this work is to establish if the new CT‐based total body irradiation (TBI) planning techniques used at University College London Hospital (UCLH) and Royal Free Hospital (RFH) are comparable to the previous technique at the Middlesex Hospital (MXH) by analyzing predicted and measured diode results. TBI aims to deliver a homogeneous dose to the entire body, typically using extended SSD fields with beam modulation to limit doses to organs at risk. In vivo dosimetry is used to verify the accuracy of delivered doses. In 2005, when the Middlesex Hospital was decommissioned and merged with UCLH, both UCLH and the RFH introduced updated CT‐planned TBI techniques, based on the old MXH technique. More CT slices and in vivo measurement points were used by both; UCLH introduced a beam modulation technique using MLC segments, while RFH updated to a combination of lead compensators and bolus. Semiconductor diodes were used to measure entrance and exit doses in several anatomical locations along the entire body. Diode results from both centers for over five years of treatments were analyzed and compared to the previous MXH technique for accuracy and precision of delivered doses. The most stable location was the field center with standard deviations of 4.1% (MXH), 3.7% (UCLH), and 1.7% (RFH). The least stable position was the ankles. Mean variation with fraction number was within 1.5% for all three techniques. In vivo dosimetry can be used to verify complex modulated CT‐planned TBI, and demonstrate improvements and limitations in techniques. The results show that the new UCLH technique is no worse than the previous MXH one and comparable to the current RFH technique.

PACS numbers: 87.55.Qr, 87.56.N‐

Total body irradiation with step translation and dynamic field matching

The purpose of this study is to develop a total body irradiation technique that does not require additional devices or sophisticated processes to overcome the space limitation of a small treatment room. The technique aims to deliver a uniform dose to the entire body while keeping the lung dose within the tolerance level. The technique treats the patient lying on the floor anteriorly and posteriorly. For each AP/PA treatment, two complementary fields with dynamic field edges are matched over an overlapped region defined by the marks on the body surface. A compensator, a spoiler, and lung shielding blocks were used during the treatment. Moreover, electron beams were used to further boost the chest wall around the lungs. The technique was validated in a RANDO phantom using GAFCHROMIC films. Dose ratios at different body sites along the midline ranged from 0.945 to 1.076. The dose variation in the AP direction ranged from 96.0% to 104.6%. The dose distribution in the overlapped region ranged from 98.5% to 102.8%. Lateral dose profiles at abdomen and head revealed 109.8% and 111.7% high doses, respectively, at the body edges. The results confirmed that the technique is capable of delivering a uniform dose distribution to the midline of the body in a small treatment room while keeping the lung dose within the tolerance level.

Implementation of a lateral total body irradiation technique with 6 MV photons: The University of Texas Health Science Center in San Antonio experience

Purpose: Total body irradiation (TBI) involves delivery of marrow-ablative or suppressive dose to the entirety of the marrow habitus. In its current practice, TBI often involves positioning the patient in an uncomfortable upright body position for extended periods of time while delivering radiation dose via anteroposterior/posterioanterior (AP/PA) fields. In an effort to maximize reproducibility and patient comfort, especially for paediatric patients, a supine lateral total body irradiation (LTBI) protocol was implemented as preparatory regimen for bone marrow transplant.

Methods and Materials: One hundred and forty-five patient charts were reviewed. Patients were treated in supine position with hands clasped over the upper abdomen in a comfortable position. They were placed in a methylcrylate body box and irradiated with opposed lateral fields at extended distance of 350 cm to the midplane of the patient. Each field delivered 100 cGy with a midplane dose of 200 cGy per fraction. Dose regimes varied from 200 to 1,200 cGy total doses. Custom lead compensating filters were utilized. A 6 MV photon beam produced by a Varian Clinac 600c linear accelerator was applied. In vivo thermoluminescent dosimeter (TLD) readings were taken for anatomical regions of interest (ROI). TLDs were placed in each ROI under a 1.5-cm-thick bolus for maximum dose build-up.

Results and Conclusion: The resulting data demonstrate a dosimetric variability of anatomical ROI from reference prescription dose of less than 3%. LTBI has been used for more than ten years in our institution and produced favourable results for more than 100 patients. We suggest this LTBI approach to facilitate successful treatment of children who require TBI while maintaining dose uniformity as recommended by the American Association of Physicists in Medicine Report 17.

