Development of a translating bed for total body irradiation

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

Pulmonary toxicity following total body irradiation for acute lymphoblastic leukaemia: The Ottawa Hospital Cancer Centre (TOHCC) experience

Purpose

To review the incidence of clinically significant pulmonary toxicity following total body irradiation (TBI) as a part of the conditioning regimen for acute lymphoblastic leukaemia (ALL) patients undergoing bone marrow transplantation (BMT) at The Ottawa Hospital Cancer Centre.

Methods

This is a retrospective review of ALL patients who received TBI in The Ottawa Hospital Bone Marrow Transplant Program (TOH-BMT) as part of their conditioning regimen from 1991 to 2011 inclusive. The patients were treated using a locally developed translating-couch irradiation technique. We have analysed all available data for the first 100 days following TBI to determine the incidence of radiation-induced pulmonary toxicities.

Results

Of the total 622 patients undergoing TBI during the specified period, 88 had ALL. Median age at BMT was 30 years and the conditioning regimens varied. A total of 74 (84%) patients received 12 Gy/6 F/BID of TBI. A total of 55 (63%) patients have died, 32 (36%) within the 1st year after BMT. In the 1st year, pulmonary events were reported for 24 (27%) patients, and the follow-up notes were unavailable for seven (8%). Pulmonary toxicities were reported as the cause of death for six patients, five (6%) within the 1st year. It is estimated that the total number of deaths in the 1st year possibly attributed to radiation-induced lung injury was four (4·5%). Eight (9%) patients had symptoms suggestive of non-lethal grade 2–3 radiation-induced pneumonitis.

Conclusions

TBI continues to be an important component of the conditioning regimen for ALL patients undergoing BMT, and the incidence of radiation-induced pulmonary injury, using our technique and lung dose, is comparable to the published literature.

Accelerating total body irradiation with large field modulated arc therapy in standard treatment rooms without additional equipment

PURPOSE

The aim of this study was to develop a generic and ultra-efficient modulated arc technique for treatment with total body irradiation (TBI) without additional equipment in standard treatment rooms.

METHODS

A continuous gantry arc between 300° and 70° composed of 26 subarcs (5° per subarc) using a field size of 40 × 40 cm(2) was used to perform the initial beam data measurements. The profile was measured parallel to the direction of gantry rotation at a constant depth of 9 cm (phantom thickness 18 cm). Beam data were measured for single 5° subarcs, dissecting the individual contribution of each subarc to a certain measurement point. The phantom was moved to 20 measurement positions along the profile. Then profile optimization was performed manually by varying the weighting factors of all segments until calculated doses at all points were within ± 1 %. Finally, the dose distribution of the modulated arc was verified in phantom thicknesses of 18 and 28 cm.

RESULTS

The measured profile showed a relative mean dose of 99.7 % [standard deviation (SD) 0.7 %)] over the length of 200 cm at a depth of 9 cm. The measured mean effective surface dose (at a depth of 2 cm) was 102.7 % (SD 2.1 %). The measurements in the 28 cm slab phantom revealed a mean dose of 95.9 % (SD 2.9 %) at a depth of 14 cm. The mean dose at a depth of 2 cm was 111.9 % (SD 4.1 %). Net beam-on-time for a 2 Gy fraction is approximately 8 min.

CONCLUSION

This highly efficient modulated arc technique for TBI can replace conventional treatment techniques, providing a homogeneous dose distribution, dosimetric robustness, extremely fast delivery, and applicability in small treatment rooms, with no need for additional equipment.

Inverse-planned modulated-arc total-body irradiation

PURPOSE

To develop a simple and robust method for inverse-planned total-body irradiation (TBI) that is more comfortable and has better dose homogeneity than the conventional forward-planned techniques and that can be delivered in a standard-sized treatment vault.

METHODS

Modulated-arc TBI (MATBI) utilizes an arc of static open-field beams to irradiate patients as they lay on a stationary couch beneath the gantry, with cerrobend blocks suspended over organs at risk to provide shielding. Prior to treatment, full-body computed tomography (CT) images are acquired of each patient and imported into the PINNACLE(3) planning system, which modulates the monitor units for the open-field beams to optimize the body dose uniformity. The volume of the body within 10% of the prescription dose, V(±10), is used as a metric to evaluate the dose uniformity. For comparison to MATBI, the dose distribution of a conventional forward-planned treatment is also calculated. Quality assurance measurements are acquired before treatment by delivering the plans to a phantom and during treatment with an ionization chamber inside a buildup block, placed between the patient's ankles.

RESULTS

For MATBI, the achieved values of V(±10) were 75.8%, 90.2%, 84.6%, and 79.8% compared to 60.3%, 77.4%, 65.6%, and 68.5% for the conventional TBI technique, respectively. The pretreatment ion chamber measurements in the phantom had an average error of 1.2%. Those acquired during treatment had larger errors, with most points being within 3% of predictions.

