Whole Pelvis Flashcards
Rationale for WPIRT/VMAT
More conformal
Reduce dose to small bowel - can escalate dose without harming OARs
3DCRT whole pelvis beam arrangement
AP/PA
PA and opposed lateral
Four fields
3DCRT limitations
Cannot deliver dose to paracervical or nodal tissue
Can increase bowel dose - bowel has high a/b ratio hence more sensitive to radiaiton
Cannot escalate dose
Which sites are often treated with WPRT
Gynae
Rectum and LN
Anal canal and pelvic and inguinal LN
Prostate and LN
EBRT dose fraction
50.4GY IN 28 (1.8gy per fraction) followed by boost of 5.4GY in 3#
Or
45Gy to nodal volumes
1.8Gy per fraction reduces severity of side effects
Considerations for rectum WPRT
SIB plan - 54_51_47Gy
Primary GTV and Mesorectum and inguinal CTV
Complex plan - do not try to achieve dose to skin as it increases skin reactions
EXT_genitalia should be considered when planning
Folding in skin causes skin reactions
WPRT techniques
✓3D-conformal radiotherapy (3D-CRT)
✓Intensity-modulated radiotherapy (IMRT)
✓Volumetric Modulated Arc Therapy (VMAT)
✓Intensity Modulated Proton Therapy (IMPT)
✓Helical Tomotherapy (HT)
✓Varian Halcyon
Most beneficial - VMAT
Robust evaluation
Impact on patient’s safety due to possible difference in patient setup
IMPT
Up to 3 beams - 2 ant oblique, 1 post
Intensity modulated proton therapy
Can be conformal but any movement can severely increase dose to OARs
What is EXP ext 1CM USED FOR
Assess weight loss or gain
Where should iso be located
In line with midline/ant tattoo
Close to BB markers
Middle of PTV
IMRT beam arrangements
Non opposing gantry angles
Collimator should be sup inf oriented as it assist in shielding bowels and lowers MUs
How many arc should be used in VMAT WPRT
2/3 depending on protocol on field size
Gantry stop and start if there is a 4 degree spacing
178 - 182
182-178
Colli 15 on 1st arc, 345 on 2nd arc
Which machine has less interleaf leakage and why
Elekta - lower jaw is primary collimator which means less interleaf leakage compared to Varian which has MLCs situated lower
Max gantry rotation speed
6 degrees
Why can’t we use a 45 degree collimator
Dose to OARs
IGRT considerations
Could be:
KV Daily
CBCT daily
Kv+CBCT day 1,2,3 and then KV daily and CBCT weekly
Imaging fields
Can be done on TPS or mosaiq
Copy IMRT fields and delete MLCs and change angles
What structures to contour
Small bowel, bowel bag, bladder, femoral head and neck, bone marrow
Bowel bag contour
Outlined on every slice which has visible bowel, extending 2cm above PTV
Includes bowels out to the edge of the peritoneum. Bowel bag contour to include small bowel, colon and sigmoid
Rectum contour
Structure should extend from anal verge to recto-sigmoid flexure
Bone marrow contour
The pelvic bone will be contoured as a surrogate for the bone marrow. Femoral heads included but not necks
IMRT beam arrangements
7-9 beams
7 beams: 45, 90, 115, 180, 245, 280 and 310