6.0 RADIATION THERAPY

6.1 Standard Radiotherapy (EBXRT)

6.1.1 Dose

One treatment of 2 Gy will be given Monday through Friday except on the four SRT treatment days for a total dose of 50 Gy. All portals shall be treated during each treatment session.

6.1.2 Treatment Factors

Treatment shall be delivered with megavoltage machines of energy ranging from 4-10 MV photons. Selection of appropriate photon energy(ies) should be based on optimizing the RT dose distribution within the target volume and minimizing dose to non-target normal tissue. Photon energies > 10 MV should be used only in dual energy beam arrangements using at least one beam with energy < 10 MV. Source skin distance for SSD techniques or source axis distance for SAD techniques must be at least 80 cm. Electron, particle or implant boost is not permissible.

6.1.3 Localization, Simulation, and Immobilization

The patient shall be treated in the supine or other appropriate position for the location of the lesion. A head-holding device that is transparent to x-rays must ensure adequate immobilization during therapy and ensure reproducibility. The target volume for the standard radiotherapy shall be based on the preoperative CT/MRI. The initial target volume shall include the contrast-enhancing lesion and surrounding edema (if it exists) demonstrated on CT/MRI plus a 2.0 cm margin. If no surrounding edema is present, the initial target volume should include the contrast-enhancing lesion plus a 2.5 cm margin. Should significant change in anatomic architecture occur postoperatively then the postoperative scans should be used for planning.

6.1.4 Treatment Planning

Treatment plans may include a wedge pair of fields, arcs, or multiple field techniques. Straight opposed lateral fields are not recommended and should be avoided. CT or MRI guided treatment planning is necessary to assure accuracy in the selection of field arrangements. Inability to achieve field placement as defined by the protocol will result in variation scores at headquarter reviews. Isodose distribution for the initial target volume and conedown target volume is required on all patients, including those treated with parallel opposed fields. The inhomogeneity across the target volume should be kept to a minimum. The minimum dose to the target volume should be kept within 5-10% of the dose at the center of the target. The use of a vertex field requires either a diagram or photograph of the treatment position to be submitted to RTOG Headquarters. If possible, send a film of the vertex field portal (without the patient). This will ensure that the vertex field portal shape/blocks matches that drawn on the DRR.

6.1.5 Dose Specification

Doses are specified as the target dose which shall be at the center of the target volume; this normally will be the isocenter in an isocentric technique (ICRU 50).

6.2 Dose Limitation to Critical Structures

The lens and cervical spine must be shielded from the direct beam at all times. When possible to do without shielding gross tumor, attempts should be made to limit dose to the optic chiasm to 55 Gy (including contribution from SRT), the retina of at least one eye (but preferably both) to 50 Gy, and the brain stem to 60 Gy.

6.3 Stereotactic Radiotherapy Boost (SRT)

SRT boost must start during week 3 of EBXRT. Patients will receive 4 total SRT treatments, one per week during weeks 3-6. Patients will not receive EBXRT on the SRT treatment days. Only those patients whose residual enhancing tumor plus tumor resection cavity is < 60 mm are eligible to enter the study. The method of SRT delivery on this study must allow for a high degree of dose homogeneity and conformality. Treatment must be performed with 3D conformal radiotherapy techniques using multiple coplanar/noncoplanar static conformal beams (or other conformal treatment delivery method, see Section 6.3.4) . The fields should be shaped by a multileaf collimator or by custom fabricated cerrobend blocks. Traditional SRS/SRT linear accelerator arc-based treatment delivery with circular collimators is not allowed. Gamma knife treatment is also not allowed due to significant dose inhomogeneity and the non-availability of relocatable frames.

6.3.1 Dose Specification

The dose prescribed to the PTV (see Section 6.3.3) will be based on the maximum diameter as follows:

Maximum PTV Diameter Dose Per Fraction
< 40 mm 7 Gy
> 40 mm 5 Gy

Dose will be prescribed to 80-90% of maximum dose. The prescription isodose volume should encompass the entire PTV.

6.3.1.1 The 100 % (maximum dose), and minimum doses to the PTV will be recorded for each patient using PTV dose volume histogram information.

6.3.2 Equipment Requirements

Participating institutions must complete the RTOG stereotactic radiotherapy facility questionnaire (Appendix V ) prior to enrolling any patients in the study. This is to ensure that RTOG participants understand the study and have the appropriate tools for delivery of SRT. Prior approval for other RTOG studies is not acceptable for RTOG BR-0023.

6.3.2.1 Immobilization/Relocalization: A non-invasive, stereotactic, relocatable immobilization system will be used for treatment simulation and delivery. These systems may include modified stereotactic frames, camera-based localization systems, etc. The immobilization/ relocalization system should be capable of reproducing the patient setup to within 3 mm.

Treatment Planning: 3-D conformal radiation therapy capabilities. The ability to plan and deliver multiple non-coplanar fields, or other highly conformal dose conformality methods (see Section 6.3.4) is mandatory.

Image Acquisition: Precise delineation of residual tumor/resection cavity is greatly improved with MRI imaging. This may be accomplished with the use of MRI compatible immobilization devices and treatment planning systems or by MRI digital image registration procedures with the SRT treatment planning CT. If the later option is used then the MRI need not be performed with the headframe in place. If such an MRI is performed for image registration procedures, it must be done within 2 weeks of the treatment planning CT. If these options for MRI target delineation are unavailable and if the tumor treatment area is well visualized on CT, then CT only may be used.