Validation of the Eclipse AAA algorithm at extended SSD

The accuracy of dose calculations at extended SSD is of significant importance in the dosimetric planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi-leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with machine-specific beam data for a 6 MV photon beam, at standard SSD (100 cm). The model was then used for dose distribution calculations at extended SSD (179.5 cm). Two sets of measurements were acquired for a 6 MV beam (from a Varian linear accelerator) in a water tank at extended SSD: i) open beam for 5 x 5, 10 x 10, 20 x 20 and 40 x 40 cm2 field sizes (defined at 179.5 cm SSD), and ii) identical field sizes but with a 1.3 cm thick acrylic spoiler placed 10 cm above the water surface. Dose profiles were acquired at 5 cm, 10 cm and 20 cm depths. Dose distributions for the two setups were calculated using the AAA algorithm in Eclipse. Confidence limits for comparisons between measured and calculated absolute depth dose curves and normalized dose profiles were determined as suggested by Venselaar et al. The confidence limits were within 2% and 2 mm for both setups. Extended SSD calculations were also performed using Eclipse AAA, commissioned with Varian Golden beam data at standard SSD. No significant difference between the custom commissioned and Golden Eclipse AAA was observed. In conclusion, Eclipse AAA commissioned at standard SSD can be used to accurately predict dose distributions in water at extended SSD for 6 MV open beams.

Total Body Irradiation, Toward Optimal Individual Delivery: Dose Evaluation With Metal Oxide Field Effect Transistors, Thermoluminescence Detectors, and a Treatment Planning System

PURPOSE

To predict the three-dimensional dose distribution of our total body irradiation technique, using a commercial treatment planning system (TPS). In vivo dosimetry, using metal oxide field effect transistors (MOSFETs) and thermoluminescence detectors (TLDs), was used to verify the calculated dose distributions.

METHODS AND MATERIALS

A total body computed tomography scan was performed and loaded into our TPS, and a three-dimensional-dose distribution was generated. In vivo dosimetry was performed at five locations on the patient. Entrance and exit dose values were converted to midline doses using conversion factors, previously determined with phantom measurements. The TPS-predicted dose values were compared with the MOSFET and TLD in vivo dose values.

RESULTS

The MOSFET and TLD dose values agreed within 3.0% and the MOSFET and TPS data within 0.5%. The convolution algorithm of the TPS, which is routinely applied in the clinic, overestimated the dose in the lung region. Using a superposition algorithm reduced the calculated lung dose by approximately 3%. The dose inhomogeneity, as predicted by the TPS, can be reduced using a simple intensity-modulated radiotherapy technique.

CONCLUSIONS

The use of a TPS to calculate the dose distributions in individual patients during total body irradiation is strongly recommended. Using a TPS gives good insight of the over- and underdosage in a patient and the influence of patient positioning on dose homogeneity. MOSFETs are suitable for in vivo dosimetry purposes during total body irradiation, when using appropriate conversion factors. The MOSFET, TLD, and TPS results agreed within acceptable margins.

Independent calculations to validate monitor units from ADAC treatment planning system

Current standards of practice are based on the use of an independent calculation to validate the monitor units (MUs) derived from a treatment planning system. The ADAC PINNACLE treatment planning system has shown discrepancies of 10% or more compared to simple independent calculations for highly contoured areas such as tangential breast and chest wall irradiation. The ADAC treatment planning system generally requires more MUs to deliver the same prescribed dose. Independent MU calculation methods are based on full phantom conditions. On the other hand, the MUs from the ADAC treatment planning system are derived using realistic phantom scatter. As such, differences exist in TMR factors, off-axis wedge factors, and the phantom scatter factor. To systematically study the discrepancies due to phantom conditions, experimental measurements were performed with various percentages of tissue missing. The agreement between the experimental measurements and ADAC calculations was found to be within 2%. Using breast field geometry, a relationship between missing tissue and the dosimetric parameters used by ADAC was developed. This relationship, when applied, yielded independent MU calculations whose values closely matched those from the ADAC treatment planning system.