PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): The use of intensity modulated proton therapy (IMPT) for high risk prostate cancer patients may achieve substantial sparing of radiation exposure to normal tissues in the pelvis. However, patient anatomic variation, such as increased bowel volume, can create large, unacceptable uncertainties. This work evaluates the ability of a previously described IMPT planning technique, known to be robust against bowel density changes, to also account for variation in rectal volume.
Materials/Methods: Data from 15 patients treated for prostate cancer were used. The robustness optimization of the originally-treated IMPT plans included density modified copies of the planning CT images, as well as a 5mm isotropic setup uncertainty and a ±3.5% range uncertainty. Doses were recalculated on CT scans with four uniform circumferential expansions or contractions of the rectum (+15mm, +10mm, +5mm, -5mm) that were edited off anatomical boundaries to assess robustness in regards to volumetric changes. In order to determine that these expansions were accurately modeling patient anatomic variation, the rectal doses were also calculated on Quality Assurance CT (QACT) scans taken at various time points during the treatment course of each patient. Percent rectal doses at levels of 60 Gy, 65 Gy, 70 Gy, and 75 Gy were recorded from the original plan and with expanded rectal volumes. Differences between these values and mean values from corresponding QACTs were compared using a two sample t-test.
Results: The IMPT plans generated rectal doses that sufficiently met our institutional guidelines on both the adjusted planning CT volumes and the QACT scans. Table 1 displays the average difference between rectal doses of adjusted volumes and mean doses of the QACT scans, which ranged from -0.624 (+10mm expansion) to -10.064 (-5mm contraction). Mean rectal dose differences using 0mm and -5mm contractions were significantly different from mean doses observed in QACT scans (p-value 0.0002 and 9.0 x 10-9, respectively), whereas means from +15mm and +10mm expansions were similar to QACT scans (p-value 0.495 and 0.67, respectively).
Conclusion: Our IMPT planning technique demonstrated robustness to variations in rectum size, as evaluated by the doses recalculated on the adjusted volumes and QACT scans. Expansions of +10mm and +15mm best account for naturally occurring variations in rectal volume, and should be examined during plan evaluation.
Differences in Percent Rectal Dose
|Rectal Volume Expansions/ Contractions (mm)||+15||+10||+5||0||-5|
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