Are moving junctions in craniospinal irradiation for medulloblastoma really necessary?

A low gradient junction technique of craniospinal irradiation using volumetric-modulated arc therapy and its advantages over the conventional therapy

A technique using volumetric-modulated arc therapy (VMAT) fields for craniospinal irradiation with low dose gradients at the field junction was tested for its sensitivity to positional inaccuracy. It was compared against the conventional three-dimensional (3D) conformal radiotherapy in terms of dose uniformity at the junction. Treatment plans generated for ten patients who received craniospinal irradiation (35Gy in 21 fractions) by VMAT technique at our centre were included in this study. For these patients, 3D conformal radiotherapy plans were also generated in addition to the VMAT treatment plans. Intentional shifting of the cranial field in the superior and then in the inferior directions was done, creating a gap or overlap between the fields. Consequent changes in dose distributions in these two plans to positional inaccuracies were studied. The 3D conformal radiotherapy plans showed large dose variations at the junction due to positional shifts as compared to the VMAT plans. With a 5mm superior shift of the cranial field isocentre creating a gap between the cranial and spinal fields, the magnitudes of under-dosing were 13.9±3.6Gy and 4.8±2.0Gy for 3D conformal radiotherapy and VMAT respectively. When the cranial field was moved by 5mm inferiorly creating an overlap between the fields, overdose to the effects of 10.3±4.0Gy and 4.9±1.3Gy were observed for the 3D conformal radiotherapy plans and VMAT plans respectively. The VMAT technique is insensitive to longitudinal setup errors (1-3mm) in patients because of the existence of low dose gradients at the junction between fields. This is unlike the 3D conformal radiotherapy plans which have steep dose gradients at the field edges and thus are highly sensitive to setup errors. Such an advantage for VMAT circumvents the need for dose feathering often practiced with the 3D conformal radiotherapy technique and makes the technique simpler to follow.

A novel supine isocentric approach for craniospinal irradiation and its clinical outcome

OBJECTIVE

To report a novel approach for craniospinal irradiation (CSI) using a supine isocentric technique.

METHODS

Patients were treated in the supine position using CT simulation. Half-beam-blocked lateral cranial fields and superior spinal fields have the same isocentre, and their beam divergences match. Tangential irradiation provides a non-divergent junction for the other two full-beam spinal fields. Shielding for cranial fields was generated, and dose distribution was calculated using a three-dimensional planning system. When sacral spinal fields were required, two lateral opposite fields were designed to protect the urogenital organs. All treatment portals were filmed once per week.

RESULTS

At a median follow-up of 49.8 months, 5 relapses and no cases of radiation myelitis developed in 26 consecutive patients. In the junctions of the brain-spine or spine-spine field, no failure occurred. Three failures occurred in the primary site alone, two in the spinal axis alone.

CONCLUSION

The results of our study have shown that our novel approach for CSI was not associated with increased failures at the field junction and deaths. In addition, no radiation myelitis, pneumonia, severe damage to the heart and gastrointestinal tract, and second cancers occurred in our study.

ADVANCES IN KNOWLEDGE

This new approach is an optimal alternative in cancer centre without tomotherapy because of its convenience for immobilization, repeatability, optimal dose distribution and satisfactory clinical outcome.

Craniospinal irradiation using a forward planned segmented field technique

Craniospinal irradiation is technically demanding due to the complex shape of the planning target volume (PTV). Radiotherapy treatment techniques have evolved over time as imaging and radiotherapy treatment technology have improved. However, most are variations on a class solution utilizing a prone patient position with two shaped lateral cranial portals and a matched posterior spinal portal with moving junctions. Major areas of difficulty remain with the accurate definition of the PTV and achieving a homogeneous dose within it, especially at the junctions. We describe a three-dimensionally (3D) planned craniospinal radiation technique that permits rapid image acquisition with reduced localization time, simplified spinal PTV definition and standardized cranial PTV definition. Improved dose homogeneity within the PTV is achieved by use of a segmented "field-in-field" technique (forward planned intensity-modulated radiotherapy (IMRT)) in place of customized compensators. This has negated the requirement for constructing physical compensators. Autosequencing for field delivery enables the junction to be "moved" during a single fraction and reduces the overall treatment time, an important consideration when treating very young patients.

