A strategy to correct for intrafraction target translation in conformal prostate radiotherapy: simulation results

A strategy is proposed in which intrafraction internal target translation is corrected for by repositioning the multileaf collimator position aperture to conform to the new target pose in the beam projection, and the beam monitor units are adjusted to account for the change in the geometric relationship between the target and the beam. The purpose of this study was to investigate the dosimetric stability of the prostate and critical structures in the presence of internal target translation using the dynamic compensation strategy. Twenty-five previously treated prostate cancer patients were replanned using a four-field conformal technique to deliver 72 Gy to 95% of the planning target volume (PTV). Internal translation was introduced by displacing the prostate PTV (no rotation or deformation was considered). Thirty-six randomly selected isotropic displacements of magnitude 0.5, 1.0, 1.5 and 2.0 cm were sampled for each patient, for a total of 3600 errors. Due to their anatomic relation to the prostate, the rectum and bladder contours were also moved with the same magnitude and direction as the prostate. The dynamic compensation strategy was used to correct each of these errors by conforming the beam apertures to the new target pose and adjusting the monitor units using inverse-square and off-axis factor corrections. The dynamic compensation strategy plans were then compared to the original treatment plans via dose-volume histogram (DVH) analysis. Changes of more than 5% of the prescription dose (3.6 Gy) were deemed clinically significant. Compared to the original treatment plans, the dynamic compensation strategy produced small discrepancies in isodose distributions and DVH analyses for all structures considered apart from the femoral heads. These differences increased with the magnitude of the internal motion. Coverage of the PTV was excellent: D5, D95, and Dmean were not increased or decreased by more than 5% of the prescription dose for any of the 3600 simulated internal motion shifts. Dose increases for adjacent organs at risk were rare. D33 of the rectum and D20 of the bladder were increased by more than 5% of the prescription dose in 9 and 1 instances of the 3600 sampled internal motion shifts, respectively. Dmean of the right femoral head increased by more than 5% of the prescription dose in 651 (18%) internal motion shifts, predominantly due to the projection of the lateral beams through the femoral head for anterior prostate motion. However, D2 was not increased by more than 5% for any of the internal motion shifts. These data demonstrate the robustness of the proposed dynamic compensation strategy for correction of internal motion in conformal prostate radiotherapy, with minimal deviation from the original treatment plans even for errors exceeding those commonly encountered in the clinic. The compensation strategy could be performed automatically with appropriate enhancements to available delivery software.

A dynamic compensation strategy to correct patient-positioning errors in conformal prostate radiotherapy

Traditionally, pretreatment detected patient-positioning errors have been corrected by repositioning the couch to align the patient to the treatment beam. We investigated an alternative strategy: aligning the beam to the patient by repositioning the dynamic multileaf collimator and adjusting the beam weights, termed dynamic compensation. The purpose of this study was to determine the geometric range of positioning errors for which the dynamic compensation method is valid in prostate cancer patients treated with three-dimensional conformal radiotherapy. Twenty-five previously treated prostate cancer patients were replanned using a four-field technique to deliver 72 Gy to 95% of the planning target volume (PTV). Patient-positioning errors were introduced by shifting the patient reference frame with respect to the treatment isocenter. Thirty-six randomly selected isotropic displacements with magnitudes of 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 cm were sampled for each patient, for a total of 5400 errors. Dynamic compensation was used to correct each of these errors by conforming the beam apertures to the new target position and adjusting the monitor units using inverse-square and off-axis factor corrections. The dynamic compensation plans were then compared with the original treatment plans via dose-volume histogram (DVH) analysis. Changes of more than 5% of the prescription dose, 3.6 Gy, were deemed significant. Compared with the original treatment plans, dynamic compensation produced small discrepancies in isodose distributions and DVH analyses. These differences increased with the magnitudes of the initial patient-positioning errors. Coverage of the PTV was excellent: D95 and Dmean were not increased or decreased by more than 5% of the prescription dose, and D5 was not decreased by more than 5% of the prescription dose for any of the 5400 simulated positioning errors. D5 was increased by more than 5% of the prescription dose in only three of the 5400 positioning errors, all three occurring with a positioning error of 10.0 cm. Dose increases for adjacent organs at risk were more common. D33 of the rectum and the periprostatic rectum was increased by more than 5% of the prescription dose in 235 (4.4%) and 212 (3.9%) of the 5400 positioning errors, respectively. D10 of the right femoral head increased by more than 5% of the prescription dose in 444 (8.2%) positioning errors, and the degree of change was highly related to individual patient anatomy and simulation position. For the bladder D20, there were three increases of more than 5% of the prescription dose. These data demonstrate the robustness of dynamic compensation for correction of patient-positioning errors in four-field conformal prostate radiotherapy, with minimal deviation from the original treatment plans even for errors greatly exceeding those commonly encountered in the clinic. Dynamic compensation can be performed remotely, thus eliminating errors that may result from unnecessary increases in treatment time or from secondary patient motion induced by couch motion during the repositioning process. Further, the ability of dynamic compensation to correct large positioning errors has implications for the accuracy necessary during the initial patient setup and, hence, patient throughput for prostate radiotherapy.

