The Dilemma of Target Delineation with PET/CT in Radiotherapy Planning for Malignant Tumors

Currently there are many unanswered questions concerning contouring a target with PET/CT in radiotherapy planning. Who should contour the PET volume-the radiation oncologist or the nuclear medicine physician？ Which factors will contribute to the dual-observer variability between them？ What should be taken as the optimal SUV threshold to demarcate a malignant tumor from the normal tissue？ When the PET volume does not coincide with the local area CT findings, which portion should be contoured as the target？ If a reginal lymph node,draining area or a remote region is shown to be PET positive but CT negative, or PET negative but CT positive, how is the target identified and selected？ Further studies concerning the relationship between PET/CT and the cancerous tissue are needed. The long-term clinical results showing an increased therapeutic ratio will finally verify the applicability of guidelines to contour the target with PET/CT in radiotherapy planning.

Influence of Position and Radiation Technique on Organs at Risk in Radiotherapy of Rectal Cancer

The influence of the position and radiation technique on the organs at risk（OARs） in radiotherapy of rectal cancer was evaluated. The relationship between the volume of irradiated small bowel（VSB） and acute bowel toxicity was determined. A total of 97 cases of rectal cancer were retrospectively randomized to receive radiotherapy with the designated treatment positions and radiation plans. Among 64 patients in the supine position, 32 patients were given three-dimensional conformal radiotherapy（3DCR） and 32 patients were subjected to intensity-modulated radiation therapy（IMRT） respectively. The rest 33 patients were treated with 3DCRT in the prone position with a belly board. The VSB was calculated for doses from 5 to 45 Gy at an interval of 5 Gy. With prescription dose in planned target volume（PTV） of 50 Gy, the dose distribution, conformal index for PTV（CIPTV）, dose-volume histogram（DVH） of OARs, the correlation of VSB and the acute toxicity were compared. The results were shown as follows：（1） Among the 3 methods, there were no differences in PTV＇s converge including V95 and D95;（2） For IMRT under a supine position, CIPTV was closest to 1, the mean dose of small bowel decreased（P〈0.05）, and the mean VSB from V30 to V45 significantly decreased（P〈0.05）.（3） For 3DCRT with a belly board under a prone position, the mean dose and the mean VSB from 40 to 45 Gy were less than those for 3DCRT under a supine position（P〈0.05）;（4） Mean proportion of VSB was significantly greater in the patients experiencing diarrhea grade 2-4 than in those with diarrhea grade 0-1 at dose levels from V30 to V45（P〈0.05）. It was concluded that for the radiotherapy of rectal cancer, IMRT technique might decrease the high-dose VSB to reduce the risk of acute injury. 3DCRT with a belly board under a prone position is superior to 3DCRT under a supine position, which could be a second choice for radiation of rectal cancer.

Evaluation of pelvic lymph node coverage of conventional radiotherapy fields based on bony landmarks in Chinese cervical cancer patients using CT simulation

Objective： To evaluate the pelvic lymph node coverage of conventional pelvic fields based on bony landmarks in Chinese patients with cervical cancer by using computed tomography （CT） simulation images to contour pelvic vessels as substitutes for lymph nodes location. Methods： A retrospective review of CT simulation images and conventional pelvic radiation planning data sets was performed in 100 patients with cervical cancer at the International Federation of Gynecology and Obstetrics （FIGO） Stage liB to IIIB in our hospital. Pelvic arteries were contoured on CT simulation images, and the outlines of conventional pelvic fields were drawn as defined by the gynecologic oncology group （GOG） after hiding the contours. The distances between the following vessel contours and field borders were measured： D1, the superior border of the anterior/posterior （AP） field and the bifurcation of abdominal aorta; D2, the ipsilateral border of the AP field and the distal end of external iliac artery; and D3, the anterior border of the lateral （LAT） field and the distal end of the external iliac artery. The distances were recorded as positive values if the measuring point was within the conventional pelvic fields, or they were recorded as negative values. Lymph nodes coverage was considered adequate when D1≥0 mm, D2〉17 mm or D3≥7 mm. Results： All patients had at least 1 inadequate margin, 97 patients （97.0%） had 2, and 22 patients （22.0%） had all the 3. On the AP field, 95 patients （95%） had the measuring point, the bifurcation of the abdominal aorta, out of the field （D1〈0 mm）, and all the patients had a distance less than 17.0 mm between the distal end of the external iliac artery and ipsilateral border （D2〈1 7.0 mm）. On the LAT field, 24 patients （24%） had a distance less than 7.0 mm between the distal end of the external iliac artery and anterior border （D3〈7.0 mm）. Conclusion： We observed that conventional pelvic fields based on bony landmarks provided inadequate coverage of pelvic lymph nodes in our patients with cervical cancer. CT simulation may be a feasible technique for planning pelvic fields optimally and individually.

Three-dimensional organ modeling based on deformable surfaces applied to radio-oncology

This paper describes a method based on an energy minimizing deformable model applied to the 3D biomechanical modeling of a set of organs considered as regions of interest (ROI) for radiotherapy. The initial model consists of a quadratic surface that is deformed to the exact contour of the ROI by means of the physical properties of a mass-spring system. The exact contour of each ROI is first obtained using a geodesic active contour model. The ROI is then parameterized by the vibration modes resulting from the deformation process. Once each structure has been defined, the method provides a 3D global model including the whole set of ROIs. This model allows one to describe statistically the most significant variations among its structures. Statistical ROI variations among a set of patients or through time can be analyzed. Experimental results are presented using the pelvic zone to simulate anatomical variations among structures and its application in radiotherapy treatment planning.