A simple technique for craniospinal radiotherapy in the supine position

PURPOSE

Craniospinal irradiation poses technical difficulties that may be addressed with the use of the newer technologies that have become available over the past decade. The use of CT simulation allows improved target localisation and beam geometry definition while significantly reducing the treatment simulation time. We have developed a CT-based technique for whole CNS irradiation in the supine position that uses fixed field parameters, asymmetric jaws for field matching and drastically reduces simulation and treatment times.

METHODS

The patient is CT scanned and treated in the supine position. The clinical target volume and relevant critical structures are outlined on a planning CT scan. Half beam blocked lateral fields with a collimator rotation are used to match the beam divergence from the superior border of the spinal field at the C2 vertebral body. The shielding for the cranial fields is generated automatically, and the dose distribution is calculated using a 3D treatment planning system. Fixed field parameters are used for the planning and treatment. The position of the isocenter of the spine field is always a fixed longitudinal distance from the isocenter of the brain fields. If multiple posterior fields are required, the isocenter of the second spine field is always a fixed longitudinal distance from that of the first and the gap between the fields is determined using virtual simulation and feathered during treatment using the asymmetric jaws of the linear accelerator. All treatment portals are filmed daily during the first week of treatment, and after each junction change thereafter.

RESULTS AND CONCLUSION

The supine position provides numerous advantages. Patients are more comfortable, the treatment position is more reproducible, and access to the airway is possible, if necessary, for patient sedation. The use of CT simulation decreases the simulation time, allows for increased planning accuracy, and enables the use of multimodality image registration, and 3D treatment planning. The use of asymmetric jaws allows for junction feathering without changing the patient setup or using a couch angle.

The effect of an extended source-to-skin distance in the treatment of the spinal field in children receiving craniospinal irradiation

In this study, we compared a single extended source-to-skin difference (SSD) spinal field and the alternative 2-field gapped approach at 100 SSD on dose to surrounding normal tissues. Five female patients ranging in age from 3 to 20 years underwent computed tomography (CT) simulation for treatment planning of the craniospinal axis, which was treated in its entirety to 36 Gy. For each slice, the clinical target volume (thecal sac and contents), mandible, thyroid gland, esophagus, heart, lungs, liver, and ovaries were contoured. Technique A employed the use of a single posterior spinal field delivered at SSD = 140 cm (140 SSD), and Technique B employed the use of 2 gapped spinal fields using a traditional SSD of 100 cm (100 SSD). Dose-volume histograms (DVHs) were obtained for each organ contoured and for each technique used. In all patients, the average mean dose to all surrounding structures was increased with the use of a single extended SSD (Technique A) when compared to the 2 spinal fields prescribed at 100 SSD (Technique B). The average mean doses to the mandible, thyroid gland, esophagus, and heart were 78%, 19%, 6%, and 16%, respectively, higher with Technique A as compared to Technique B. In addition, the average mean doses to the lungs, liver, and ovaries were 53%, 33% and 69%, respectively, higher as compared to Technique B. However, the clinical target volume at the spinal junction site received a less homogenous dose with Technique B as compared to Technique A. We conclude that although the use of a single-field extended SSD delivered a more homogenous dose to the spine, a higher dose to the ovaries, thyroid gland, mandible, lungs, liver, and heart was seen.

Technical aspects in cerebrospinal irradiation

BACKGROUND

Craniospinal irradiation (CSI) is commonly used for medulloblastoma patients and some others; for example, those with ependymoma. It is a complex technique where accuracy of field placement and choice of irradiation source and energy can play a major role in determining success or failure.

PROCEDURE

A literature review of the techniques used and reported by treatment centers was undertaken.

RESULTS

Great variability was found in methods and practices. This is striking because many hospitals treat fewer than five patients per year.

CONCLUSION

Standardized techniques with strict central review for quality control before therapy is initiated should become the norm. Standardization is especially important when clinical trials are planned. Valid outcome data depend on CSI uniformity.