The effect of setup uncertainty on normal tissue sparing with IMRT for head-and-neck cancer

PURPOSE

Intensity-modulated radiotherapy (IMRT) is being evaluated in the management of head-and-neck cancers at several institutions, and a Radiation Therapy Oncology Group study of its utility in parotid sparing is under development. There is an inherent risk that the sharper dose gradients generated by IMRT amplify the potentially detrimental impact of setup uncertainty. The International Commission on Radiation Units and Measurements Report 62 (ICRU-62) defined planning organ-at-risk volume (PRV) to account for positional uncertainties for normal tissues. The purpose of this study is to quantify the dosimetric effect of employing PRV for the parotid gland and to evaluate the use of PRV on normal-tissue sparing in the setting of small clinical setup errors.

METHODS AND MATERIALS

The optimized nine-beam IMRT plans for three head-and-neck cancer patients participating in an institutional review board approved parotid-sparing protocol were used as reference plans. A second optimized plan was generated for each patient by adding a PRV of 5 mm for the contralateral parotid gland. The effect of these additions on the quality of the plans was quantified, in terms of both target coverage and normal-tissue sparing. To test the value of PRV in a worst-case scenario, systematic translational setup uncertainties were simulated by shifting the treatment isocenter 5 mm superiorly, inferiorly, left, right, anteriorly, and posteriorly, without altering optimized beam profiles. At each shifted isocenter, dose distributions were recalculated, producing a total of six shifted plans without PRV and six shifted plans with PRV for each patient. The effect of setup uncertainty on parotid sparing and the value of PRV in compensating for the uncertainty were evaluated.

RESULTS

The addition of the PRV and reoptimization did not significantly affect the dose to gross tumor volume, spinal cord, or brainstem. In contrast, without any shift, the PRV did increase parotid sparing and reduce coverage of the nodal region adjacent to the parotid gland. As expected, when the plans were shifted, the greatest increase in contralateral parotid irradiation was noted with shifts toward the contralateral parotid gland. With these shifts, the average volume of contralateral parotid receiving greater than 30 Gy was reduced from 22% to 4% when a PRV was used. This correlated with a reduction in the average normal-tissue complication probability (NTCP) from 22% to 7%.

CONCLUSIONS

The use of PRV may limit the volume of normal tissue structures, such as the parotid gland, exceeding tolerance dose as a result of setup errors. Consequently, it will be important to incorporate the nomenclature of ICRU-62 into the design of future IMRT studies, if the clinical gains of increased normal-tissue sparing are to be realized.

Gemcitabine following radiotherapy with concurrent 5-fluorouracil for nonmetastatic adenocarcinoma of the pancreas

Gemcitabine has been shown to be an active agent in the treatment of pancreatic cancer. This study was conducted to prospectively examine the tolerance and early efficacy of adjuvant gemcitabine following radiotherapy with concurrent 5-fluorouracil (5-FU) for nonmetastatic pancreatic adenocarcinoma. Twenty-three patients, median age 64 years, were treated with combined modality therapy. Nine patients underwent tumor resection before chemoradiation; 14 patients with locally unresectable tumors received definitive chemoradiation. Radiotherapy utilized four fields to the tumor and lymphatics to 45 Gy, plus a lateral boost to 50.4 Gy. Concurrent 5-FU 500 mg/m(2)/day was administered on days 1-3 and 29-31, followed by 4 months of gemcitabine 1,000 mg/m(2)/week for 3 weeks (fourth week break). Adjuvant gemcitabine was well tolerated. Eighty-three percent of the patients completed three to four cycles. The primary dose-limiting toxicity was leukopenia, which was observed in 10 patients (43%). Nonhematologic toxicities were reported in five patients (22%). There were no cases of gemcitabine-induced radiation recall and there have been no deaths attributed to treatment toxicity. Median follow-up for the 23 patients was 12 months (range, 5-50); the actuarial median survival was 13 months. This report confirms that adjuvant gemcitabine following radiotherapy with concurrent 5-FU for nonmetastatic pancreatic adenocarcinoma can be safely administered.