Investigation of a Novel Decision Support Metric for Head and Neck Adaptive Radiation Therapy Using a Real-Time In Vivo Portal Dosimetry System

In adaptive radiation therapy of head and neck cancer, any significant anatomical changes observed are used to adapt the treatment plan to maintain target coverage without elevating the risk of xerostomia. However, the additional resources required for adaptive radiation therapy pose a challenge for broad-based implementation. It is hypothesized that a change in transit fluence is associated with volumetric change in the vicinity of the target and therefore can be used as a decision support metric for adaptive radiation therapy. This was evaluated by comparing the fluence with volumetric changes in 12 patients. Transit fluence was measured by an in vivo portal dosimetry system. Weekly cone beam computed tomography was used to determine volume change in the rectangular region of interest from condyloid process to C6. The integrated transit fluence through the region of interest on the day of the cone beam computed tomography scan was calculated with the first treatment as the baseline. The correlation between fluence change and volume change was determined. A logistic regression model was also used to associate the 5% region of interest volume reduction replanning trigger point and the fluence change. The model was assessed by a chi-square test. The area under the receiver-operating characteristic curve was also determined. A total of 46 pairs of measurements were obtained. The correlation between fluence and volumetric changes was found to be -0.776 (P value <.001). The negative correlation is attributed to the increase in the photon fluence transport resulting from the volume reduction. The chi-square of the logistic regression was found to be 17.4 (P value <.001). The area under the receiver-operating characteristic curve was found to be 0.88. Results indicate the change in transit fluence, which can be measured without consuming clinical resources or requiring additional time in the treatment room, can be used as a decision support metric for adaptive therapy.

SU-E-T-583: Feasibility of Constraining Dose to the Nausea Center (area Postrema and Dorsal Vagal Complex) in IMRT Treatment Planning of the Head and Neck

PURPOSE

Nausea and vomiting have been known to occur in patients undergoing external beam radiation treatments for head&neck cancers. We sought to determine the feasibility of limiting the dose delivered to the nausea center, area postrema (AP) and dorsal vagal complex (DVC), for these patients without compromising target coverage and critical organ doses.

METHODS

In a retrospective study 23 oropharyngeal cancer patients were identified as being treated with definitive or adjuvant radiotherapy at Memorial Sloan Kettering Cancer Center. Patients were treated solely with external beam radiation using intensity modulated radiation therapy (IMRT). The nausea center was carefully contoured in the treatment CT with the assistance of a board certified neuroradiologist. The doses delivered to the nausea center were calculated for each plan delivered. Cases were replanned offline to determine the lowest achievable nausea center dose that does not compromise the overall PTV coverage or critical structures doses, these being brainstem, spinal cord, cochleas, and temporal lobes.

RESULTS

Patients reporting higher nausea grade had median AP and DVC doses of 38.7Gy and 40.4Gy, respectively. Patients reporting higher vomiting grade had median AP and DVC doses of 39.5Gy and 44.7Gy, respectively. Replanning resulted in reduced dose to AP by an average of 18% and to the DVC by an average of 16% while maintaining adequate target coverage and doses to the critical organs the same or decreased by 1-4% . We aim to achieve a max dose of 36Gy to AP and 38Gy to DVC for these cases.

CONCLUSIONS

It is feasible to limit the doses to the nausea center without compromising target coverage or critical organ limits for oropharyngeal cancer patients undergoing IMRT treatment. Clinical results indicating an association between radiation dose to the nausea center and development of nausea and/or vomiting can potentially be addressed by implementing this technique.

Lachancea nothofagi sp. nov., a yeast associated with Nothofagus species in Patagonia, Argentina

Six strains of a novel yeast species were isolated from Nothofagus species trees in native forests in Patagonia, Argentina. The strains were isolated from bark, fluxes and the ectomycorrhizospheric soil fraction of Nothofagus antarctica, Nothofagus nervosa and Nothofagus pumilio. Analysis of the D1/D2 large-subunit rDNA sequences indicated that the novel species belonged to the genus Lachancea and is closely related to Lachancea meyersii. The name Lachancea nothofagi sp. nov. is proposed to accommodate these strains. The type strain is UWOPS 99-807.3T (=CBS 11611T=NRRL Y-48670T).