CONCLUSIONS

MATBI provides better dose uniformity and comfort than the conventional forward-planned TBI techniques. In addition, the technique can be implemented on most linacs, in standard-sized vaults, without the use of a translating couch.

3D heterogeneous dose distributions for total body irradiation patients

One major objective of total body irradiation (TBI) treatments is to deliver a uniform dose in the entire body of the patient. Looking at 3D dose distributions for constant speed (CstSpeed) and variable speed (VarSpeed) translating couch TBI treatments, dose uniformity and the effect of body heterogeneities were evaluated. This study was based on retrospective dose calculations of 10 patients treated with a translating couch TBI technique. Dose distributions for CstSpeed and VarSpeed TBI treatments have been computed with Pinnacle3 treatment planning system in homogeneous (Homo) and heterogeneous (Hetero) dose calculation modes. A specific beam model was implemented in Pinnacle3 to allow an accurate dose calculation adapted for TBI special aspects. Better dose coverages were obtained with Homo/VarSpeed treatments compared to Homo/CstSpeed cases including smaller overdosage areas. Large differences between CstSpeed and VarSpeed dose calculations were observed in the brain, spleen, arms, legs, and lateral parts of the abdomen (differences between V100% mean values up to 57.5%). Results also showed that dose distributions for patients treated with CstSpeed TBI greatly depend on the patient morphology, especially for pediatric and overweight cases. Looking at heterogeneous dose calculations, underdosages (2%-5%) were found in high-density regions (e.g., bones), while overdosages (5%-15%) were found in low-density regions (e.g., lungs). Overall, Homo/CstSpeed and Hetero/VarSpeed dose distributions showed more hot spots than Homo/VarSpeed and were greatly dependent on patient anatomy. CstSpeed TBI treatments allow a simple optimization process but lead to less dose uniformity due to the patient anatomy. VarSpeed TBI treatments require more complex dose optimization, but lead to a better dose uniformity independent of the patient morphology. Finally, this study showed that heterogeneities should be considered in dose calculations in order to obtain a better optimization and, therefore, to improve dose uniformity.

A translational couch technique for total body irradiation

We have constructed a computer controlled translational couch to administer total body irradiation reproducibly and safely. The system has replaced the previous stationary anterior-posterior technique in our institution and 30 plus patients have been treated with it so far. In this technique, patients comfortably lie on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in an upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry, and machine dose rate. In our design, the couch velocity is continuously updated to compensate for machine dose rate fluctuations. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents a special challenge for dosimetry calculations. A dosimetry parameter is introduced that converts the moving beam output to the fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been developed that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Multiple point dose measurements within a phantom confirmed the validity of the calculation method.

The clinical application of dynamic shielding and imaging in moving table total body irradiation

The moving table technique for total body irradiation (MTT TBI) has some advantages in regard to dose homogeneity, patient positioning and comfort. However, divergence of the radiation field coupled with patient motion necessitates corresponding motion of the shielding blocks and verification film so that penumbra is minimized. MTT TBI system is presented, together with dose calculations, incorporating moving trays for shields and film to ensure dose delivery with minimal penumbra of the blocked field.

A translating-bed technique for total-body irradiation

Total-body irradiation (TBI) is a therapy modality that is being used with increasing frequency, in conjunction with chemotherapy, for patients undergoing bone-marrow transplantation. At the Ottawa Regional Cancer Centre a technique has been developed for the delivery of TBI to patients prior to bone-marrow transplantation. In this technique patients are treated on a mobile couch at approximately 195 cm SSD with a field size of 66.5 cm wide by 57 cm long. A computer-controlled stepping motor drives the patient couch at a user-selectable speed. The total dose delivered to the patient is a function of couch velocity, field size and patient separation. Treatment times are of the order of 10 min for each of the anterior and posterior fields for a 400 cGy fraction. It has been found that the conventional central axis tissue maximum ratio (TMR) and percentage depth dose (PDD) functions are not appropriate for describing dose delivered during dynamic treatment. To this end we have developed dynamic TMR and PDD functions. Extensive measurements have been performed in an anthropomorphic water phantom to determine the dose distributions in three dimensions and the efficacy of polymethyl methacrylate (PMMA) beam spoilers as a replacement for anterior and lateral bolus. It has been found that 2.4 cm PMMA spoilers do provide full skin dose and negate the requirement for lateral bolus. This TBI procedure is simple, rapid and appears to be well tolerated by the patients. 55 patients have been treated since the introduction of this technique in 1991.

Calculation of lung shielding for total body irradiation

The radiation absorbed dose to the lung is calculated for patients receiving total body irradiation. Based on this calculation the thickness of lead attenuators required to reduce the lung dose is determined. To calculate accurately the dose the lung density as well as the lung dimensions of individual patients must be considered. For total body irradiations all calculations of attenuator thicknesses should be based on measured data obtained for the large radiation field.