Simultaneous Integrated Boost In Cervix Cancer: The Impact Of Organ Motion And Tumor Regression

ObjectivesWhole-pelvis intensity modulated radiation therapy (IMRT) is increasingly used to treat cervix cancer. Encouraged by this, some groups have proposed the use of simultaneous integrated boost (SIB) to target the tumor. Our purpose was to evaluate the magnitude of SIB regression and motion and to monitor treatment progress dosimetrically and volumetrically to see if treatment goals are met.Materials/MethodsTen patients treated with standard doses of chemoradiation therapy and instructed to have bladder full and rectum empty all along treatment were retrospectively re-planned using an IMRT- SIB technique for the hypothetical scenario of a feasibility study. Patients without significant change in outer anatomy during treatment were selected to avoid discrepancies in the inner dose grid. Contouring occurred on the planning - CT and on each weekly megavoltage computed tomography (MV-CT) based on RTOG 0418 and IMRT consortium guidelines. An internal consensus was met for contouring the clinical target volume-SIB (CTV-SIB). The center of mass and Dice's similarity coefficient were used to evaluate volumes displacements and overlaps. The planned dose was 50.4 Gy, 1.8 Gy for pelvis PTV and 59.36 Gy, 2.12 Gy for the PTV-SIB. A rigid registration of the DICOM-RT dose file was performed. The baseline and weekly CTV-SIB and organs at risk (OARs) were then used to plot a secondary dose cumulative histogram to model estimated delivered doses.ResultsThe relative reduction in the CTV-SIB volume from baseline to the end of treatment was 31-70%. Extensive CTV-SIB motion was observed mainly in the antero-posterior and supero-inferior position with Dice's similarity coefficient of 0.67. Despite bowel and bladder preparation, substantial variation occurred during treatment. The maximal relative change in bladder volume compared to baseline was 34 - 91%, recto-sigmoid 8 - 73.8%, and bowel 5-52%. There was no significant difference between planned vs. delivered doses to CTV-SIB. However, individually two patients received less than 95% of the prescribed dose. Much higher doses than expected to the OARs were observed. A multiple regression analysis showed significant interaction between CTV-SIB reduction and OARs dose increase.ConclusionsThe CTV-SIB had a marked interfraction variation due to tumor regression and anatomical motion. Daily image guided radiation therapy and considerable PTV margins could not compensate for CTV-SIB positional changes with lower therapeutic doses than expected. The OARs had unpredictable shifts and dynamics with higher than expected doses. Failure to take these variations into account during the application of new IMRT techniques like SIB exposes patients to the unpredictable risk of under dosing the target or overdosing adjacent critical structures. ObjectivesWhole-pelvis intensity modulated radiation therapy (IMRT) is increasingly used to treat cervix cancer. Encouraged by this, some groups have proposed the use of simultaneous integrated boost (SIB) to target the tumor. Our purpose was to evaluate the magnitude of SIB regression and motion and to monitor treatment progress dosimetrically and volumetrically to see if treatment goals are met. Whole-pelvis intensity modulated radiation therapy (IMRT) is increasingly used to treat cervix cancer. Encouraged by this, some groups have proposed the use of simultaneous integrated boost (SIB) to target the tumor. Our purpose was to evaluate the magnitude of SIB regression and motion and to monitor treatment progress dosimetrically and volumetrically to see if treatment goals are met. Materials/MethodsTen patients treated with standard doses of chemoradiation therapy and instructed to have bladder full and rectum empty all along treatment were retrospectively re-planned using an IMRT- SIB technique for the hypothetical scenario of a feasibility study. Patients without significant change in outer anatomy during treatment were selected to avoid discrepancies in the inner dose grid. Contouring occurred on the planning - CT and on each weekly megavoltage computed tomography (MV-CT) based on RTOG 0418 and IMRT consortium guidelines. An internal consensus was met for contouring the clinical target volume-SIB (CTV-SIB). The center of mass and Dice's similarity coefficient were used to evaluate volumes displacements and overlaps. The planned dose was 50.4 Gy, 1.8 Gy for pelvis PTV and 59.36 Gy, 2.12 Gy for the PTV-SIB. A rigid registration of the DICOM-RT dose file was performed. The baseline and weekly CTV-SIB and organs at risk (OARs) were then used to plot a secondary dose cumulative histogram to model estimated delivered doses. Ten patients treated with standard doses of chemoradiation therapy and instructed to have bladder full and rectum empty all along treatment were retrospectively re-planned using an IMRT- SIB technique for the hypothetical scenario of a feasibility study. Patients without significant change in outer anatomy during treatment were selected to avoid discrepancies in the inner dose grid. Contouring occurred on the planning - CT and on each weekly megavoltage computed tomography (MV-CT) based on RTOG 0418 and IMRT consortium guidelines. An internal consensus was met for contouring the clinical target volume-SIB (CTV-SIB). The center of mass and Dice's similarity coefficient were used to evaluate volumes displacements and overlaps. The planned dose was 50.4 Gy, 1.8 Gy for pelvis PTV and 59.36 Gy, 2.12 Gy for the PTV-SIB. A rigid registration of the DICOM-RT dose file was performed. The baseline and weekly CTV-SIB and organs at risk (OARs) were then used to plot a secondary dose cumulative histogram to model estimated delivered doses. ResultsThe relative reduction in the CTV-SIB volume from baseline to the end of treatment was 31-70%. Extensive CTV-SIB motion was observed mainly in the antero-posterior and supero-inferior position with Dice's similarity coefficient of 0.67. Despite bowel and bladder preparation, substantial variation occurred during treatment. The maximal relative change in bladder volume compared to baseline was 34 - 91%, recto-sigmoid 8 - 73.8%, and bowel 5-52%. There was no significant difference between planned vs. delivered doses to CTV-SIB. However, individually two patients received less than 95% of the prescribed dose. Much higher doses than expected to the OARs were observed. A multiple regression analysis showed significant interaction between CTV-SIB reduction and OARs dose increase. The relative reduction in the CTV-SIB volume from baseline to the end of treatment was 31-70%. Extensive CTV-SIB motion was observed mainly in the antero-posterior and supero-inferior position with Dice's similarity coefficient of 0.67. Despite bowel and bladder preparation, substantial variation occurred during treatment. The maximal relative change in bladder volume compared to baseline was 34 - 91%, recto-sigmoid 8 - 73.8%, and bowel 5-52%. There was no significant difference between planned vs. delivered doses to CTV-SIB. However, individually two patients received less than 95% of the prescribed dose. Much higher doses than expected to the OARs were observed. A multiple regression analysis showed significant interaction between CTV-SIB reduction and OARs dose increase. ConclusionsThe CTV-SIB had a marked interfraction variation due to tumor regression and anatomical motion. Daily image guided radiation therapy and considerable PTV margins could not compensate for CTV-SIB positional changes with lower therapeutic doses than expected. The OARs had unpredictable shifts and dynamics with higher than expected doses. Failure to take these variations into account during the application of new IMRT techniques like SIB exposes patients to the unpredictable risk of under dosing the target or overdosing adjacent critical structures. The CTV-SIB had a marked interfraction variation due to tumor regression and anatomical motion. Daily image guided radiation therapy and considerable PTV margins could not compensate for CTV-SIB positional changes with lower therapeutic doses than expected. The OARs had unpredictable shifts and dynamics with higher than expected doses. Failure to take these variations into account during the application of new IMRT techniques like SIB exposes patients to the unpredictable risk of under dosing the target or overdosing adjacent critical structures.