2001 CTOS Annual Meeting Posters— Radiation Oncology

USE OF INTENSITY MODULATED RADIOTHERAPY (IMRT) IN THE TREATMENT OF SARCOMA
Martin H Robinson¹,  Barbara-Ann Millar²,  John Conway³,  Christopher M Bragg^1
1YCR Unit of Clinical Oncology, Weston Park Hospital,  ²Department of Clinical Oncology, Weston Park Hospital,  ³Department of Medical Physics, Weston Park Hospital

METHODS: The diagnostic and planning images of 9 patients treated pre or post operatively for sarcoma (4 limb extremity, 3 pelvic, 1 nasal sinuses, 1 trunk). Using radiotherapy planning computerised tomography (CT) images and diagnostic magnetic resonance imaging (Mm) and CT imaging for each patient, the gross original tumour, clinical target volume, and planning target volume were outlined along with normal tissue structures. Normal tissues were assigned appropriate tolerances for radiotherapy. Radiotherapy plans were then produced using conformal and IMRT planning techniques.

RESULTS: Regret analysis, normal tissue complication probability and Conformity Index (CI) were used to compare the ability of each technique to optimise dose to tumour whilst sparing normal structures. Extremities: The CI was improved with IMRT in both PTV-I and PTV-II. In one case, the PTV-I with 3D-CRT gave a value of 0.14 improving with IMRT to 0.32 (5f-IMRT). PTV-I was also shown to improve with initial 3D-CRT value of 0.58, increasing to 0.84 (5f-IMRT). However, it was not possible to utilise all of the multiple fields in one case due to the field length being greater than 25 cm and therefore the maximum number of fields was limited to five. This is felt to have contributed to the poorer overall results for this case. In extremity sites, where the treatment volumes include considerable amounts of normal tissue, it was possible to limit the dose to bone and subcutaneous tissue using IMRT by up to 20%. Pelvic: In planning treatment for pelvic sarcoma again improvement in conformity was demonstrated with IMRT, especially when using greater than 5 fields. A deterioration in homogeneity and increase in under- and overdosing of both PTV-I and PT V-Il may have been due to a compromise in optimisation due to the position of the target relative to the surrounding normal tissue structure such as the rectum. Trunk and paranasal sinuses: In these cases, due to the site of the original tumour, margins were added to the original tumour volume and this was treated in a single phase. In the case of soft tissue sarcoma of the trunk, IMRT showed a considerable improvement in homogeneity, conformity index and underdosing. The conformity index increased from 0.52 (3D-CRT) to 0.85 (6f-IMRT). The majority of organs at risk demonstrated a consistent reduction in maximum dose received, e.g. the heart received 19.7 Gy (3D-CRT) compared to 6.2 Gy (7f-IMRT). There was a slight increase in the dose received by the spinal cord but this remained below 8 Gy in all cases. In the case of the osteosarcoma arising in the paranasal sinuses, again conformity was improved but at the expense of homogeneity due to the close proximity of the organs at risk to the tumour volume. It was possible to reduce the mean dose to organs at risk, in particular the lens and the eye. In the right lens, a complication probability of 100% with 3D-CRT was reduced to 75% and in the left lens it was reduced from 100% to 30%. Reviewing the tumour control probability (TCP) for IMRT, there was a trend towards improvement in TCP with an increased number of fields and this was generally mirrored in the regret scores and conformity index.

CONCLUSION: IMRT has the potential to improve quality of radiotherapy in sarcoma practice. This benefit appears to vary with tumour site.