Either the treatment planning CT or MRI should be acquired with the patient in the same position, immobilization device, and conditions as will be used during treatment delivery. Each patient will be immobilized in a relocatable stereotactic frame or with another precision, non-invasive stereotactic system. Standard thermal-plastic masks may not be used. The treatment planning CT/MRI scan should start at the top of the cranial vertex and proceed down to the neck to encompass the entire intracranial contents. The scan slice thickness should be < 0.5 cm (preferably 0.3 cm) through the region that contains the target volume. The regions above and below the target volume may be scanned with a slice thickness of < 1 cm. The GTV, PTV, and normal tissue structures must be outlined on all CT/MRI slices in which the structures exist.

6.3.3 Volume Definitions

6.3.3.1 The gross tumor volume (GTV) will include the contrast-enhancing residual tumor and the operative tumor resection cavity with no margin. The operative tumor resection cavity should be contiguous with or include the residual tumor although it may be difficult to distinguish between them because of postoperative rim-enhancing changes, etc. On occasion, a corticotomy tract is performed at the time of surgery to access deep seeded tumors; such tracts should not be included in the GTV. Surrounding areas of edema will also not be considered part of the GTV.

NOTE: If the GTV has increased in size beyond 60 mm since registration, the patient will not receive stereotactic boost and will be considered removed from study treatment even if the criterion for formal progression (Section 11.3.5) is not met. Radiotherapy will be at the discretion of the treating physician. (Please notify RTOG Headquarters and the study chairman if this occurs).

Gross total resection: GTV = tumor resection cavity
Partial resection: GTV = residual enhancing tumor + resection cavity
Biopsy only: GTV = enhancing tumor only

6.3.3.2 The planning target volume (PTV) will be equal to the GTV + 5 mm in all directions to incorporate adjacent high-risk disease and treatment set-up uncertainty. Note: The PTV does not necessarily indicate the block edge. An additional margin around the PTV to define the beam aperture may be needed to meet PTV dose homogeneity requirements because of the effect of beam penumbra.

6.3.3.3 Critical normal structures. Normal tissues to be contoured include the eyes, optic nerves, optic chiasm and brain stem.

6.3.4 Treatment Planning

6.3.4.1 3-D Conformal Radiotherapy Using Static Coplanar/Noncoplanar Beams (3DCRT): Multiple vertex and coplanar/noncoplanar beams should be used and arranged with the goal of excluding as much normal brain tissue as possible outside of the PTV at high and intermediate dose levels. The beam’s eye view displays must be used to design beam apertures. Wedges, compensators, and multileaf collimators may be used. Dynamic conformal arcs and other beam shaping techniques which modulate the field aperture to further improve dose conformality may be used as long as the dose homogeneity requirement is maintained.

6.3.5 SRT Treatment Planning Data

6.3.5.1 Submit isodose distributions calculated through the center of the target in the transverse, coronal, and sagittal planes. The isodoses shall be superimposed on MRI/CT anatomy and shall include isodose lines that correspond to 100%, 90%, 80% and 50% of the prescription dose. It is intended that these data are of sufficient quality that the reviewer can judge the adequacy of target coverage by the dose distribution.

6.3.5.2 Submit dose-volume data in tabular form, showing the accumulated volumes of those elements within the planning target volume and within the entire global treatment volume receiving dose in 1 Gy dose intervals. These data may be differential or cumulative dose-volume statistics.

6.3.6 SRT Quality Assurance Review

The protocol chairman and the protocol physicist will perform a final review of the stereotactic radiotherapy treatment. The review process will evaluate the SRT Summary Form, and the stereotactic CT/MRI with superimposed isodoses at required levels, and dose-volume data. Based on the evaluation and verification of data submitted, the following Quality Assurance scores will be assigned to each case.

6.3.6.1 The isodose line to which the dose is prescribed is considered the “prescription” or 100% isodose line. If the 90% isodose line completely encompasses the target, the case is considered per protocol. If the 90% isodose line does not completely cover the target, but the 80% isodose line does completely cover the target, this shall be classified as a minor variation. If the 80% isodose line does not completely cover the target, this shall be classified as a major deviation.

6.3.6.2 The maximum dose delivered shall be determined. Dose homogeneity within the planning target volume shall be determined as the maximum dose divided by the prescription dose (ratio MDPD). This ratio should be less than 1.25. MDPD ratio greater than 1.25 and less than 1.4 will be classified as a minor deviation. MDPD ratio greater than 1.4 will be classified as a major deviation.

6.3.6.3 The volume of the prescription isodose surface shall be determined from the dose volume histogram. A figure of merit for conformation of the prescription isodose surface divided by the target volume (ratio PITV). Every attempt should be made to keep the ratio as close to 1.00 as possible while maintaining target coverage and target homogeneity criteria. A value between 1.00 and 2.00, if achieved, will not be a minor deviation from protocol. PITV ratios > 2.00, but less than 2.5, shall be classified as a minor deviation. PITV ratios > 2.5 shall be classified as a major deviation.

6.3.6.4 Optic chiasm dose including contributions from all fields >55 Gy will be considered a major violation.