Accelerated Partial Breast Irradiation Techniques and Clinical Implementation - part 3 doc

4 B-39 Protocol Requirements  of imaging would include simulator x-rays, c-arm x-rays, ultrasound or CT scan For users without x-ray projection imaging, note that CT is acceptable, but for this purpose it need not be a complete scan A single slice through the balloon center is sufficient For patients with iodine allergy, instillation of contrast medium into the balloon is contraindicated In such cases, either CT or ultrasound work well If balloon deflation occurs, a new balloon may be inserted, and the course of treatment completed 4.4 3D Conformal External Beam Intensity-modulated radiation therapy (IMRT) is not allowed For protocol purposes, this means that any form of segmented fields with weights determined via inverse planning would not be acceptable Simple segmented fields, used in the manner of wedged fields may be acceptable The primary reasons for these exclusions are that IMRT methodologies are not entirely standardized at this time, and providing oversight for the technical aspects of delivery is inordinately time-consuming with current tools It is to be expected that once this form of treatment becomes routine, off-protocol treatments will frequently make use of IMRT Tissue inhomogeneity correction must be used for all external beam calculations 4.4.1 Contouring These structures must be outlined on all CT slices in which they appear: Excision/lumpectomy cavity Skin Ipsilateral breast Contralateral breast Thyroid Heart • • • • • • 4.4.2 Volumes of Interest The CTV is defined as the contoured excision cavity plus a 15 mm uniform margin Pectoralis muscles are excluded, as well as any breast tissue within mm of the skin surface If the excision cavity is defined only by surgical clips, the radiation oncologist will construct a volume to represent the cavity This may introduce some variability into the definition of the subsequent volumes of interest In the large majority of cases, however, it is expected that the cavity will be easily visualized via CT, as the protocol mandates CT scan within 42 days of surgery, and recommends that it be performed within 14 days The PTV consists of the CTV plus an additional uniform 10 mm expansion to accommodate daily setup error and breathing motion This volume will be used to generate the actual beam apertures (with a further margin added for beam penumbra which will depend on the accelerator used) This implies that this volume will be allowed to extend outside the patient and into the chest wall structures  Gregory K Edmundson Since such a target volume is not appropriate for dose calculations (due both to including nontarget tissues and also occupying the build-up region, where the dose calculation becomes uncertain), an additional volume is defined: the PTV_EVAL This volume starts with the PTV, but excludes any portion outside the patient, or lying within mm of the skin surface, and re-excludes the pectoralis and ribs It is the PTV_EVAL which is used for DVH analysis of the target, and the generation of constraints for coverage (There are separate constraints for normal tissues, which are based on other regions of interest; see below.) 4.4.3 Beam Angles/Treatment Position Typically three-, four-, or five-field non-coplanar beam arrangements using high-energy photons can be used (Baglan et al 2003) No beams may be directed toward critical normal structures (heart, lung, contralateral breast) This implies that only quasitangential beams are allowed Depending on the gantry- and table-angle limitations imposed by the geometry of the particular accelerator, this will make it difficult to meet the normal tissue dose constraints listed below for patients with small breasts or large cavities Bolus should not be used 4.4.4 Appropriateness for Treatment There are a few more dosimetric constraints for this method than for the brachytherapy methods (see Table 4.3) V90 is the coverage parameter, and has the same definition and allowable values described above for the brachytherapy methods Of all the appropriateness parameters for 3D-CRT, it is the only one evaluated on the PTV_EVAL The uniformity criterion is the breast maximum dose This can be demonstrated from the whole breast DVH, and is also commonly reported directly by the planning system 4.4.5 Dose Prescription and Delivery Dose prescription is 385 cGy per fraction, ten fractions, twice daily The minimum time between fractions is hours, and duration of treatment is 5–10 days The dose in this case is prescribed to a reference point, usually the isocenter Ipsilateral breast constraints are defined at the 50% and 100% dose levels (V50 and V100) In addition to the V50 constraint defined for brachytherapy, no more than 35% of the breast reference volume is allowed to receive the prescription dose (385 cGy per fraction) Like the V50 constraint, the V100 constraint derives from a retrospective study of interstitial brachytherapy at William Beaumont Hospital (Baglan et al 2003) In this study, the maximum percentage breast volume receiving the prescribed dose (40 Gy in eight fractions) was 38% Other constraints are as shown in Table 4.3 All of these should be relatively easy to meet It is the constraints on the ipsilateral normal breast which will consume most of the planning effort  B-39 Protocol Requirements Table 4.3 Evaluation parameters for 3D-CRT Parameter Allowed value Evaluation of V90a ≥90% of PTV_EVAL DVH of PTV_EVAL Ipsilateral breast maximum ≤120% of prescription dose (462 cGy/fraction) Point dose V50a