Standard Target Volume Research Articles

139. Parallel-opposed fields vs. four fields, and two- (2D) vs. three-dimensional (3D) radiotherapy planning in thin patients with gynecological malignancies

The superiority of 3D planning and the use of multiple field techniques allow more selective delivery ot radiotherapy in the majority of patients. In pelvic irradiation of very thin patients, however, some centers still use the parallel opposed field technique, due to its economic and logistic benefits and doubtful advantage of more sophisticated techniques. This study compares dose distribution for various treatment techniques in cervical or endometrial cancer patients with AP diameter of less than 20 cm. Material and methods In 6 patients CT-based 3D treatment planning was conducted using 4-field “box” technique. For study purposes, 2D simulation portals were also obtained for parallel opposed fields and 4-field “box” techniques. All plans were prepared both for Cobalt-60 unit (Co) and for 9 or 18 MV photons generated by linear accelerator (LA). The median fields’ area and the dose-volume histograms for critical organs’ including bladder, rectum and small intestine were compared for three treatment methods and for all energies. The 2D portals were transferred into 3D system to assess the accuracy of PTV (primary tumor and nodal areas) coverage. Results On average, 3D AP/PA and lateral fields were 76 cm 2 (23%) and 43 cm 2 (16%) bigger than 2D portals, respectively. Bladder volume covered by 95% isodose was similar with all methods, while both 2D and 3D “box” technique spared additional 10–14% the of rectum volume. The small intestine volume covered by 95% isodose was similar in 2D-plans (with both 2- and 4-field techniques), while 3D planning spared additional 10% volume. The mean dose for bladder was 98% for all plans. Up to 10% of the rectum was spared in standard 4-field and Co conformal plans as compared to 14% in LA conformal plans. Median small intestine doses were 76%, 84% and 76% for 2D 2-, 4-fields and conformal fields, respectively. PTV was not covered effectively with 2D Co 2 fields, LA 2 fields and with Co box technique in 5, 1 and 5 pts, respectively. Conclusion The standard target volume defined on CT scans is not covered effecively by 2-field and Co 2D 4-field plans. 4-field plans allows for better sparing of the rectum. 3D planning does not allow for better bladder protection, but spares additional volume of the rectum and small intestine.

Metabolic imaging of low-grade gliomas with three-dimensional magnetic resonance spectroscopy

Purpose: The role of radiotherapy (RT) seems established for patients with low-grade gliomas with poor prognostic factors. Three-dimensional (3D) magnetic resonance spectroscopy imaging (MRSI) has been reported to be of value in defining the extent of glioma infiltration. We performed a study examining the impact MRSI would have on the routine addition of 2–3-cm margins around MRI T2-weighted hyperintensity to generate the treatment planning clinical target volume (CTV) for low-grade gliomas. Methods and Materials: Twenty patients with supratentorial gliomas WHO Grade II (7 astrocytomas, 6 oligoastrocytomas, 7 oligodendrogliomas) underwent MRI and MRSI before surgery. The MRI was contoured manually; the regions of interest included T2 hyperintensity and, if present, regions of contrast enhancement on T1-weighted images. The 3D-MRSI peak parameters for choline and N-acetyl-aspartate, acquired voxel-by-voxel, were categorized using a choline/ N-acetyl-aspartate index (CNI), a tool for quantitative assessment of tissue metabolite levels, with CNI 2 being the lowest value corresponding to tumor. CNI data were aligned to MRI and displayed as 3D contours. The relationship between the anatomic and metabolic information on tumor extent was assessed by comparing the CNI contours and other MRSI-derived metabolites to the MRI T2 volume. Results: The limitations in the size of the region “excited” meant that MRSI could be used to evaluate only a median 68% of the T2 volume (range 38–100%), leaving the volume T2c. The CNI 2 volume (median 29 cm 3, range 10–73) was contained totally within the T2c in 55% of patients. In the remaining patients, the volume of CNI 2 extending beyond the T2c was quite small (median 2.3 cm 3, range 1.4–5.2), but was not distributed uniformly about the T2c, extending up to 22 mm beyond it. Two patients demonstrated small regions of contrast enhancement corresponding to the regions of highest CNI. Other metabolites, such as creatine and lactate, seem useful for determining less and more radioresistant areas, respectively. Conclusion: Metabolically active tumor, as detected by MRSI, is restricted mainly to the T2 hyperintensity in low-grade gliomas, but can extend outside it in a limited and nonuniform fashion up to 2 cm. Therefore, a CTV including T2 and areas of CNI extension beyond the T2 hyperintensity would result in a reduction in the size and a change in the shape of the standard clinical target volumes generated by adding uniform margins of 2–3 cm to the T2 hyperintensity.